# What to look for when choosing subs for infinite baffle



## PottersField (Mar 18, 2011)

As the title states, what do you look for when choosing subs for infinite baffle? I'm looking at doing an IB install, perhaps with a pair of 15's. So far I've read a Qts of .6 or higher is ideal, and I've also read a lower Qts is okay. Some say pretty much any sub can be run IB, others say not so much. Can I get a little help from on this one?

As a reference, here's the T/S parameters from one sub I'd like to use if the specs will allow:

Electrical Q Value (Qes): 0.55
Mechanical Q Value (Qms): 4.3
Total Speaker Q Value (Qts): 0.49
Free Air Resonance (Fs): 22 Hz
Equivalent Compliance (Vas): 209 liters
One-Way, Linear Excursion (Xmax): 18 mm
Efficiency (SPL 1W/1m): 88 dB SPL
Effective Piston Area (Sd): 810 cm2
DC Resistance (Re): 2.8 ohm
Nominal Impedance (Znom): Dual 2 ohm
Thermal Power Handling (Pe): 600 W
Force Factor (Bl): 17.7

I'd be using a pair of them and giving them around 250w each, which I would think should be acceptable for running them IB.


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## azngotskills (Feb 24, 2006)

hasnt this been discussed many times before in various threads?


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## PottersField (Mar 18, 2011)

I've read as much as I could find but I haven't been able to find a definitive answer. I just want some hard facts, if they even exist, so I can make an informed decision.

But thanks for the help.


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## HiFiAudioGuy34 (Mar 30, 2011)

PottersField said:


> I've read as much as I could find but I haven't been able to find a definitive answer. I just want some hard facts, if they even exist, so I can make an informed decision.
> 
> But thanks for the help.


Yeah I was looking around on the site too. I haven't found any hard facts either. They say look for low X specs and Low Y specs and High Z specs but give no examples of number (Minimum/maximum) range to look for. 

I don't even know where to start.


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## DaveRulz (Jun 22, 2006)

From evertything that I have read:

Yes, any speaker can be used IB. The results however will vary greatly depending on the specs. The closer you get to an appropriate IB driver, the less eq you need to use. 

The two major factors that I understand to be important are:

*You want the driver to be as close to .707 qts as possible*
Since there is no enclosure you want to get your qts as close to the ideal enclosure qtc of .707

_As a side note: I remember reading that when AE designed their IB series they shot for a qtc closer to .6 because once coupled with the car environment it rose closer to .707_​
*High X-max*
Without suitable throw, and with no box controlling cone movement, you can reach a driver's mechanical limits very easily IB. 

Also, a good parametric EQ is a must to dial it in correctly (I guess this holds true for most subwoofers really)

The specs you have listed look like a pretty nice IB driver to me.


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## fish (Jun 30, 2007)

Read these two posts...

http://www.diymobileaudio.com/forum/1535217-post68.html

http://www.diymobileaudio.com/forum/1535241-post69.html

He always posts up good info on IB topics.


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## robert_wrath (Apr 24, 2011)

+1 Subscribed. This thread has been rather useful. Thanx for clarifying a few IB issues.


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## HiFiAudioGuy34 (Mar 30, 2011)

Ok I still feel lost on the subject. It would be great to just get all the specs out in the open easy clear cut info.

Example: (im just making up numbers here) I'm looking for minimum and maximum or the "range" I need to look for NO LOWER THAN or NO HIGHER THAN

Xmax- 15-30mm or 15mm+ (and up)
Vas- 100+ Liters or more
FS- 25Hz-50Hz


To see what the maximum and minimum requirements for each spec would be great. 

so on and so forth. You get the idea. If there's such a stencil or list to reference to I'd appreciate the link. 

I am a true rookie when it comes to IB. Starting from the ground up with this info.


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## DaveRulz (Jun 22, 2006)

There is no range, and there is a reason that there is no clear cut info. The best way to do it is to look at your options and then compare

Xmax - More is better
Vas - More Is better
FS - Lower is better
QTS- As close to .707 as possible

As was mentioned before, ANYTHING can be used IB with success. It's just the degree to which it will perform. 

If you want the most output: Lots of XMAX and VAS are a good thing
If you want the best sound quality: QTS as close to .707 as possible
If you want them to dig deep: low FS

It's a balancing act. You need to choose what drivers you think you might use, then compare the specs to see which ones will fit your needs best. 

Again, as all things in car audio: THERE IS NO RECIPE


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## sqshoestring (Jun 19, 2007)

It really depends on what you will do with it, what sound you want. Like I just posted on that other IB thread you have two issues: tuning and capability. For tuning you can model it just like a box since IB is just a large sealed. You want roughly a large driver like a 15 with a low Fs 20Hz and figure it will play down to there, and you want qts .7 about. If you model that you see that qts will push the Fr up above 20 and give you a flat FR down to 30Hz or so. That is IF you want a flat FR, if you want an spl hump you can get a lower qts or higher Fs sub, model it. Naturally smaller subs have a higher Fs and can't get as low, they just can't move the air no matter the tuning and output will be lacking down there.

