# Infinite Baffle – Theory vs. Reality



## Schramm (Mar 12, 2012)

Hello all from Minnesota. I am planning an installation (first one) in a gas sipping daily driver sedan (Honda Civic) and have most of the equipment already. I may be getting a slightly newer Civic soon, so the project is temporarily on hold now. However, I need some help.

*There are a lot of existing threads regarding infinite baffle design and construction examples scattered here on this forum. I have glanced many of them. While I am convinced of the potential advantages enough to begin experimentation with some low frequency transducers, I have a few major concerns not addressed elsewhere, at least that I can find. Quite frankly, I have never heard such a design in real life. Rather than jump in on someone else’s thread, I am going to start my own. This is a long first post, so if you have made it thus far, please keep reading. *

Allow me to introduce myself as I am relatively new to the DIYMA site. Most of my experience and interest is actually in home audio as the car environment is terrible for acoustics. However, I love music and have been fixing, designing, and building things since a young age. I was most recently employed in the aerospace engineering industry for the last few years. In my free time, I enjoy studying engineering and technical aspect of acoustics and loudspeaker design. I am most interested in dynamic, low distortion, and cohesive wave front approaches common to pro audio, so am partial to large drivers, concentric point source drivers, horns, synergy horns, acoustically ideal room spaces, etc. I also enjoy furniture making as a hobby. With limited means, lots of tools, and excellent fabrication skills; the DIY approach is addictive. It is good to see pictures of installations, experimental approaches, and passionate; intelligent dialogue with those who have spent real money on equipment to do scientific measurement/analysis to back up opinions on this forum. I have to say that because there are so many variables involved, making subjective comparison difficult. Thanks to all for contributing.

Here we go. *The primary design goal is sound quality – low distortion, flat frequency response, dynamic up to a loud conversational volume level.* I listen mostly to acoustic and classical music from CD’s, rarely any louder for safety reasons while driving. This will be a relatively low power system (6 channels, actively driven, 30W to tweeters, 50W to midbass, 100W to sub). Scanspeak 6.5” midbass and 1” tweeter will be used for the front stage. I have yet to hear a good system that gives impression of up front bass. However, I am convinced there is no substitute for cone surface area when it comes to reproducing the lowest octave (32-65 Hz) efficiently and with realism. There is no sense in rowing the boat really fast with a small paddle. It can be done with smaller low Fs high xmax drivers, but at the cost of efficiency and heavy loads on a vehicle electrical system. I prefer to take the road less traveled and infinite baffle is promising in theory at many levels, i.e. minimal trunk space, ability to use large drivers. 

Q1
*Has anyone purposely built a "partially sealed" infinite baffle design, sealing everything except the air vents located on the rear deck of most sedans?*

The reason I ask is because MN weather conditions are very harsh and present additional challenges with this type of installation. Wide temperature and humidity variations require the heating/AC and ventilation system work effectively. Air flow around vents must be retained at all costs because in the winter temps typically range from -25F to 40F and lots of snow and ice will blanket a vehicle. The temp differential inside the cabin will cause moisture build up and frosting if air flowing into the cabin if not also exited at the same rate. Hot air flow on the base of the windshield, door glass near side mirrors, and base of rear window glass is a must to defrost inner glass and thaw ice build-up on exterior. A functional A/C system is not as critical, but still important since in the summer from July to August temps are often 85-95 degrees F with humidity 90-95%. Therefore, sealing the two 12” long rectangular air vents on my rear deck is not an option for me as it will prevent air and moisture circulation. I could seal everything else in the cabin tight, at the rear OEM 6X9 location, just in front of the trunk spring torsion bars to cabin area. With that said, I am still experimenting with infinite baffle design, convinced of its theoretical advantages. 

Attached picture shows an 18” JBL 2241G and two 15” TOA BST-126. They are very efficient pro audio drivers. I may keep the JBL for the house as it is a far superior driver in every aspect. That is what my wife wants! I cannot find specs on the TOA woofers, but they have very light paper cones and vented pole. I am not sure if they will play deep enough, below 45Hz.

Q2
*Can a “partially sealed” infinite baffle system blend well with a front stage without being completely sealed? *

With a very large cone surface area, I assume output can still be more than adequate even with some unavoidable back wave cancellation. However, I have read others who experience nulls in up front midbass response, even when the subwoofer(s) are crossed very low around 45Hz (?), time aligned, and EQ applied to flatten the frequency response. This combined with a highly resonant trunk and somewhat band pass effect raises serious concern.

Q3
*Is 18dB per octave crossover slope with time alignment enough to avoid pulling the sound to the rear? Midbass will likely be crossed over at 65-70Hz. Again loud conversational volume level, nothing too crazy.*

Q4
*Will an IB design firing up through empty OEM 6X9 location create a Helmholtz resonance effect?*

That’s it for now. Hope this has been a worthwhile read. Please accept my apologies if such questions have been addressed elsewhere.

Cheers


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## sqshoestring (Jun 19, 2007)

I left a couple of vents open in my rear deck, I doubt it makes much difference with two 15s. You can blow through holes but should have something like half the cone area maybe more with big xmax maybe less is possible with lower power use. Try to keep the holes shallow unlike a port, so they tune way higher than the sub. Try to keep the manifold area small as you can but don't restrict air flow that will happen at lowest bass tones. The chamber can tune like a bandpass and you don't want that.

15s are ideal because they can have an Fs of 20Hz....they get lower. IB is just a really large sealed, or large enough the enclosure does not affect the tuning much if at all. You can see it in a model if you change volume the FR stays about the same.

I'd say focus on getting more midbass in the front of your car, use larger mids, midbass, deaden the doors, do anything to enhance 60-100hz in the front. Xover the subs at 50-80hz depending on FR and xover used. Have mids that will pick up there, that is the hardest part usually.

I ran an old alpine amp that was 4x25 and 1x100 @12v. It worked ok on my 15s. They didn't pound but the SQ was great they still played flat to 25hz or so. But normally I run 500rms on them, in the case that I need to rock some bass to relieve tensions lol. But seriously I rarely run it that loud. I had a 350rms that did 98% of what I need.

Be careful of what sub you select, I recommend you model it in your trunk volume. High Vas subs will raise the Q in a small trunk or with multiple subs, and you lose the bottom that way. You don't want a really inefficient sub on 100w, but not a really efficient one either because most of those will roll off more on the bottom. Most pro drivers are that way, they are designed to play much higher range than size suggests. They play low vocals through 12s. My 15s are fs20 and qts .7, this gives a bump at about 30hz, its perfect bottom in a car long as your trunk is large enough to keep the Q down. I have 16cf it works great. Even better they were cheap, and using two keeps xmax low and SQ high in the normal range of listening. Also gives two locations of bass, I like more than one.


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## TOOSTUBBORN2FAIL (Jan 31, 2011)

Resonance and rattles will likely be your biggest contributor to rear bass. I had my last set up crossed at 80hz, both at 12db and 24db, with time alignment, and until I really cranked it, you could never tell where it was coming from. When I cranked it, there were some rattles that pointed to the back.


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## Schramm (Mar 12, 2012)

sqshoestring said:


> I left a couple of vents open in my rear deck, I doubt it makes much difference with two 15s. You can blow through holes but should have something like half the cone area maybe more with big xmax maybe less is possible with lower power use. Try to keep the holes shallow unlike a port, so they tune way higher than the sub. Try to keep the manifold area small as you can but don't restrict air flow that will happen at lowest bass tones. The chamber can tune like a bandpass and you don't want that.
> 
> 15s are ideal because they can have an Fs of 20Hz....they get lower. IB is just a really large sealed, or large enough the enclosure does not affect the tuning much if at all. You can see it in a model if you change volume the FR stays about the same.
> 
> ...


