# Low Qts + eq vs High Qts for IB



## Orion525iT (Mar 6, 2011)

I have been thinking about attempting IB. I never really considered it because the project car is a wagon, but I think I can pull it off. I have heard that some people have had great success with "value" subs. My idea was to use multiple cheap 12" (4) or 15" (3) subs, keep excursion extremely low and linear and go for pure cone area. 

In looking for IB subs it has been suggested that high Qts (>.6) work best. Low Qts subs roll off much more on the low end. But I have read some information that low Qts subs are "faster" (hate that terminology) and better in transient response. It happens that many of the "value" subs I am looking at are low Qts (~.3).

So what is better? High Qts sub or low Qts sub with eq to compensate for the roll off? Any other factors to consider?


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## minbari (Mar 3, 2011)

I think both can work. If you look at subs that have "IB" in the name quite alot of them are more than 0.5 qts. I am sure BuickGN will chime in cause he has alot more experience on this. he has said that lower QTS works just as well. seems to be a turkey shoot.

how do you intend to isolate the front and back wave in a wagon?


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## Orion525iT (Mar 6, 2011)

minbari said:


> I think both can work. If you look at subs that have "IB" in the name quite alot of them are more than 0.5 qts. I am sure BuickGN will chime in cause he has alot more experience on this. he has said that lower QTS works just as well. seems to be a turkey shoot.
> 
> how do you intend to isolate the front and back wave in a wagon?


I was under the impression that any sub could work, but ran across info on home audio forums that suggested there is a difference in transients and roll off and maybe a few more finer points. 

I am only running them up to 50hz, if it makes any difference, the RS225-4 up front will take over from there.

I plan to make a sealing flange down the C-pillar, which mates to the shock tower, around the back of the split fold seats. I could make a single panel to mate to the flange all the way around. But the hope is to make a separate panel that will mount the the rear seat back, seal tightly, and still allow me to fold the seats down. Trunk space should be over 20 ft3.


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## Hanatsu (Nov 9, 2010)

It will be more or less efficient. Qt together with Fs determine the natural highpass shape of the rolloff. 

EQ the response to the same shape and don't think you'll hear any difference if all other parameters remain the same. It might even be the opposite and you'll require more EQ with a high Q sub since the lows can be too boosted due to cabin gain. If you don't have access to EQ you can still shape the FR somewhat with the amps built-in crossover filters.

Sent from my Samsung Galaxy 3 via Tapatalk.


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## Orion525iT (Mar 6, 2011)

Hanatsu said:


> It will be more or less efficient. Qt together with Fs determine the natural highpass shape of the rolloff.
> 
> EQ the response to the same shape and don't think you'll hear any difference if all other parameters remain the same. It might even be the opposite and you'll require more EQ with a high Q sub since the lows can be too boosted due to cabin gain. If you don't have access to EQ you can still shape the FR somewhat with the amps built-in crossover filters.
> 
> Sent from my Samsung Galaxy 3 via Tapatalk.


Its always a challenge to determine in cabin response, I suppose. But, I think you are maybe correct. The subsonic filter on the amp is set at 25hz 24db. Since I plan to only run the subs through a single octave, maybe I am splitting hairs a bit? I guess my main thought was the suggestion that transients are different from low q to high q subs. But then again, I am not sure it can be perceived and/or noticed.


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## Oscar (Jun 20, 2010)

Low Qts means the oscillations die out faster. So it's not the transient part that is faster, it's the decay. This is just the regular ol' definition of damping for an oscillatory system.


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## Hanatsu (Nov 9, 2010)

...and decay times in lowest octaves are dominated by modal ringing that far exceeds that of a high Q alignment. I'd say it's an inaudible effect.

Just EQ the response, try to get it as efficient as possible.

Sent from my Samsung Galaxy 3 via Tapatalk.


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## BuickGN (May 29, 2009)

To me with the system Q in the .5 area sounds more natural, less boomy. Both ways will work, espcially with EQ. There is one area I still wonder about. If high Q gives more ringing, more overshoot, how can it ever be the same as a low Q sub even with eq? Isn't the ringing a mechanical problem that can't be removed with eq? Even if you remove the bump, it's still not damped as well. Wouldn't it be better to start low and eq the response you want and have a better damped system? Unless I'm wrong and EQ can change damping. 

