# The truth about high QTC



## cvjoint (Mar 10, 2006)

Lessons learned here from home theater may not apply in a car. So what does matter in a car? Is high QTC good for some applications like midranges but bad for subwoofers. Can low end peaking and all it's downsides be EQd away? When it comes down to it, what is the ideal QTC?


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## cvjoint (Mar 10, 2006)

Bump. 3 days no go huh?


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## sqshoestring (Jun 19, 2007)

Ideal is what fits your needs. To me its just how peaky is the bass section going to be, in some installs that is ok or good if you are spot on your xover point for example. Much of the time for my cars I don't like it because its just more work for me to tune. I'd rather it fit the curve I want for my whole system. But you never know sometimes you don't have room for X enclosure you have to consider Y that might have different tuning than you prefer. You might be pushing the power envelope and want a peak right before you cut that midrange off with a 24dB slope, etc. My IB subs for example are likely around .7 or under, very smooth hardly any EQ needed, very easy to tune I could even run them without any EQ at all they are that close....it would be not perfect but listenable. On the other hand 15s with a Fs 20Hz are going to be smooth with a qts .7. If you are running 8" subs it may be entirely different.

Sure you can EQ it away, but not everyone has a 31 band or PEQ, or equipment to measure. I only like high Q in particular situations.


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## BuickGN (May 29, 2009)

I've run the 12W6, Tempest X, and two IB15s in the IB configuration and they just don't do what the numbers would suggest... or maybe I'm interpreting the numbers wrong.

The W6 and IB15s have awesome transients, great low end, and great top end for the IB15s and very good for the W6 up to about 65hz. The IB15s play 80hz very cleanly and with authority. Both hit below 20hz easily. The IB15s might be consdiered a little "faster".

The Tempest X required 10db of cut to the 20-40hz range and 10db boost above that to even resemble flat. It sounded a little "slower" and transients weren't that good. Removing the weatherstripping from the trunk really turned things around. 

The Tempest and W6 have similar specs with VAS being the biggest difference. I would love to hear any speculation as to what caused the single W6 to sound similar to the two IB15s and the Tempest so different.


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## cvjoint (Mar 10, 2006)

If differences in QTC are indeed all frequency response differences and nothing more the effect of using a smaller box would be almost negligible in the car. Cabin gain varies from 0 to 25db in some cars bellow 100hz. That's going to shape the frequency response a lot more than the 1 or 2db or gain from using a super tight box. Cutting 1 or 2 db at the peak with a decent Q is going to make the two sound the same from a frequency response point of view. Indeed 1 db and over is audible, but in the context of cabin gain it's not much at all. I know some heads can't EQ. down low as well. 

What about "transient" response and group delay? If you do manage to EQ. the frequency response of a 1/2 cube box to look like 1 cube (anybody with 1 band of parametric can do this, and graphics will do ok too) are they going to sound the same? Does fixing frequency response fix transient and group delay at the same time? Are they all sort of the same thing?

If we throw in lots of speakers in a comparison, W6, Tempest, AE IB, etc there are too many variables to look at. I was thinking of a simple experiment. Take on sub and mount it in a 3 ft3 box, then in a 2 ft3, then in 1/2 ft3. If you EQ. away to make them look the same on the RTA are they going to sound the same? Is QTC all about frequency response?


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## sqshoestring (Jun 19, 2007)

I just don't worry about that stuff for the most part, doing daily installs. Look at the delay on ported but does it really matter....in a car? How many don't run ported because of it? What about expensive home systems with ports? I go for response first and the output I want, which is likely less than many here. Most people can't hear 10%THD in subs, really they should perfect the rest of the system while running pyramid subs before they worry about it...figuratively, because pyramids might not have the output they want lol. But your acuity is going to be far higher for 80Hz and above is my point.

You can EQ it, until you run out of power. The worse your Q/response for the real response you want, the worse your efficiency. Patrick likes to recommend 3" midbass or whatever it was, cool for him but I'd never bother with it. I have a sawzall, jigsaw, welder, shears, air hammer, whatever I'll install some larger midbass one way or another. I really respect what he says, its just not the way I want to go about getting my sound.