Well said on driver selection. There are trade offs. I looked at AE Speakers IB15AU, which is near ideal and better choice for deeper bass output with extremely high volume capability. They have very well engineered motors and are still fairly sensitive. They do not appear to be available on AE site now and I am curious of price. I hope to get around the earlier 12dB per octave roll off with EQ. With such a low power setup, I am not concerned about reaching xmax at all. Were only talking about a few mm of cone movement at 100W RMS. However, I am not sure if the DRZ9255 with its 5 band EQ will be sophisticated enough. If only Dolby Lake processors were made for car audio, tuning would be easy.

I cannot find specs anywhere for the 15" TOA drivers and have no idea what cabinet they came out of either. They only weigh about 7lbs each. I am guessing they were used specifically for a low power subwoofer cabinet (similar to model SLB) use based on design. Not sure if they will play deep enough, maybe with EQ, but Japanese built drivers for $25 each, it is worth a try. 

The 18" JBL 2241G is a different animal. Has Fs of 35Hz which is still quite high for its size. It can play flat out to 500Hz, but is typically used for sub only cross at <100Hz. QTS of 0.40, with 98db SPL, 1W, 1m at 2.0V from 100-500Hz. Ideal cabinet volume is 10 cubic feet. My trunk is 11.9 cubic feet. Subtracting approx 1 cubic foot for the baffle and driver, volume is 10.9 cubic feet. JBL spec sheet response curve is measured using a 10 cubic foot sealed and ported box. I modeled parameters in WinISD, Qtc of sealed box is 0.57 (deep dry bass). So in theory, it is very good match for the trunk volume. If sensitivity holds same in cabin at lower frequencies (it won't due to roll off, cabin gain), at 100W RMS, assuming perfect sealed box and baffle (not possible), here are some numbers.

F3 = 65Hz, -12db at 27Hz (no cabin gain)
106dB 27Hz
107dB 30Hz
110dB 40Hz
113dB 50Hz
114dB 60Hz
115dB 70Hz

I need to get a mic, USB computer inferface, and RTA software for some testing. Normally this driver can take 600W RMS no problem in a proper enclosure like JBL SR4718X.

The Scanspeak 18W/4531G Revelators I have are a very good drivers for midbass in two way set-up, but are still limited by 6.5" size and enclosure volume. I have a set of Scanspeak 12mu/4731t00 midrange drivers also that I am playing with, so could go 3-way. I am trying to avoid complexities of 3-way set up though and system is already over budget.


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## Schramm (Mar 12, 2012)

TOOSTUBBORN2FAIL said:


> Resonance and rattles will likely be your biggest contributor to rear bass. I had my last set up crossed at 80hz, both at 12db and 24db, with time alignment, and until I really cranked it, you could never tell where it was coming from. When I cranked it, there were some rattles that pointed to the back.


Thanks for response. Resonances are indeed the reality in a tin can. Every time I hear one note wonders with sheet metal buzzing wildly, it makes me cringe and think how stupid is that. It sounds absolutely terrible. Why bother? I have no intention of such a set up or to ever approach such volume levels. Some damping will be needed, but as little as possible to keep weight down.


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## sqshoestring (Jun 19, 2007)

You are losing 3db at 40Hz, it will sound like a giant midbass or spl....at least without EQ it will. Up to you but I'd be wary of pro sound drivers, they rarely get under 50hz with subs far as I know.

More drivers is free efficiency, low xmax is better SQ, that is why more sub is win-win with IB. Larger subs have more cone, but also lower Fs which helps them get down lower. 

Model a pair of these in 16cf and see what the curve looks like. Pyle - PLWB155 - 15" 1000 Watt 4 Ohm Subwoofer

Some trunks are larger than stated because they list the usable luggage space so they don't include the space in the quarter panel around the wheel well, under the rear deck, inside the body parts/tubes, in the spare tire area, in the tail light areas, etc. Add to that they are not rigid or sealed, so I always model 10% larger or so.

I like the AE a lot, though they are not tuned exactly like I want but entirely workable and very well designed/built. Even they try to say you don't need that much bottom because cabin gain will make up for it, and I don't agree. Maybe in some cars but not the one I drive. Guess it depends on what kind of bass you want. I like too much 30hz, because I can get rid of it with zero effort....but its hard to make more when you don't have it. Which brings up the best weapon for IB subs, a PEQ, they rule. I have a 16 band in my HU and the PEQ was much nicer on the subs. I'm still looking at bass processors as some of them are nothing more than a PEQ and SS. Hard to find one with a low enough SS or defeatable one.


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## Fricasseekid (Apr 19, 2011)

A guy like you may have a hard time taking those Pyles seriously. They have a look that screams teenage marketing. 
But having just installed 2 Pyle PLBW 15s in my Honda (http://www.parts-express.com/pe/showdetl.cfm?partnumber=267-087), I can vouch for the SQ of these speakers. They dig very deep, like organ music deep and at the lower listening levels they blend in beautifully and become very transparent. I have mine crossed @ 80 and another cross @ 125. They even fill in the gaps due to my lack of midbass up front. The AE are great speakers, but there is no way you'll ever use that much Xmax, save your money. 

And with your saved money, buy some processing. Don't know if you've seen these but they may interest you:
http://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4


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## BuickGN (May 29, 2009)

I highly suggest the IB15AU. They're still being made, just not on the website. I believe the price is around $199 each. Extremely low inductance, 91db-1w/1m efficiency, and a very low moving mass if you're into that sort of thing. I've played them out past 4khz not that you would want to do that. 

100w to a single one will give you plenty of output. 200w will reach xmax in the lower octave FWIW. 

I do have to mention that a 12 or 13W6 or 12" or 13" W7 will give you a VERY nice crisp, clean, quick sound and dig deep effortlessly when installed infinite baffle. Still one of my all time favorite setups. The W7 subs have a lot of cone area for their physical size and more linear excursion than you could ever use. They usually pop up in the used section on here pretty cheap. I'm not sure if the low VAS would make a difference in how it would interact with your proposed leaks.

I've left the factory 8" sub hole in the rear deck completely unsealed for a week or two and the main difference noticed when I sealed it up was a little more low end bass. It was a subtle difference in output but when you consider the amount of added power it takes to get just a little more output in the really low stuff it's worth it to have it fairly sealed.

It seems like leaks around the baffle near the subs make more of a difference than leaks on the rear deck assuming your subs are firing through the rear seat. If you have to have airflow I would do it on a different plane from the subs.

I don't know if this would help, it would probably make your problem worse but I removed the rubber one way flappers that vent the trunk to atmosphere to give the trunk a little more volume. They were covered with screen of course. I discovered that my 16.5 cubic foot trunk is not large enough to give me the low Qtc I like so much. I would highly recommend a single 15 in the trunk of a Civic, especially with your goals.

The one place I might suggest something different is in the mid-bass. I've gone from an extremely nice pair of Dynaudio Esotar 650 6.5" mid-basses that sounded great with 150w on them to a pair of Dynaudio MW182 9" mid-basses with 300w. I have 150w on each mid-range and 150w on each tweeter. The reason being is dynamics. Each time I've added more power to the system it's become more dynamic and life like, even at reasonable listening levels. The larger mid-basses really help keep up with the 15s so I can truly level match them at their transition from the subs. It will get loud if I want it to but the goal of lots of power is to get that realism even at lower volumes. 

I know you can't go with 9" or 10" midbass in a 2-way but if you think it lacks dynamics when it's up and running you might consider a little more power on your existing midbasses. 

With lots of power and lots of cone area, you get effortless dynamics and a very life-like presentation. As I've learned tuning is probably the most important part but at least you'll have the tools.