While I don't fully understand the science, there's absolutely no doubt that I prefer the lower Qtc systems. Of couse Fs plays a role too. I know in my TL I've never had to boost the 20hz area even when I had the 12W6s with a .46 Qts and a low vas where system Q modeled the same as the sub Q. It would dig down effortlessly. I know the system Q is higher with my IB15s and they don't have as much low end as the W6 did, not in total output but in FR.

My opinion> In the end I just don't think it matters as much as most people seem to believe. You rarely ever hear about Q being talked about in sealed boxes where you're a lot more likely to end up with too high of a Q and a boomy, "slow" sub stage. For some reason it's focused on in IB and after hearing some .3Qts subs and even a pair of .2 Qts subs I just don't worry about Qts being too low. Most will disagree with me but I think the Qts is less important when going IB. The ones I've tried both sealed and IB, IB always had more low end than sealed even with the low Qts subs. I just don't think it's that important and I would definitely want to error on the low side. Just my opinion and experience.


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## Oscar (Jun 20, 2010)

BuickGN said:


> To me with the system Q in the .5 area sounds more natural, less boomy. Both ways will work, espcially with EQ. There is one area I still wonder about. If high Q gives more ringing, more overshoot, how can it ever be the same as a low Q sub even with eq? Isn't the ringing a mechanical problem that can't be removed with eq? Even if you remove the bump, it's still not damped as well. Wouldn't it be better to start low and eq the response you want and have a better damped system? Unless I'm wrong and EQ can change damping.
> 
> While I don't fully understand the science, there's absolutely no doubt that I prefer the lower Qtc systems. Of couse Fs plays a role too. I know in my TL I've never had to boost the 20hz area even when I had the 12W6s with a .46 Qts and a low vas where system Q modeled the same as the sub Q. It would dig down effortlessly. I know the system Q is higher with my IB15s and they don't have as much low end as the W6 did, not in total output but in FR.
> 
> My opinion> In the end I just don't think it matters as much as most people seem to believe. You rarely ever hear about Q being talked about in sealed boxes where you're a lot more likely to end up with too high of a Q and a boomy, "slow" sub stage. For some reason it's focused on in IB and after hearing some .3Qts subs and even a pair of .2 Qts subs I just don't worry about Qts being too low. Most will disagree with me but I think the Qts is less important when going IB. The ones I've tried both sealed and IB, IB always had more low end than sealed even with the low Qts subs. I just don't think it's that important and I would definitely want to error on the low side. Just my opinion and experience.


I agree 100%. Unless you have a very very flat BL curve, or only keep the excursion within a very flat range, the BL variation will cause Qts to increase anyways as there is more and more power; so IMO it's better to start out low---that way if you "lose" Q (for lack of a better word), you'd still [likely be] within an acceptable range.


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## Orion525iT (Mar 6, 2011)

BuickGN said:


> To me with the system Q in the .5 area sounds more natural, less boomy. Both ways will work, espcially with EQ. There is one area I still wonder about. If high Q gives more ringing, more overshoot, how can it ever be the same as a low Q sub even with eq? Isn't the ringing a mechanical problem that can't be removed with eq? Even if you remove the bump, it's still not damped as well. Wouldn't it be better to start low and eq the response you want and have a better damped system? Unless I'm wrong and EQ can change damping.
> 
> While I don't fully understand the science, there's absolutely no doubt that I prefer the lower Qtc systems. Of couse Fs plays a role too. I know in my TL I've never had to boost the 20hz area even when I had the 12W6s with a .46 Qts and a low vas where system Q modeled the same as the sub Q. It would dig down effortlessly. I know the system Q is higher with my IB15s and they don't have as much low end as the W6 did, not in total output but in FR.
> 
> My opinion> In the end I just don't think it matters as much as most people seem to believe. You rarely ever hear about Q being talked about in sealed boxes where you're a lot more likely to end up with too high of a Q and a boomy, "slow" sub stage. For some reason it's focused on in IB and after hearing some .3Qts subs and even a pair of .2 Qts subs I just don't worry about Qts being too low. Most will disagree with me but I think the Qts is less important when going IB. The ones I've tried both sealed and IB, IB always had more low end than sealed even with the low Qts subs. I just don't think it's that important and I would definitely want to error on the low side. Just my opinion and experience.