Another thing is some drivers are made to have the Q pushed up, like low QTS subs. They are engineered to have a small box make them work properly. Maybe you can use them for something else sure, but something to keep in mind when they do things differently than other drivers.

The most important thing is you use what works for your system, the way you want or need to do it. Sometimes you need a tiny box and a low Q sub is the way to go. If you understand how they work, then you can better pic what you need. What really helps is just plug them into winISD and play with the box and watch the response change. Compare to the sub you have in your car, you can learn a lot from fiddling with it.


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## cvjoint (Mar 10, 2006)

The audibility of group delay for sub frequencies seems to be ongoing research. There are some guys who claim that it generally audible, some say it has to be in the 30ms range, some say it's never audible. There is no good study I could find. I was modeling a pair of 4"s in the pillar pods and noticed the group delay is 2ms for a 1.1Qtc, but that's at 200hz. Interestingly enough a 4th order Butterworth HP added 3ms of group delay! 

I know 10% THD is generally inaudible for sub frequencies and remember reading it somewhere at some point in time. I wonder what test they ran, 2nd through 5th order total? I would think third order distortion at 50hz is quite audible in the 150hz range. Add to that if the sub is 15db over the midrange output, which is very common, that distortion should be very audible. So while in HT the subwoofer plays somewhat blended in, in a car the much higher output might render its harmonics audible. Recent studies for audible distortion seem to emphasize the importance of woofers. We also don't have brick filters so a +15db sub can still play at the same level as the midbass way past it's LP.


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## sqshoestring (Jun 19, 2007)

cvjoint said:


> We also don't have brick filters so a +15db sub can still play at the same level as the midbass way past it's LP.


Ha, it sure can but you will never find that in my car. 80hz is the limit often lower, in response not a number on a dial or display. IMO you need midbass if you want more 80 and up.

Also in a car, or most cars, you get a lot of road noise down there. I don't think most people have the sensitivity to discern. Some people may in the driveway sure, but not on the road. Maybe if you have a really quiet car, I sure don't this econo car needs more and I have some deadening to go in it if I ever get to it.

QTC is just how its tuned for whatever reason you tune it that way. I like smooth and less EQ, others go the other way with small drivers and small enclosures. If you use a driver properly and not overpowering it/etc I think in most cases either will work when properly done. Seems like preference is more important, some people like tight high Q bass and even 8" subs sound grand to them. I want 20Hz, or close as I can get so 8s suck in my book....for a car sub. But I have run them when I had to they are better than nothing lol.

I could be wrong I suppose, what I have seen is in a normal system (not a comp car/etc) most of the problem is response or using the driver improperly. If I run my pyles up to xmax I know they are distorting, they are cheap subs with old motor technology. The idea with a pair of 15s is I don't have to for SQ, only do that for screwing off even then its a bit much for my tastes these days. 500rms on many different IB 15s will vibrate some stuff. But most cheap lower or normal xmax subs are fairly good at half xmax or less. I measured over an inch on these when I got them. And most people characterize response issues as fast/slow/etc when they are simply response problems. I had a PEQ and I could make the subs sound any way I wanted. For me it is a pain to deal with a peaky high Q sub, I hate them lol, but they can be dealt with. My IB quad 12s would pound at 50Hz I had to run two xovers at 50 and cut the EQ (16 band) at 50 to flatten them out. Then they worked fine. Somebody into an SPL sound would have loved them.

I don't mean to say there are not sound issues with subs, but being its the least audible to humans, its in a noisy reflecting and confined car, subject to install limitations....its the last thing on my list. I've had actual pyramid subs that sounded quite good in the right use, given they had limitations just like the better pyles I have now.


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## BuickGN (May 29, 2009)

I enjoy these conversations but ever notice how it's usually the ib guys talking about qts?


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## cvjoint (Mar 10, 2006)

sqshoestring said:


> Ha, it sure can but you will never find that in my car. 80hz is the limit often lower, in response not a number on a dial or display. IMO you need midbass if you want more 80 and up.
> 
> Also in a car, or most cars, you get a lot of road noise down there. I don't think most people have the sensitivity to discern. Some people may in the driveway sure, but not on the road. Maybe if you have a really quiet car, I sure don't this econo car needs more and I have some deadening to go in it if I ever get to it.
> 
> ...