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## BuickGN (May 29, 2009)

Fricasseekid said:


> A guy like you may have a hard time taking those Pyles seriously. They have a look that screams teenage marketing.
> But having just installed 2 Pyle PLBW 15s in my Honda (Pyle PLBW154 Blue Wave 15" Woofer 267-087), I can vouch for the SQ of these speakers. They dig very deep, like organ music deep and at the lower listening levels they blend in beautifully and become very transparent. I have mine crossed @ 80 and another cross @ 125. They even fill in the gaps due to my lack of midbass up front.* The AE are great speakers, but there is no way you'll ever use that much Xmax, save your money. *
> 
> And with your saved money, buy some processing. Don't know if you've seen these but they may interest you:
> miniDSP 2x4 | miniDSP



The AE subs are about a whole lot more than just xmax and offer several advantages over the Pyles. Whether the AEs are worth the extra money is up to the OP but let's not act like excursion is the only difference between the two. If 100w is all the sub(s) are going to get, I also recommend the AEs due to their higher efficiency. I also recommend just one in the trunk of a Civic unless you're willing to vent to atmosphere. Put them in a large enough trunk and you'll be rewarded with one of the most musical and accurate subs out there.


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## Schramm (Mar 12, 2012)

Wow, thanks for all the great feedback guys! My knowledge is still theoretical at this point. I just need to start building, testing, and taking some measurements. It has taken awhile buying all used gear. What I am hearing is that having the air vents on the deck open should be a workable compromise. That was my biggest concern.

I am partial to low power and sensitive drivers because the 70A alternator and small battery does not leave much headroom once the vehicle's electrical system is factored in. I have a McIntosh MC420 and MC423 that I am trying to use, but realize more power would be ideal for dynamics. A major design goal is to keep added weight down also - something many people consider as an after thought.

In WinISD, the Pyle's frequency response is flat to 30Hz making it easy to blend with front stage. Assuming 10.9 cubic ft. trunk volume, the box Qtc is higher than desired at .85. With 100W RMS, SPL is flat at 106dB across the usable frequency response. However, the tradeoff is 86dB SPL to begin with. If measured at 2.83V that is more like 83dB per 1W,1M. Terrible! A 300-500W amp would be required. There is no point in choosing an inefficient driver for IB design. Low efficiency is nearly impossible to avoid with small sealed box playing deep .It negates one of the primary advantages. 

The JBL 2241G will require PEQ to flatten the response due to earlier roll off. However, the end result will require significantly less power for the same SPL above 30Hz. This makes all the difference. It is an $800 driver if purchased new and in a different category than most car audio drivers. Weighs 23.5 lbs. The AE15AU specs indicate a low distortion motor, reasonably efficient driver, optimized for IB, and at a good price.

I learned a few things from this site. Check out the true Infinite Baffle Sub. William Cowan's Homepage

The MiniDSP could be a very useful low cost solution for the sub channel. Thanks guys. Cheers.


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## req (Aug 4, 2007)

this should waste some of your day 

"Cult of the Infinitely Baffled"Hear The Bass, Not The Box The definitive online resource for Infinite Baffle subwoofer designEstablished 1999 - Home


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## 14642 (May 19, 2008)

Infinite baffle is really not so difficult. There's no need to seal up the entire trunk--in fact, don't do it. All cars since the 60s have been designed to vent exhaust gas through the cabin and out through the trunk. Don't fill the sides with expanding foam or go crazy with sealing stuff. 

Xmax is important because there won't be anything to limit excursion, but you don't need some super-Xmax woofer if you'll be using 100W. Choose one with a Qts of .7 or below. Mount the sub to the rear package tray--don't "fire" it through the back seat. In your Honda, I recommend an MDF baffle attached to the package tray. Then, Use a couple of layers of dynamat or some such stuff on the top. Put something (foam, jute or dacron) between the trim panel and the deck before you put that back on. That will help to eliminate rattles. 

You may or may not need to put some kind of a baffle over the back of the rear seat. The need to seal this is a big fat myth. Cabin gain will boost low frequencies, but this big leak will reduce the magnitude of the boost. No big deal.


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## minbari (Mar 3, 2011)

Schramm said:


> Wow, thanks for all the great feedback guys! My knowledge is still theoretical at this point. I just need to start building, testing, and taking some measurements. It has taken awhile buying all used gear. What I am hearing is that having the air vents on the deck open should be a workable compromise. That was my biggest concern.
> 
> I am partial to low power and sensitive drivers because the 70A alternator and small battery does not leave much headroom once the vehicle's electrical system is factored in. I have a McIntosh MC420 and MC423 that I am trying to use, but realize more power would be ideal for dynamics. A major design goal is to keep added weight down also - something many people consider as an after thought.
> 
> ...


by the number you are right. but dont forget cabin gain. I have two of the "blue label" varieties. PL1590bl and they are being powered by a JL 500/1. gain is all the way down and EQ is nearly flat. believe me when I say I am not using 1/2 this amplifier and they get stupid loud! have not even reached xmax on them yet because there is more bass there than I need. playing a test CD they start playing at a level you can hear and feel around 22hz. ( I have a 22hz SSF) by 28 hz they shake everything lose on the rear dash.

With normal listening levels, they are very musical and blend with my front stage to the point that I have questioned if they are on a couple times, lol.

are they as good as other subs? maybe not, but for $50 each they are amazing in IB. I wouldnt use them for anything else, as you have mentioned, the QTS is pretty high on them and a VAS of 5cuft means they love alot of space (again, perfect for IB)


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## Fricasseekid (Apr 19, 2011)

Andy Wehmeyer said:


> Infinite baffle is really not so difficult. There's no need to seal up the entire trunk--in fact, don't do it. All cars since the 60s have been designed to vent exhaust gas through the cabin and out through the trunk. Don't fill the sides with expanding foam or go crazy with sealing stuff.
> 
> Xmax is important because there won't be anything to limit excursion, but you don't need some super-Xmax woofer if you'll be using 100W. Choose one with a Qts of .7 or below. Mount the sub to the rear package tray--don't "fire" it through the back seat. In your Honda, I recommend an MDF baffle attached to the package tray. Then, Use a couple of layers of dynamat or some such stuff on the top. Put something (foam, jute or dacron) between the trim panel and the deck before you put that back on. That will help to eliminate rattles.
> 
> You may or may not need to put some kind of a baffle over the back of the rear seat. The need to seal this is a big fat myth. Cabin gain will boost low frequencies, but this big leak will reduce the magnitude of the boost. No big deal.


What is so bad about firing a sub through the back seat that it justifies rebuilding the entire back deck?


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## 14642 (May 19, 2008)

The back deck is "leakier" than the back seat and is far more prone to rattles. I prefer to only rebuild one--and since the deck is worse than the seat, it's just easier. Plus, I prefer not to have to deal with the acoustic low pass filter that IS the rear seat. 

Finally, if the rear seat folds down so you can carry long stuff, you get to keep that feature.


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## Fricasseekid (Apr 19, 2011)

Andy Wehmeyer said:


> The back deck is "leakier" than the back seat and is far more prone to rattles. I prefer to only rebuild one--and since the deck is worse than the seat, it's just easier. Plus, I prefer not to have to deal with the acoustic low pass filter that IS the rear seat.
> 
> Finally, if the rear seat folds down so you can carry long stuff, you get to keep that feature.


That makes good sense. I definitely get the best sound with my seat folded down. But when the seat is folded down is when the back deck rattles the most, fold the seat back up 90% of the resonance goes away. I still have the little ski pass door that folds down behind the armrest though and there is a night and day difference with that up and down. I just removed the little door and leave the armrest down. I'm very happy with the way it sounds. Now if I drove a BMW that had a plate of tank armor behind the back seat I'm sure it would be a different story.