I think this coincides with my thoughts. Most of the IB info I ran across comes from home audio or home theater knowledge. In those cases, it makes sense to focus on Qts, were IB is true IB and room gain is less of a factor. I think this is where the focus on Qts originates.

But move to car audio and things change. I was initially worried about the earlier roll off of the low Qts subs. I didnt like the thought of adding a bunch of eq to get flat response down low. But I completely disregarded cabin gain. If you look at plots of typical cabin gain, they are somewhat inversely proportional to the roll off you would see with a low Qts dirver in IB. It may turn out that very little eq is needed to achieve flat response when a low Qts driver is employed. I think Hanatsu alluded to this.

Now turn it around. If you choose a high Qts driver, which has a much later roll off on the low end, what looks good on paper, becomes peaky once cabin gain is factored. You could end up with > 10db gain on the low end. Its seems it would be be very difficult to tame that much gain.

Part of what is confusing was the sources of most information for IB; home theater / home audio. The other was that when looking for IB subwoofer candidates, I naturally looked to examples of true IB subs. Interestingly, the AE IB15 is close the "ideal" Q, which seems a bit odd. The AE IB12 is lower Qts.

It does seems that using a low Qts, low fs sub, with good efficiency coupled with cabin gain will naturally get you closer to linear response. Of course eq will play a roll, but you won't need it to dial back 12-15db of gain on the low end.


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## BuickGN (May 29, 2009)

One interesting note that John at AE has talked about is the main change between the home and car version of those subs. The home version has a .7 Qts, the auto has a .4 Qts. In a car, Qtc usually does not equal Qts like it does in home "real" IB. That .4 will go up in a trunk and it will go up even more with a pair of subs in a trunk. John lowered the Qts so Qtc would not go over .7 in a trunk. 

In my trunk, mine model to around .62 Qtc if I remember right but I've made sure they can breathe to the outside world because I like how they sound in a larger airspace and with a lower Qtc better. Some of this is just personal preference. Some of it depends on the car itself. Some subs are close to real IB in a trunk and some aren't. 

Also, if I remember right, the lower Qtc will have more of the low, low bass. There are times some material gets close to 20hz and it's a fun experience when the system can play it.... audibly. 

I'm just rambling now because I have to work this weekend and I'm bored. When I get home I'll have all of my info from my current setup and my old setups.


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## Hanatsu (Nov 9, 2010)

Orion525iT said:


> I didnt like the thought of adding a bunch of eq to get flat response down low. But I completely disregarded cabin gain. If you look at plots of typical cabin gain, they are somewhat inversely proportional to the roll off you would see with a low Qts dirver in IB. It may turn out that very little eq is needed to achieve flat response when a low Qts driver is employed. I think Hanatsu alluded to this.
> 
> Now turn it around. If you choose a high Qts driver, which has a much later roll off on the low end, what looks good on paper, becomes peaky once cabin gain is factored. You could end up with > 10db gain on the low end. Its seems it would be be very difficult to tame that much gain.
> 
> ...


It's basically the same deal with vented alignments. I tune them pretty low generally, I get a TON of low end - too much low end due to cabin gain. I lower those regions by adding a highpass filter and cut with EQ. Why do I do this? Simple - It's because of the immense gain in efficiency. The Qts value is the sum of the mechanical and electrical Q and basically defines energy storage and the dissipation of energy around resonance (Fs/Fsc). The lower Q you have, the more energy you have to push into the driver to get it to move, the efficiency goes down as less energy is transferred into moving air. When the sub moves it created bass and that's what we want right? 

The natural response to this would be; OK, but I want good sounding bass. So it's basically about, how much can I get away with and still sound good? And here's where everyone starts arguing, I say it's not very audible, lots of things are not very audible in the lowest two/three octaves. People like to rave about group delay and non-linear distortion with subs but the audible threshold is actually pretty high, unless the driver is plain crap and the enclosure is mismatched, I'd say you can get away with a great deal. Also, IMHO you can get several types of alignments (IB/Sealed/Vented...) to sound similar just by EQing them to the same FR, irregular FR is DISTORTION, linear distortion. Whatever system you using, the FR won't be flat, nor does it have to be flat to sound good. The modal ringing in those lower regions can easily exceed 1000ms with still audible sound "bouncing around". The energy storage or lack of energy dissipation around Fs due to a high Q will NOT contribute to that amount by far. It will however contribute to a peak in FR, that peak is the main reason why the alignment sound "boomy", imo. 