I agree with most of this. There are only two points I wold make:
Cheap driver at xmax and top dog driver at xmax have the same amount of distortion. That's the point of xmax: 10% distortion, at least if they were rated properly. The better sub of course with have a lot more stroke and output at xmax but still 10% distortion. 

Subwoofers are not the least audible distortion wise. Tweeters past 10khz are documented to be inaudible in harmonics. I only care about frequency response and output past 10khz. With subs you still have to think about distortion, and imo they are just as important as midbasses and low midrange. 

Of course QTC talk doesn't have to be about subs. If we were to talk midbasses how would the recommendations change?

In most of my testing I often find left midbasses have a dip around 100hz that is fairly wide. But, does anybody make a different box for left versus right midbass? I mean, if we truly should use the box to get a flat FR out of the box, doesn't that mean unequal boxes based on car acoustics? 

Another thing to note is that I was never able to get a decent impedance curve testing IB subs yet. The QTC of IB subs is a mixed bag in applications as it can't be pinned down. There is too much leakage, I would think.


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## ErinH (Feb 14, 2007)

is the impedance spike (where the Qtc is derived) outside the passband of the driver? how does the Qtc work to the HPF (or LPF) filter you could implement (natural rolloff)?


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## sqshoestring (Jun 19, 2007)

BuickGN said:


> I enjoy these conversations but ever notice how it's usually the ib guys talking about qts?


Only because that is part of how you determine tuning for IB subs, instead of building an enclosure. QTS is QTC in case of IB, unless you get down near Vas with your trunk. Like I posted I lose 1dB at 20Hz and nothing at 30Hz, running 1.5X Vas with my IB 15s. So outside that, QTS is QTC.


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## sqshoestring (Jun 19, 2007)

cvjoint said:


> I agree with most of this. There are only two points I wold make:
> *Cheap driver at xmax and top dog driver at xmax have the same amount of distortion.* That's the point of xmax: 10% distortion, at least if they were rated properly. The better sub of course with have a lot more stroke and output at xmax but still 10% distortion.


What, is that legal to say in here....:laugh: I don't talk about cheap subs much because many don't want to hear this. I have read some subs are very linear up to rated xmax, so they should perform better. Xmax is mostly a factor of capability, providing it is built properly. Some cheap big xmax subs do not work that well.


cvjoint said:


> Subwoofers are not the least audible distortion wise. Tweeters past 10khz are documented to be inaudible in harmonics. I only care about frequency response and output past 10khz. With subs you still have to think about distortion, and imo they are just as important as midbasses and low midrange.


That might be true, but most tweeters are going to play under 10K so you still need a reasonably good tweeter...though again a DIY tweeter can sound very good. Maybe what I mean is if I had to save coin on my install, the subs would be the last thing I would spend on. I'd much rather have quality mid and highs before subs, when it comes to SQ.


cvjoint said:


> Of course QTC talk doesn't have to be about subs. If we were to talk midbasses how would the recommendations change?
> 
> In most of my testing I often find left midbasses have a dip around 100hz that is fairly wide. But, does anybody make a different box for left versus right midbass? I mean, if we truly should use the box to get a flat FR out of the box, doesn't that mean unequal boxes based on car acoustics?
> 
> Another thing to note is that I was never able to get a decent impedance curve testing IB subs yet. The QTC of IB subs is a mixed bag in applications as it can't be pinned down. There is too much leakage, I would think.


QTS is the driver IB. None of it is imperative anyway because a car install changes things so much, IMHO it just gives me the idea of what kind of driver it is and how it will react to tuning, what it needs to be tuned like X, etc. Same with midbass or subs, though tuning needs may be different.

Sure make different enclosures if you want, if you are not going to do it with electronics as most do. When I got into this hardly anyone had an EQ, we had to try subs IB to find what worked because they had no T/S specs published. If they didn't work right and you had no EQ, yeah they sounded ugly. It made you work on the install much more, while today you can slap a 31 band in and use any drivers you feel like. Again that is two ways to solve the problem and both can work. Most people don't have time to screw with swapping drivers/install all the time anyway.