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## BuickGN (May 29, 2009)

gregerst22 said:


> To the op. I live in MN as well and drive an acura with an IB setup. all though I think the term trunk baffle is more accurate. I lose 10db of output measured with an RTA when I open my trunk just 1". You won't need to seal up the rear air vents or vents to the outside. Mine are not. Just seal any major leaks from the trunk to the cabin and you should be fine. Short of cutting a big hole in your trunk to vent outside, IB in a car is just a really big semi-leaky box. A lot of air volume allows subs to play smoother with less distortion especially with the right subs. My subs play almost perfectly flat down to 20Hz without any EQ.
> 
> Half the battle with getting upfront bass is to eliminate any localization cues. It's absolutely necessary to eliminate all rattles or noises coming from the rear. Once you've accomplished that you can play with slopes and t/a to get upfront bass. I had to step up to 9" mid-basses in order to get realistic upfront bass that could match my 15's at moderately-high to high volumes. I'm very happy with it now.


Not if the 15s had enough breathing room and the TL's trunk just doesn't cut it. They need to be vented to atmosphere. 

Where do you lose that 10db of output, probably in the 50hz range. Since you complained about these subs being sloppy I suggest you open the trunk wide open, windows up, and give it another go. Once you see how good they can sound, you have to figure out if you want to mod the car to give them the air space they need, EQ them, or swap subs.


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## Schramm (Mar 12, 2012)

req said:


> this should waste some of your day
> 
> "Cult of the Infinitely Baffled"Hear The Bass, Not The Box The definitive online resource for Infinite Baffle subwoofer designEstablished 1999 - Home


Thanks for sharing.


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## Schramm (Mar 12, 2012)

gregerst22 said:


> To the op. I live in MN as well and drive an acura with an IB setup. all though I think the term trunk baffle is more accurate. I lose 10db of output measured with an RTA when I open my trunk just 1". You won't need to seal up the rear air vents or vents to the outside. Mine are not. Just seal any major leaks from the trunk to the cabin and you should be fine. Short of cutting a big hole in your trunk to vent outside, IB in a car is just a really big semi-leaky box. A lot of air volume allows subs to play smoother with less distortion especially with the right subs. My subs play almost perfectly flat down to 20Hz without any EQ.
> 
> Half the battle with getting upfront bass is to eliminate any localization cues. It's absolutely necessary to eliminate all rattles or noises coming from the rear. Once you've accomplished that you can play with slopes and t/a to get upfront bass. I had to step up to 9" mid-basses in order to get realistic upfront bass that could match my 15's at moderately-high to high volumes. I'm very happy with it now.


Welcome from MN! I am glad to know real people exist nearby. My wife is concerned my "internet friends" are just imaginary. Strange, what do you think is causing the 10dB loss when trunk is opened?


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## Schramm (Mar 12, 2012)

Whoa there fellas,

This discussion raises some valid points. In vehicle frequency response and SPL is difficult to model due to 1) trunk being neither perfectly sealed nor truly infinite baffle, 2) partially effective barrier between trunk and cabin leads to cancellations and can act as an acoustic filter at some frequencies, and 3) effects of cabin gain or standing waves.

With that said, I understand it is always a good idea to model the response as best as possible using the traditional sealed approach. I think this is what BuickGN is saying. The qts of the driver is not important by itself. The Qtc including the enclosure is what really matters in the model. If truly infinite baffle, Qtc will equal Qts, which is why many drivers designed for IB use have Qts of .7. Case in point, 2 AE IB15AU's in an 06 Acura TL with trunk volume 12.5ft3 shows box qtc of 1.25. This suggest the trunk is too small and will lead to a bump in response around 50Hz. See model attached.

Regardless of intentional or not, the response can be further changed by EQ. I have read that too much EQ can degrade sound quality. Not sure if this is true.


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## BuickGN (May 29, 2009)

Very good points. 50hz is exactly where I have to cut and depending on time alignment I've had to cut around 2db at 40hz and 6db at 50hz. Because I installed this in stages, losing interest several times I might put the carpet back in on one side one day and not get to the other side until a couple months later so some of the changes happened gradually. 

I was reading over some of my old posts on here about how quick and snappy these subs are yet they were sounding a little on the sloppy side, at least compared to how I initially described them. After talking to Greg on here I decided to investigate by going big first- Opening the trunk. It got a little thin but the punch and speed was back. I started removing trunk carpet one piece at a time and each piece changed the sound a little closer to the open trunk sound. 

With no carpet in the trunk they sound as I described them back when I installed them which actually makes sense. . This isn't a permanent solution obviously especially considering the carpet helps knock down road noise and road noise seems to make it through the cones as if they're not there. I may end up going with the JL subs. 

If I learned anything its that a sealed space is not required to drastically change the Qtc. There's plenty of wasted space behind my carpet in the trunk and I always assumed that any pressure would find its way through the many gaps in the carpet and the effective volume of the trunk would be the same as with the carpet removed. I was wrong. 

With the carpet removed no EQ is required on the subs. Trying to simulate the larger trunk response with EQ never fully worked for me. While I could get it to sound about the same, that same effortless low end and ultra quick kick just wasn't as effortless or quick. Maybe I need more practice with the EQ but I swear there's a different sound to the more well damped subs in the larger "box" that EQ can't entirely replicate.


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## BuickGN (May 29, 2009)

Schramm said:


> Wow, thanks for all the great feedback guys! My knowledge is still theoretical at this point. I just need to start building, testing, and taking some measurements. It has taken awhile buying all used gear. What I am hearing is that having the air vents on the deck open should be a workable compromise. That was my biggest concern.
> 
> I am partial to low power and sensitive drivers because the 70A alternator and small battery does not leave much headroom once the vehicle's electrical system is factored in. I have a McIntosh MC420 and MC423 that I am trying to use, but realize more power would be ideal for dynamics. A major design goal is to keep added weight down also - something many people consider as an after thought.
> 
> ...


About the amps, have you considered going with a good class D like the JL XD or HD series? You could lessen the load on your charging system or retain roughly the same load but with a little more power to the speakers. A single HD or XD amp could supply power to your whole system. I love the McIntosh amps, I have one collecting dust but when I could hear absolutely no difference in sound quality I went with the smaller and much more efficient amps.


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## Schramm (Mar 12, 2012)

Although infinite baffle extends low frequency response, there are some disadvantages. Believe it or not, I am going to come out and say I still prefer large acoustic suspension enclosures over true infinite baffle if optimal conditions exist. The reason why is that compressed air is a far more linear spring than the spider of most drivers and offers better transient response. Manufacturers have a very hard time designing low compliance suspensions while retaining linearity across the frequency response and xmax range. To compensate, the driver has to have a lot of electrical control over the cone to damp it properly. Yet, strong magnet and light cone leads to higher roll off point and not deep bass. True infinite baffle requires a lot of surface area to work due to reduce output.

My goal is to treat the entire trunk as an enclosure, and match the volume available to the VAS of the driver as best as possible to approach Qtc of 0.5 to 0.7. Hopefully, this alignment (assuming parametric equalization to flatten the response of the JBL 2241G), will lead to optimal flat response, good cone control, with good transient response.

Thanks for all the responses. I think most of my questions have been answered. 

Now its on to fabrication design. See attached pic of the trunk. Yes, it snowed today. There is no way I will ever cut into the rear deck of any vehicle, as it is a structural part of the frame. I may use modest amounts of 3M 2-part rigid cure polyurethane foam in between key parts of the sandwiched deck sheet metal to make it stiffer. That's it for foam. The rest of the seal will consist of other preferred materials, i.e. epoxy, silicone, EPDM foam. Laying a few thin sheets of epoxy/fiberglass is also up for consideration to stiffen the deck. I plan to build an exterior frame. The main baffle will bolt to it, and be easily removable.


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## BuickGN (May 29, 2009)

I should just shut up but I can't sleep tonight. 