I don't promote using very high Q alignments, at some point it will sound like crap and no amount of EQ will fix it. IME, this happens when you reach very high Qt values of 1.2-1.3 or more but I don't have any real proof to support it. Using lower Q drivers opens up more options, they can be put in several types of enclosures. High Q drivers however, are limited to either very large sealed boxes, IB or AP, several of these drivers also has a limited Xmax, NOT a good combo. 

So what am I saying? You don't really have to put that much thought into this, IB is pretty straight forward. You want high displacement, since cone movement increase four times for each drop in octave you need large drivers with high Xmax/Xmech. In my opinion, you don't have to give up efficiency to gain SQ. Just don't overdo it, use a mid Q driver, with low/mid Fs (for its size). VAS should be at least 4x higher than the apparent enclosure size (on either side of the cone) for the driver to "see" an IB config, so a huge VAS value might not be the best option either. But I limit my rant to this.


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## Orion525iT (Mar 6, 2011)

Hanatsu said:


> It's basically the same deal with vented alignments. I tune them pretty low generally, I get a TON of low end - too much low end due to cabin gain. I lower those regions by adding a highpass filter and cut with EQ. Why do I do this? Simple - It's because of the immense gain in efficiency.


Interesting approach, but I can see how that works.



Hanatsu said:


> VAS should be at least 4x higher than the apparent enclosure size (on either side of the cone) for the driver to "see" an IB config, so a huge VAS value might not be the best option either.


 That seems more like a sealed alignment. Don't you want the apparent enclosure size to be...infinite...?


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## ErinH (Feb 14, 2007)

Honestly, I didn't read the replies here (I'm limited on time) so this may have been said already...

The high Qts actually means the driver's low end response will roll off sooner than a low(er) Qts driver, given the same conditions. So, really, if anything you'd be wanting to EQ the higher Qts driver; not the lower Qts driver, to achieve more low frequency output. This is, however, at a cost. Does the driver have the ability to handle the excursion and is the amplifier capable of putting out that increased (via EQ) response cleanly. Basically, If you're sending the sub a square wave you're going to kill it. 

Then you have to consider the 'enclosure'. The enclosure will alter the Qts and provide you with the installed Qt*c*. Basically, when you target a Q, you're targeting in-box response which is Qtc. Not Qts. 

And then there's the fact that not all 'IB' subs are the same. Some are better suited for a given application but not solely due to its Qts. You have to look at the other parameters (namely Vas and Fs). Vas is a means of determining how much volume a driver needs at it's given Qts. If you have a driver with a Qts of 0.7 and a Vas of 1 ft^3 then putting it in a large enclosure like your trunk may not be the best idea; you'll keep that Q with a Qtc of roughly the same thing. But it may not handle power well. If you have a driver with a Qts of 0.7 and a Vas of 10ft^3 then it may be more well suited for the enclosure you're putting it in (where I'm going off the rule of thumb that the trunk has in the ballpark of 10ft^3), putting you at a Qtc of 0.7. 

On the flip side, you can have a driver with a Qts of 0.4 and a Vas of 24ft^3. This Qts may seem low, but the two values work in tandem. You're raising the Qtc... you may wind up at Qtc=0.7 with this setup (I'm pulling numbers out of the air for explanation purposes). Looking at the specs, you'd say "heck no", but it actually may work in the car. Much moreso than a driver with a Qts of 0.7 with the same Vas in the same enclosure. 

The trunk is always the smallest enclosure size but the cabin vs the trunk typically are so large in regards to the subs we (in the car audio community) use for IB that it's kind of a wash. Your response will be driven by the trunk, but it's nothing to sweat over. Not unless you're using a subwoofer that's designed to have a whole lot of airspace around it to keep it's suspension damped properly. 

So, to reiterate: it's the combination of the parameters and the enclosure that drive what you should be using. If you're a numbers guy, then you should buy or borrow something like the Dayton DATS which will allow you to do an in-car sweep to determine the Qtc. I do this. I've got a Qtc of 0.758 and it's very, very nice. The response is 'quick' and 'tight'. Much better than the previous alignment that was closer to 0.5 Qtc. I wouldn't necessarily sweat the difference in 1/10, but beyond that I believe there's legitimate differences in apparent response. And the measurements back it up. 