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## sqshoestring (Jun 19, 2007)

There was a post here not that long ago about a KEF driver available someplace. It is very efficient, I modeled it the specs were posted. It performs but wow what a peaky thing, I'd hate to have to EQ that down in my car better to have it RTA'd with a big EQ. It was like a giant midbass driver in the model. Another pain is when a driver like that overruns your xover and you try to EQ it with a system EQ, you start cutting down the next driver. For example that KEF makes piles of 100Hz, you xover at 80Hz and EQ it down then you are cutting your midbass at 100 too. You may end up with the 100Hz still coming from your subs not the midbass/mids...unless you have enough slope in your xover to tame it. Either way its more work.


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## cvjoint (Mar 10, 2006)

bikinpunk said:


> is the impedance spike (where the Qtc is derived) outside the passband of the driver? how does the Qtc work to the HPF (or LPF) filter you could implement (natural rolloff)?


The spike is in the passband for subs, sometimes in the passband for midbasses, and almost never for low crossing mids. I never tested the QTC with a filter on, so I don't know.



sqshoestring said:


> What, is that legal to say in here....:laugh: I don't talk about cheap subs much because many don't want to hear this. I have read some subs are very linear up to rated xmax, so they should perform better. Xmax is mostly a factor of capability, providing it is built properly. Some cheap big xmax subs do not work that well.
> 
> That might be true, but most tweeters are going to play under 10K so you still need a reasonably good tweeter...though again a DIY tweeter can sound very good. Maybe what I mean is if I had to save coin on my install, the subs would be the last thing I would spend on. I'd much rather have quality mid and highs before subs, when it comes to SQ.
> 
> ...


Some high Xmax subs can have more than 10% distortion at Xmax for various reasons: the manufacturer doesn't rate xmax properly, straight up lie or they don't take into account LE and CMS nonlinearities. Second, while Klippel Wolfgang claims the parameter measure aligns well with the performance based method, with high excursion subs you can get unusually high motor noise and soft parts resonance that may not show up in a Xmax figure. You'll see a lot of HT dudes always going for high efficiency low excursion drivers for this reasons. Surface area never fails to please, big subs always steal the show. 

We are still at an infant stage with T/S and Xmax parameters. While a lot of this stuff is published it's not always accurate or paints the entire picture. We're not far from old school. 

Imo almost nothing costs a lot of money in this hobby if you know what you are doing. The only component that commands a lot of $ in this hobby is the headunit.


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## sqshoestring (Jun 19, 2007)

cvjoint said:


> Some high Xmax subs can have more than 10% distortion at Xmax for various reasons: the manufacturer doesn't rate xmax properly, straight up lie or they don't take into account LE and CMS nonlinearities. Second, while Klippel Wolfgang claims the parameter measure aligns well with the performance based method, with high excursion subs you can get unusually high motor noise and soft parts resonance that may not show up in a Xmax figure. You'll see a lot of HT dudes always going for high efficiency low excursion drivers for this reasons. Surface area never fails to please, big subs always steal the show.
> 
> We are still at an infant stage with T/S and Xmax parameters. While a lot of this stuff is published it's not always accurate or paints the entire picture. We're not far from old school.
> 
> Imo almost nothing costs a lot of money in this hobby if you know what you are doing. The only component that commands a lot of $ in this hobby is the headunit.


I have to say I lean heavily towards the big cone area/low xmax IB in cars. But you don't see anyone tossing my $40 pyles in do you lol. It avoids so many issues. I have no doubt they could do well in HT IB as well, given they are less capable meaning you don't feel the need to vibrate your home into pieces. Not as much with 15s, but high efficiency can peak the response up and make it harder to get low or harder to EQ just like a car.

I love to model drivers, but in the end its just a model. It takes more time to science out car audio then it is worth; you could have installed all the top contenders and have your system tuned by then.....if you are not a manufacturer or otherwise in the field. It is a fun hobby though and great to gain knowledge in the subject for a better understanding that can help steer you in the right direction.

Have to agree there too a HU and any processing need to be good, the rest you can do a lot of things that are low cost.


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