About the sealed vs IB, it sounds like you know more than I do about audio so I don't want to insult your intelligence but I wanted to bring up one point. The air spring in a sealed enclosure reduces cone control. The smaller the volume of air, the stiffer the air spring and the more overshoot and less cone control you have.

I've never had better transient response than I have now. I ran some JL 12W6s for about 6 years and in that time they went from sealed to large sealed to podted to bandpass and finally infinite baffle. I tried IB because I wanted my trunk space back but what a surprise when I first heard it. 

It sounds like with your goals even a single 15 is barely going to be moving, more than likely always being in a very linear part of its travel. 

The Ib15au has a very light (for a car sub) 142g moving mass and a very "loose" suspension so it retains a low Fs and digs deep. When mine first arrived and I pushed on the cone of one of them I thought they forgot to attach the spider lol. 

IB has the same output potential as sealed all else being equal. We normally do lots of cone area to enjoy the more efficient low end without running out of displacement. Throw a subsonic filter on there to match a sealed box's roll off and feed it lots of power and its going to get just as loud. 

I think you're going to be pleasantly surprised when yours is done.


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## Fricasseekid (Apr 19, 2011)

Don't know if you've saw my IB build thread but the pass through between my trunk and cabin looks nearly identical to yours. I have lots of pictures and maybe you could use some the ideas I came up with or give me some pointers. 
Check it out here:
http://diymobileaudio.com/forum/showthread.php?t=147692


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## Schramm (Mar 12, 2012)

BuickGN said:


> I should just shut up but I can't sleep tonight.
> 
> About the sealed vs IB, it sounds like you know more than I do about audio so I don't want to insult your intelligence but I wanted to bring up one point. The air spring in a sealed enclosure reduces cone control. The smaller the volume of air, the stiffer the air spring and the more overshoot and less cone control you have.
> 
> ...


Sorry if my technical response seemed know it all. It is definitely not the case. I read a lot of technical papers on acoustics in effort to learn and my parroted explanation was incomplete. In reality, I have almost no experience in car audio. I do not know anyone locally with a high end car audio system and have no interest in competing. Having tried many installation approaches and gear is a much better teacher. I cannot afford the time or cost of trying out various set-ups. Many people have done this on DIYMA site and that is why I am on this forum. I am aiming for excellent transient response, deep effortless bass, high efficiency, and maximum trunk space. Hope to get it right the first time around. Thanks again for confirming success with the infinite baffle approach, for lack of a better description.

The AE Ib15au characteristics you describe are exactly why many people use them for this approach and I would love to test it. Yes, one would be far more capable than needed for SPL and best match the air volume in the trunk. Inductance is also extremely low due to copper shorting rings.

Yes, I am considering full range class D amps. Specifically, the JL HD series and ZED Audio. I would have to hear in person though prior to purchase as they are expensive and a I would be concerned about using them full range. Historically, class D design has not been well regarded for sound quality. The bridged channels of the McIntosh (max 100RMS) do show clipping at musical peaks when the driver was tested free air. Ideally the JBL 2241G would see more power, maybe 300-600 watts to capture the dynamics. I won't really know how it will sound until built. So stay tuned.


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## Schramm (Mar 12, 2012)

Fricasseekid said:


> Don't know if you've saw my IB build thread but the pass through between my trunk and cabin looks nearly identical to yours. I have lots of pictures and maybe you could use some the ideas I came up with or give me some pointers.
> Check it out here:
> First go at an infinite baffle install. - DIYMA Car Audio Forum


Yes, I am following your build closely already. You are way ahead. It is an inspiration. Keep up the good work. I will try to add some ideas on the fabrication given my talents in engineering and I have lots of hands on experience in woodworking and construction. 

I tend to go to the extremes at whatever project I undertake, so it is easy to lose sight of the big picture and budget. My advice on fabrication is to keep things lightweight as much as possible via wise choice of materials, composite construction, and mechanical advantages. More is not always better. If you integrate it well with the OEM appearance and functionality, it will be something you can enjoy for many years. Otherwise, you can go far fast, and end up with a not so functional car that attracts too much attention. Cheers.


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## Schramm (Mar 12, 2012)

gregerst22 said:


> When I've model a pair of them in Bassbox pro I get a QTC of .634 not 1.25 You should also factor in cabin gain.


Bassbox is better program than WinISD, with more capabilities. I am not aware WinISD (basic version) gives option for cabin gain. Are you certain you modeled two drivers, because .63 X 2 = 1.25? The programs can't be off by 100%.


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## Schramm (Mar 12, 2012)

gregerst22 said:


> Yes that was with two. I'll double check my parameters though. It looks like AE actually has the subs with specs listed on their website now. When I looked them up back in January I had to get the specs from a post in their forum. Maybe they are different?


See screenshot. Specs taken from AE.


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## RNBRAD (Oct 30, 2012)

The only disadvantage I can see with an IB system as opposed to a sealed is the output from the rear of the driver causing structural resonances behind the sub that can then transfer into the cab or listening area. This should be a non-issue or at least a lesser issue with a properly dampened sealed box. Whatever advantages an IB can bring can easily be negated by a greater possibility of excess resonance.


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## Schramm (Mar 12, 2012)

Has anyone applied acoustically absorbant material to dampen the rear wave, lining the trunk like a large speaker enclosure? If so, what was the effect?


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## minbari (Mar 3, 2011)

But those resonances are in the trunk. You can't hear them

Sent from my motorola electrify using digital farts


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## thehatedguy (May 4, 2007)

Wouldn't do anything.



Schramm said:


> Has anyone applied acoustically absorbant material to dampen the rear wave, lining the trunk like a large speaker enclosure? If so, what was the effect?


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## 14642 (May 19, 2008)

Guys, you're making way too much of this trunk thing. The effect of the box on the suspension of the woofer (the box is a spring that adds stiffness to the speaker's suspension) gets smaller as the box gets larger. In addition, the flexible nature of the car's body, the leakiness of the trunk to the outside (designed to eliminate exhaust gas) and the leakiness of the trunk into the listening space all increase the apparent size of the box. Leakiness into the cabin increases at low frequencies, so that reduces the effect of cabin gain. Assuming that the removal of the carpet reduced the Qtc because it made the box leakier is only part of the story. It also reduced peaks in the measured acoustic response in the listening space because more of the back wave was allowed to mingle with the front wave causing cancellation (more accurately, because of the leaks, the pressure between the two seeks equilibrium).

All of this modeling based on supposed trunk size, talk about lining the trunk with dacron, cutting a hole to the outside etc. is ridiculous. Mount the woofer and EQ the response.


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## subwoofery (Nov 9, 2008)

gregerst22 said:


> Makes sense and yeah you're probably right.


Probably? 

:laugh:

Kelvin


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## 14642 (May 19, 2008)

Have you measured the 10dB when you pop your trunk? I'd postulate a different reason for the change in response.


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## sqshoestring (Jun 19, 2007)

Schramm said:


> See screenshot. Specs taken from AE.


My quad 12s modeled just like that and sounded like dog sh** to me. They were infinity .46 qts and 24hz fs. I had to put a PEQ on there to get them usable. Lots of output for sure but what a terrible native tune. Also makes me think this car has little cabin gain. That peak 40-50 is SPL all the way in my eyes. If you have to take that because you are putting many subs in a small trunk then you plan to EQ, fine, but why use those subs for an SQ setup IB when something with a better FR is available....assuming you are buying them and there are other subs out there with better specs.


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## BuickGN (May 29, 2009)

sqshoestring said:


> My quad 12s modeled just like that and sounded like dog sh** to me. They were infinity .46 qts and 24hz fs. I had to put a PEQ on there to get them usable. Lots of output for sure but what a terrible native tune. Also makes me think this car has little cabin gain. That peak 40-50 is SPL all the way in my eyes. If you have to take that because you are putting many subs in a small trunk then you plan to EQ, fine, but why use those subs for an SQ setup IB when something with a better FR is available....assuming you are buying them and there are other subs out there with better specs.