If you want a real blaster then you need to go to HD subs with huge xmax and braking rings so they don't bottom, etc, things a JBL GTi will have. Even though it is more tuned for a box it can take a beating and push more air, it will work IB it just will need EQ depending on how far off the specs are.

You can run more subs, personally I like dual 15s or quad something with tons of cone area. With low xmax you get low distortions even with lesser subs like my pyles. There is a reason high xmax subs cost more, it is hard to make them not distort.

The lower you want to go the more cone area/xmax you need, life is easy if you don't need 30Hz or less.

If you run cheaper subs you will have to be careful you don't bottom them, or run a SS filter, you have to eyeball them for xmax there is no other way...or don't flog them that hard lol.

Now as you get to Vas of the subs with your trunk, you start to push Q up. So a qts .7 will go higher in a small trunk and will happen sooner if the Vas is huge. That is good reason to model them. If you have a tiny trunk a lower qts might work better like ~.5 or so. Remember high qts above .5 will naturally inflate the output above Fs, and small enclosure will push that 'inflation' higher above Fs as you reach Vas and smaller...which is pushing the Q up that is what boxes do and why box subs have low qts to start with. 

That is why you start with a low Fs and the high Q will boost lower. If you had a Fs 40 sub then the high Q would start to boost at 50 instead of 30Hz...see? Now Qts is just Q of the sub with no box, and when you get to 1.0 Q or more subs can get strange and peaky. That is why they use low qts subs for small boxes, then the box pushes the Q up to a normal range of .5-1 more or less. So the Qts you want can depend on your trunk size and the Vas of the sub, which both affect the response if you get into them.

Still if you want all out thunder bass, you have to go to a better sub. You tune best you can but most of them are not IB ideal the Q will be lower for a box. So you do what you can and I find a PEQ works best to manipulate the FR and get the sound right.

All this also depends on what you want, I like low flatter bass but if you like spl tight/etc bass then the tune should be different for you. For example you could use more smaller subs to get more output but they are all tuned higher in general....compared to my pair of 15s tuned low. Box or IB they both will go louder if you tune a higher spl peak into them. My quad infinity 12s would really bang at 50Hz they seemed to tune there, but dual LP and PEQ got them cleaned up. But also you can get a 15 that is tuned higher too, so it is not a set rule far as sub size. Most people say that because most people going IB want good bottom that gets low with authority and low Fs with higher Qts is where its at. I hardly EQ mine with Fs 20 and Qts .7. Like the AE IB15 that is about ideal in a car. These are 5.1cf Vas so I am about 1.5X vas, and lose 1dB at 20Hz and gain 1dB at 30 and above iirc but 1dB is not much so limited effect. If my trunk were smaller it would start to cut the bottom off and I would not get 30Hz.


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## BuickGN (May 29, 2009)

DaveRulz said:


> There is no range, and there is a reason that there is no clear cut info. The best way to do it is to look at your options and then compare
> 
> Xmax - More is better
> Vas - More Is better
> ...


I fully agree, especially with the last sentence. The only exception is the Qts. I still prefer the lower Qts subs. Maybe my ears are weird but they seem to blend better, smoother overall, and better low end. I also hate too much 50-80hz so maybe that's why I like the low Qts.


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## sqshoestring (Jun 19, 2007)

BuickGN said:


> I fully agree, especially with the last sentence. The only exception is the Qts. I still prefer the lower Qts subs. Maybe my ears are weird but they seem to blend better, smoother overall, and better low end. I also hate too much 50-80hz so maybe that's why I like the low Qts.


Could be your car has more cabin gain. My qts .7 is pretty flat, sometimes I boost 20Hz.

A lower qts will not have a boost right above Fs it will tend to come on later in higher frequency. For example a typical Fs 20-25 with a .4 qts might have a peak at 50Hz instead of 35 like a .7 qts. But don't forget to account what your trunk does to the q, it may raise it up anyway....so model it lol.

Good thing to do is make or pick some various subs with high/low Fs and qts, see what changes in the models.


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## Gary S (Dec 11, 2007)

sqshoestring said:


> For tuning you can model it just like a box since IB is just a large sealed


 - Right. Maybe model a known IB sub in a sealed box as a reference, and then look for other subs that also work well in over sized-boxes.


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## sqshoestring (Jun 19, 2007)

There are not that many subs with pure IB specs. I liked the AE IB15 a lot but 250 was a lot of coin I don't even drive this car all the time and its not special. I found these pyles with near identical specs, knowing pair of 15s was overkill for me so they would not be stressed I gambled and bought a pair for <100. Turns out they are perfect for me. But just saying there are not many low Fs high Q subs out there to choose from in the <100 each range. The AE, Fi, Dayton, and a couple others are most of them at any price. But if you like a different tune or have cabin gain other subs will work, with EQ just about anything will work that will handle the xmax. Some subs do not work well at all however, in fact cheap subs (like <50) actually tend to work better as you don't need a huge motor on an IB sub. Big inductance is not good for IB. So if you go high output built sub you have to watch for those issues.


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