I'm sure I'm leaving some things out, but hopefully that helps.


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## Hanatsu (Nov 9, 2010)

Orion525iT said:


> That seems more like a sealed alignment. Don't you want the apparent enclosure size to be...infinite...?


LOL. Meant to say the exact opposite. Fixed ^^



> The apparent "enclosure" should be at least 4x higher than the VAS (on either side of the cone) for the driver to "see" an IB config


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## Orion525iT (Mar 6, 2011)

Well based on the information here (thanks) I went ahead and bought a whole bunch of low Qts subs with knowledge that I can manipulate Q, through various methods, and even fs to get me where I want to be. Where that is, I am not sure, but the low Qts should give me plenty of variables to mess with.

IB PPSL .


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## Xaborus (Feb 1, 2012)

QTS of 0.5 is the fastest any speaker driver can be without oscillating. Basically this means this is the fastest a speaker driver can be while still being accurate. This is like a door with an automatic door closer that perfectly closes the door to a dead stop.

A higher QTS than .5 means the door gets to the resting position faster, but wobbles back and forth a few times before coming to a stop. The speaker driver is technically faster to the close position, but isn't necessarily accurate to the input signal.

A lower QTS than .5 makes the speaker driver a bit slower to get to the resting position.

So higher than .5 rings like a bell, lower than .5 is a bit slow/ dead like a hand on a bell, and .5 is exactly accurate to the input and rings once, the fastest it can accurately.

The problem is that in a sealed enclosure, your QTC can never go below QTS. Therefore, speakers designed with sealed enclosures in mind lower the Q to around .45 allowing a sealed system to approach 0.5x. I believe .577 is a critically damped sealed enclosure. Dunno why its .577 rather than .5 honestly.

For IB or OB use, A QTS of .5 is perfect. A higher QTS/QTC will give you better low frequency response with accuracy as a tradeoff. Most consider the max QTS/QTC of .707 as it has the flattest low frequency response. No reason to go any higher, there aren't any benefits. Going lower than .5 has no benefits IMO, your just getting worse low frequency response and getting a slower driver.

In summary, if you want a driver that can be put in any enclosure and sound great, a QTS of .45 is probably your best bet, especially if you want to approach .57 in a sealed enclosure. It's no coincidence msost people build sealed enclosures to .707, stuff it with polyfill, and find it sounds better. What you just did was lower the QTC closer to .5.

If you don't care about approaching .577 in a sealed enclosure and are going for .707, a driver QTS of .5 will better suit you. Or if using OB or IB, you definitally want a driver with QTS 0.5.

Going over .5 will give a boomy/muddy sound, going under .5 will probably sound to lean/slow.

Its worth noting that while 0.5 is the fastest,the other question is "how fast can the fastest be?". This is where inertia comes in. A lower driver mass has less inertia, and subsequently is easier to get to come to a dead stop. This is usually known as Mms. Lower Mms means lower inertia. Le or inductance, also stores energy. So drivers with a lower Le will also be faster.

The more you know


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## Oscar (Jun 20, 2010)

Xaborus said:


> QTS of 0.5 is the fastest any speaker driver can be without oscillating. Basically this means this is the fastest a speaker driver can be while still being accurate. This is like a door with an automatic door closer that perfectly closes the door to a dead stop.
> 
> A higher QTS than .5 means the door gets to the resting position faster, but wobbles back and forth a few times before coming to a stop. The speaker driver is technically faster to the close position, but isn't necessarily accurate to the input signal.
> 
> ...


If I'm not mistaken, 0.577 is a Bessel alignment. 1/sqrt(3)=0.577.....

I think it comes from the coefficients of the solutions of certain 2nd order ODEs of the transfer function magnitude.


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## Xaborus (Feb 1, 2012)

Oscar said:


> If I'm not mistaken, 0.577 is a Bessel alignment. 1/sqrt(3)=0.577.....
> 
> I think it comes from the coefficients of the solutions of certain 2nd order ODEs of the transfer function magnitude.


Holey crapola. Have an engineering degree by any chance? I didn't understand any of that Lol.

Your probably right though, with you being well educated and all


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