That's how mine is with carpet in. For what its worth mine is sealed very well from the cabin. I was never one to believe it needed to be sealed really well so after a year of having it semi sealed I went crazy and got every single hole with pretty much no change in response as I expected. 

The MS8 always had trouble with these subs, maybe because the pair in my trunk are a little peaky in the 50hz area. 

Back to pulling the carpet out, the main reason I believe the change in sound is due to the change in QTC is the result sounds very similar to opening the trunk. A single IB15 might have been the better choice in a trunk this size. I'm back to being happy again but I'm obviously not going to drive around with the carpet missing forever. 13W7s here I come.


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## Oliver (Jun 25, 2007)

Andy Wehmeyer said:


> Guys
> 
> All of this modeling based on supposed trunk size, talk about lining the trunk with dacron, cutting a hole to the outside etc. is ridiculous. Mount the woofer and* EQ the response.*


*Can U do this with a ruler?*



subwoofery said:


> *Probably? *
> 
> :laugh:
> 
> Kelvin


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## Schramm (Mar 12, 2012)

Guys,

I am happy with the responses and many good points have been raised. Modeling is only an approximation. I will just have to build it, take some measurements with a microphone, figure out how to implement parametric equalization to adjust for the drivers roll off and cabin gain, play with crossover slopes, time delay, and be creative in getting rid of unwanted resonances. 

I am hopeful to experiment and compare to the sound of a single 12" in a sealed box with qtc of 0.707. Many guys want ultra low bass down to 30Hz. While that is ideal, most of the music I listen to has little content that goes that low. I am only looking for flat in car response down to 40Hz, which should be easier to achieve. As long as I can hear lowest to highest note of a double bass and have it sound real, with some good dynamic range, I will be happy.


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## minbari (Mar 3, 2011)

think you would be surprised. stuff like Metallica "enter sandman" has some very low stuff.


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## Fricasseekid (Apr 19, 2011)

minbari said:


> think you would be surprised. stuff like Metallica "enter sandman" has some very low stuff.


X2

Think about ambient tweeters. Those extreme high frequencies that add that "sparkle" and clarity to music. The same goes for subwoofers playing under 40hz. They add a deep fullness and rich sound to the music. Mine do so to the point that sometimes I dont think my subs are playing any content at all. Then when I cut the subwoofers off the music falls flat on its face and sounds completely anemic. They were providing substance and foundation. Now that's transparency!


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## Schramm (Mar 12, 2012)

gregerst22 said:


> I agree 100% about being able to play sub bass below 30Hz. Without it your missing some material sorta like watching 16:9 content on a 4:3 TV. Most of it's there but you'll be missing the whole experience. Even piano notes play below 30Hz.


Very true. I forgot about the piano, ... organ, and taiko drums! I do not listen to metal, rap, pop, electronic, rock, etc., but I do like powerful music every now and then. I hope to be pleasantly surprised by using the 18" driver on music like this:
http://www.youtube.com/watch?v=C7HL5wYqAbU


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## BuickGN (May 29, 2009)

gregerst22 said:


> Those JL's model well with a Qtc of .5 or lower. I think you're going to need to give them more power than the AE's though. If you do get them I'll be interested in knowing what you think. This whole time I've been using outdated parameters with the AE's and thinking they were around .643 Qtc when they're actually closer to 1.0 This may explain why they don't sound really tight or snappy. Although I've been able to improve it with some Eq and crossover work.


Looks like we're in the same boat. I was modeling for a larger trunk this whole time. They do sound really good with the carpet missing though. I have a dead space in the wall at home, probably 20 cubes or so, that might become their new home. Then again it would be pretty easy to take one out and screw a piece of mdf over it.


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## sqshoestring (Jun 19, 2007)

Flat to 40, you only need 1cf box for that....get a dayton. There is a lot of sound under 35hz in most music, at least stuff after the 60s. You can have 30hz, why not do it?


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## Schramm (Mar 12, 2012)

sqshoestring said:


> Flat to 40, you only need 1cf box for that....get a dayton. There is a lot of sound under 35hz in most music, at least stuff after the 60s. You can have 30hz, why not do it?


Exactly. To be clear, flat to 30Hz is the goal. IB takes up less space and makes it possible to use very large and efficient drivers, with excellent transient response if parameters are properly matched to the trunk volume.


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## Schramm (Mar 12, 2012)

Actually, I thought of a few reasons why FLAT to 30Hz may be difficult, not because it is difficult with IB, but because of the equipment I have selected. 

1) I suspect the JBL 2241G's earlier roll off may require more sophisticated low end equalization than available in Clarion DRZ9255, where PEQ is 1/3 octave, adjustable in three bands with frequency centers of 50, 63, 80, 100, 125, .... through 20kHz. Cuts on these frequencies to flatten the response would also effect the midbass, assuming crossover point of 70Hz. This is a little more limiting than 31 band graphic EQ available in most dedicated DSP's with adjustments as low as 25Hz, independent for each driver. However, budget has already been exceeded.

2) Flat to 30Hz will require more power. May need a different amplifier or accept lower overall output levels.

3) Fs of 2241G is 35Hz. Most drivers exhibit inductance changes and rising impedance when approaching Fs. This may electrically limit the drivers response. Cabin gain may help quite a bit, but I am not sure how to predict the in car response curve.


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## quietfly (Mar 23, 2011)

sub'd


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## stryke23x (Jun 22, 2007)

Hi everyone. Wanted to address a few things here. First off, updated parameters for the IB15AU are posted on the main website now. Those parameters will be consistent for a long time to come. I plan to get the SBP15 parameters up soon as well. Both are available to order. 

I made a similar post in another thread but figured it was worth commenting here as well. In a vehicle, you will have a lossy enclosure due to the trunk never being completely sealed. Vents, tail lights, trunk seal, etc will never be completely air tight. As a result the apparent volume of the trunk will seem larger than it really is. Once you start putting things in the trunk though, that volume comes back in. That should also be taken into account when trying to determine what your actual volume is. 

That said, the IB15AU and SBP15 are two different options for car IB use. The IB15AU uses a 5.25" diameter motor and the SBP15 uses the 6.125" diameter motor. The SBP15 motor gives about 30% more BL than the IB15AU motor does. This extra motor strength lowers the Q of the driver allowing it to work in a smaller space. It also has a larger 19mm thick top plate instead of the 10mm top plate in the IB15AU. This gives a flatter BL curve for lower distortion. Both use the Lambda style motor with full copper sleeve on the pole. AE Speakers --- Superb Quality, Unforgettable Performance, Definitely.

The cabin effects are separate from what the drivers themselves will do. The IB15AU and SBP15 can be compared based on the volume available in the vehicle. The higher motor strength of the SBP15 will always lead to a lower Q and flatter response as the enclosure gets smaller. You can see how the two woofers compare to each other here and see how each compares based on enclosure volumes.




















You can see that the SBP15 stays flatter as the enclosure volume gets smaller. It also has more efficiency up top. This is ideal when most systems lack midbass. There are a few reasons for this lack of midbass. Sometimes you deal with all of the issues at once and get the perfect storm where your midbass totally disappears. 

Many drivers with large, heavy VC have such high inductance that they already begin to roll off by 60hz or so. By 100hz they can be several dB down already. Choosing a driver that can cleanly play to a full octave above your crossover point is ideal to eliminate rolloff and inductance related distortion. Secondly, you may be getting cancellations between the woofers and the front stage at these upper bass frequencies if they are not time aligned/in phase properly. DSP to correct the phase/time alignment can correct this. There is also much less cabin gain at the higher frequencies to begin with. Below is a transfer function measured in my Audi S4. You can see that from 60-100hz there is only about 10dB of gain and the response increases under 60hz. At 22hz there is almost 35dB of cabin gain. It takes a lot of output from the driver to make up for that in the upper octaves. 










This is a measured curve in a Buick Regal. You can see the effect is even more drastic here:










While the actual levels may change, the general trend is quite similar in most vehicles. Another example can be seen between the response curves of a Toyota Camry and the same Buick Regal to see how they compare.










Interesting to note is that the one big dip you see in the upper bass is very dependent upon placement of the woofer in the vehicle. The main reflection to worry about is the reflection from the back of the trunk coming forward. The other is from the front of the vehicle coming back to the listener. The position of the subwoofer will have a big effect on this upper end response. The following curves were measured with a sealed box and show the effects of positioning the subwoofer in the vehicle. 










The same happens in an IB installation to some extent. I believe bikinpunk had commented on this in some recent posts. While the main rear wave is out of phase with the front wave, there will still be a dip based on the reflection off the back of the trunk if there is a direct path. There are a couple options to deal with this this. The most simple is to just cross your front stage low enough that this is no longer an issue. Typically 70hz is easy to achieve with a decent front stage. In a smaller vehicle with less distance in the trunk the dip moves higher in frequency even so your subwoofer can play higher. 

The second option is to use something to divide up the direct path in back of the woofer or to absorb the back wave. We do this most often in midrange cabinets to avoid the same situation. A distance of 4.5" from the back of a midrange driver to back of the cabinet will give you a deep null at 700hz. Dividing up this space with bracing or internal angles can eliminate this or push it higher in frequency. In the case of a trunk it is harder to do this though but i have seen some who have done manifolds to cover the back of the woofer and block the direct path and then stuff the back side. Almost along the lines of a transmission design. This is however more difficult to do when you are talking about larger wavelengths. 

The final option is to do some kind of a cardioid system where the back wave is cancelled. This is done in live sound systems for similar reasons. You don't want bass to come backwards onto the stage. The idea then is to cancel the output going backwards with additional drivers. A small sealed subwoofer in the trunk playing backwards can cancel the output of the IB subs quite effectively over a specific range. In this case it does not need to play low as you only need to cancel the upper end of the response. If you are using something like a minidsp and have an extra dsp channel available you can really dial this in to most ideally cancel the back wave at the higher frequencies.


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## rich20730 (Feb 13, 2012)

^^^ Great information!


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## sqshoestring (Jun 19, 2007)

Schramm said:


> Actually, I thought of a few reasons why FLAT to 30Hz may be difficult, not because it is difficult with IB, but because of the equipment I have selected.
> 
> 1) I suspect the JBL 2241G's earlier roll off may require more sophisticated low end equalization than available in Clarion DRZ9255, where PEQ is 1/3 octave, adjustable in three bands with frequency centers of 50, 63, 80, 100, 125, .... through 20kHz. Cuts on these frequencies to flatten the response would also effect the midbass, assuming crossover point of 70Hz. This is a little more limiting than 31 band graphic EQ available in most dedicated DSP's with adjustments as low as 25Hz, independent for each driver. However, budget has already been exceeded.
> 
> ...


1) If you choose a driver with a lot of rolloff and have to EQ, in my experience a PEQ is very effective. My 16 band has 20/31/50/80hz bands and the PEQ I used was much more effective. I used an older EQ, but many bass processors are a PEQ and not that expensive. I used the PEQ on the sub output of my HU so it did not affect anything but the subs. I'm seriously thinking of getting a bass processor for that reason, and you get a gain knob on top of it. I find bass levels vary wildly by source when you get to 35hz and under.

2) If you want it louder you will need more power, and/or more cone area. All depends on the volume level you need to reach, but yes it does naturally reduce the lower you go.

3) Subs never seem to have the issues with playing Fs range as everyone thinks, but I know if I see -3db that is where my bass will be weak. Cabin gain I can complain about because I've had cars with little of it, or little of it under 40hz. It is more difficult to use a sub or install designed to work with cabin gain when you don't have what they assume you do. But most do not have the equipment to measure it. That is why I laugh at people who say why would you want all that 20 or 30hz in your FR....like I can't get rid of it, and if I don't have enough I know I will not be happy. Its far easier to deal with excessive bottom and rarely happens anyway so I always design for it when possible. But hey that is just me. I mean people never seem to come on this site and ask how to get rid of too much 30Hz sound from their subs...


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## Schramm (Mar 12, 2012)

Stryke23x,

Thank you very much for providing this rich explanation and extensive data models! Your post is well written and the primary data for transfer function is extremely helpful. The effect you measured is significantly larger than I imagined. If +10db is approximately twice the perceived volume and requires approximately 10x the amplifier power, that is no small effect. The rising rate in lowest octave could help provide an inverse response to a traditional driver's 12dB per octave roll off in sealed enclosure or IB. It gives me some hope to pull off a flat response without more sophisticated PEQ in my case. In the Audi S4 example, +35dB cabin gain at 22Hz is massive. In any case, more efficiency is a huge benefit. 

Your explanation on why some have noted certain mid bass frequencies canceling each other and strategies to minimize the effect makes a lot of sense. I do not have the measuring equipment or know how to test this, so again thanks for your knowledgeable feedback. Is this also related to poor impulse response in most vehicles, basically echoing in tin can near field environment? I suspect rear sub and front mid bass reflections may occur with other enclosure types also, but to a lesser extent? 

Drivers engineered by Acoustic Elegance for IB roll off incredibly low without the transfer function factored! Very impressive engineering and optimization.

Cheers


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## Schramm (Mar 12, 2012)

sqshoestring said:


> 1) If you choose a driver with a lot of rolloff and have to EQ, in my experience a PEQ is very effective...
> 
> 3) Subs never seem to have the issues with playing Fs range as everyone thinks,


That is good to know about PEQ. Hopefully, I will be able to contribute with some real measurement data once this project gets built. Regarding Fs, smaller 8-12" drivers can be quite jagged in frequency response below 100Hz as Fs is approached. This can be easily seen from manufactures spec sheet plot. By comparison, most 15" and 18" woofers have a smooth and well behaved response in the lowest octaves. It is hard to defy Physics.


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## minbari (Mar 3, 2011)

What physics are those?

Drivers tend to have Peak around fs, if you have a really low fs, this will smooth out too.

BUT, once you put any driver in an enclosure this all changes

Sent from my motorola electrify using digital farts


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## 14642 (May 19, 2008)

Hey Stryke,

Good info. I posted a bunch of cabin gain curves for a bunch of different cars here awhile back and your measurements and my measurements are very similar. I didn't post different locations for the subs, although we did measure that--our results were similar. 

In an IB, we don't really have an opportunity to choose woofer locations (they go in the rear package tray or behind the back seat). We also don't really have an opportunity to dramatically change the size or the rigidity of the enclosure. Finally, unless the woofer has a really loose suspension, the volume of the "box" is so big that it doesn't affect the response very much (this is apparent in your woofer-in-box-only graphs for the various sizes. While there is some change in the curves, it's quite small (6dB or so at 20Hz) compared to the difference between those graphs and te in-car graphs (+20dB or so) at 20Hz. 

Since the real issue is the difference between the huge peak at low frequencies and the big dip in the midbass, there's another way to address this without having to be so concerned about what happens with the back wave. We can simply EQ out much of the cabin gain so there's a smaller difference between what happens at low frequencies and the midbass. Removing a bunch of drive at low frequencies reduces the input power to the woofer at low frequencies, which helps to eliminate some distortion in the woofer since excursion at low frequencies (and around Fs) is reduced. Once the sub's response has been EQed so that the response is much flatter out to say, 150-200Hz, then application of the low pass filter according to the capabilities of the midbass speakers in the front will also remove some of the boost that may have been required to flatten the sub's upper frequencies. Then, the level of the subwoofer can be set with the gain control of the amp. 

If you look at the combination of crossover and EQ that's applied in this method as a single filter, you'll see that it's essentially a high-Q high pass filter with an F3 that's centered just below the frequency where cabin gain begins to boost the response (with a few wiggles here and there to smooth peaks and dips). It's essentially the Linkwitz transform applied to attenuate rather than to boost. 

http://sound.westhost.com/linkwitz-transform.htm


This cannot be done without separate EQ for the subwoofer. A pair of 31-band graphic EQs (one for the right and one for the left) isn't the right tool for this. The reason I suggest this method rather than imprecise modeling of the trunk and mechanical parts to "fix" the issue, is because adjusting a few filters in an EQ is MUCH easier, MUCH faster, and makes fine adjustment possible without a thousand trips to Home Depot and a trash can full of iterations.


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## sqshoestring (Jun 19, 2007)

Schramm said:


> That is good to know about PEQ. Hopefully, I will be able to contribute with some real measurement data once this project gets built. Regarding Fs, smaller 8-12" drivers can be quite jagged in frequency response below 100Hz as Fs is approached. This can be easily seen from manufactures spec sheet plot. By comparison, most 15" and 18" woofers have a smooth and well behaved response in the lowest octaves. It is hard to defy Physics.


I just don't see it as important. Its just that you have so many things going on between the trunk volume, reflections, cabin gain, hearing sensitivity in that range, road noise, and etc so I don't put much weight on what the sub does at Fs on a test graph. I agree the 15s and up do work better in this range. If I use a 10 it is because I have no other choice, so I work with it. If you find a 10 with a Fs 20hz you will likely find other issues along with it.


I really need to find one of these cars with so much gain at 20-30hz I have to cut it back. I can say in all my life I never have experienced that. While my current 15s certainly can do it, that design was on purpose and overkill in my thinking. It was an effort to extend the FR lower possibly to 20hz, which needs much more capacity. They can get 25hz with some authority, made me very pleased considering $95 worth of sub in there and a nearly empty trunk. _If_ it could successfully be ported it might get flat to 20hz.

Have had cars that midbass cancelled in a spot, found that is a bad place to have your xover at for starters.

Andy as usual explains things very well. The PEQ I used would also cut, and so could be used in a fashion to do as he explains. With the 12s I hated so the PEQ could cut 50Hz or boost <30hz for similar results after gain changes. The 12s were very strong 40Hz and up, in fact the PEQ was on my sub out and had already cut 50 on the HU EQ. Liking my far more balanced 15s much better but maybe just have a car with weird acoustic properties...and yes I do prefer the capability of strong low bass if desired some music seems to need it.


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## cruzinbill (Jul 15, 2011)

sqshoestring said:


> I just don't see it as important. Its just that you have so many things going on between the trunk volume, reflections, cabin gain, hearing sensitivity in that range, road noise, and etc so I don't put much weight on what the sub does at Fs on a test graph. I agree the 15s and up do work better in this range. If I use a 10 it is because I have no other choice, so I work with it. If you find a 10 with a Fs 20hz you will likely find other issues along with it.
> 
> 
> I really need to find one of these cars with so much gain at 20-30hz I have to cut it back. I can say in all my life I never have experienced that. While my current 15s certainly can do it, that design was on purpose and overkill in my thinking. It was an effort to extend the FR lower possibly to 20hz, which needs much more capacity. They can get 25hz with some authority, made me very pleased considering $95 worth of sub in there and a nearly empty trunk. _If_ it could successfully be ported it might get flat to 20hz.
> ...


You can successfully port it and get pull up the 20-30hz area.


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## Schramm (Mar 12, 2012)

minbari said:


> What physics are those?


Large surface area Sd is necessary to reproduce the lowest octaves and couple with air efficiently.


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## sqshoestring (Jun 19, 2007)

cruzinbill said:


> You can successfully port it and get pull up the 20-30hz area.


The model of my trunk ported looks great, it is flat to 20 with a bump at 30 I cold get rid of. But it seems that nobody has posted about doing it, some people say it will not work. Need to measure in there and make it reversible if I do something. Also want a non-straight port to combat trunk noises if there are any though it does not seem to be an issue. Finding time to do it is an issue lol.


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## Fricasseekid (Apr 19, 2011)

sqshoestring said:


> The model of my trunk ported looks great, it is flat to 20 with a bump at 30 I cold get rid of. But it seems that nobody has posted about doing it, some people say it will not work. Need to measure in there and make it reversible if I do something. Also want a non-straight port to combat trunk noises if there are any though it does not seem to be an issue. Finding time to do it is an issue lol.


You and I spoke about doing it I thought. I modeled mine and with any port area that was feasible I was left with a port that was only 2-3" long. Wasn't that you that I had a discussion about when a port becomes so short that the volume of air inside of it may not successfully couple with the speaker cone?


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## BuickGN (May 29, 2009)

Back when I modeled mine to get flat to 18hz it required a 1" long port. Don't know if it will work that way in real life though.

I might as well tack it onto this thread but since reading several of the above posts I've tried filling the trunk with polyfill and currently have two large pillows behind the subs which has made a larger change in the upper response than I ever thought it could. Instruments now sound like instruments instead of boom. It doesn't seem to matter much if the pillows are directly behind the subs or near the rear of the trunk. FWIW I tend to run my subs higher than many.


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## cruzinbill (Jul 15, 2011)

sqshoestring said:


> The model of my trunk ported looks great, it is flat to 20 with a bump at 30 I cold get rid of. But it seems that nobody has posted about doing it, some people say it will not work. Need to measure in there and make it reversible if I do something. Also want a non-straight port to combat trunk noises if there are any though it does not seem to be an issue. Finding time to do it is an issue lol.




It works.


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## Fricasseekid (Apr 19, 2011)

cruzinbill said:


> It works.


How long is that port and what is your tuning frequency? What types of subs are those? Have you metered your output with RTA?


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## BuickGN (May 29, 2009)

I like!


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## sqshoestring (Jun 19, 2007)

Oh that looks sweet! 

Did talk about it some time ago, I have ported large boxes and they seem to work differently than smaller more 'normal' size boxes. Not looked into the science of it, but the ports still worked they just seemed to act different. Maybe just a factor of less pressure change in a larger box, I don't know. Once ran a 4cf box with pair of 10s and ports worked great on it, sounded like IB after that and I had run the same subs IB in a different car.


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## cruzinbill (Jul 15, 2011)

Fricasseekid said:


> How long is that port and what is your tuning frequency? What types of subs are those? Have you metered your output with RTA?


its like 9" cant remember off the top of my head. Its tuned to the mid 20s. Looked good on the RTA, its xover to only play from 45 down, but even when full passed it sounded smooth its whole range. 

To me it feels like a ported enclosure.... but instead of having a sharp peak it pulls a wide range of freq around the tuning area up. 

They are RF T1 10s, This is in my FR-S


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## jsketoe (Aug 8, 2008)

that sub set up in the scion sounded great. I'd highly recommend that.


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## Schramm (Mar 12, 2012)

Update. Progress is slow as I save for funds for a newer vehicle for the installation. In the meantime, I managed to buy a McIntosh MC440M and hope this can handle the entire system.


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## sqshoestring (Jun 19, 2007)

I hear you, this summer has been ballistic with family stuff and work and the usual outdoor yard stuff and projects. Took me a couple years to get quad 12s IB in my car, another year to pull them and get 15s in there that work much better. Been enjoying it all this time. Hoped to put 8-10s in the front doors this summer but not out of the woods yet. Still working on other summer toys in the tiny bit of free time I have. Otherwise I'd be doing a sweet door build and porting my trunk. Just keep it on the list and eventually it will happen.


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