Qtc discussion!

[Electrodynamic]

Note, this is a discussion on Qtc, NOT QTS.

Qtc is often defined as:

Qtc - is the total resonance of the speaker system. An enclosure with a Qtc value of 0.707 will give you the flattest frequency response and the lowest possible F3 for a given driver, use this value if you want the your music to sound as close to the original recording as possible.

I used to live and die by that magical number. 0.707. 0.707! That was the end-all, be-all.

But then I learned more about Qtc's and how they really affect the response.

Come to find out, there isn't that much difference between a Qtc of .707 and a .95. I know that last sentance is grounds for permanent flaming and bashing and trashing and nit-picking. If you want to flame about that, go right ahead. But if you flame, you might want to take your head out of your ass for a split second and read what follows, you might have a different opinion about Qtc's by the time you're done reading this.

This web page describes how a lot of people view Qtc's.

However, if you look a little bit closer at the graph you'll see that things aren't exactly what they seem. Vance Dickason even said it in the coveted Loudspeaker Design Cookbook that anything above a 0.707 may be somewhat "un-natural" with a bass hump, etc, etc.

But look at the graph. Really look at the graph. Look at the scale of the graph. The scale is in 1 dB increments. The difference at 70 Hz between at .707 and a .90 is right at 1 dB. That's right, 1 dB.

You can reference the LDC and look through graphs of group delay until you're blue in the face, but anything under a Qtc of 1.0 isn't really detectable / audible. Overall you're still talking about less than 1 dB of variance.

Now, what about the F3 of the system? Isn't it higher with higher Qtc's? Yes. But how much higher? Take a look for yourself at the above web page and notice the 87 dB line and the corresponding Qtc's. There isn't much difference between the .707 and the .9.

Another thing to think about is not just Qtc, but Qts, Vas, and Fs. Just because you can reach a .707 in 0.1 cubes doesn't mean that the driver is going play lower than a driver that reaches a .707 in 0.8 cubes.

You may also be thinking, what if the 0.1 ft^3 Qtc driver has more Xmax over the 0.8 ft^3 Qtc driver? Xmax means piss if the driver has to work 2x or 3x as hard, draw 2x, 3x the power, etc, to produce the same output. You're now having to use more power to achieve the same output.

Well I think that's all for now. I'll write more later. Any thoughts? Flames?

[M-Special]

Ok. So since you haven't posted any T/S parameters for the new mag yet, What will the QTC of the mag in 1 cubic foot be? .707 or are you bracing us for the possibility of .9 or higher at 1 cube? Or don't you know yet, I'm not really sure when or how these things get measured....

Oh, also do you have any idea how accurate programs like WinISD, or Bassbox Pro are with regard to qtc? I've built a few boxes based on what those programs have told me with mixed results and I was just curious. I can assume they just do the formula for you and would be accurate in that respect. But the formula can't account for your car/trunk, although that wouldn't affect the response of the enclosure really would it... Too many variables for me to wrap my head around I think I confused myself.

[Electrodynamic]

Originally Posted by M-Special

Too many variables for me to wrap my head around I think I confused myself.

LOL!

Well, the new Mag and the new BM will have Qtc's higher than .707 in their sealed enclosure recommendations. They will be lower than 1.0, but higher than .707.

The point to that discussion / rant was to make people think outside the box for a little bit. It's all to easy to look at those graphs and go "man, a .707 is the best way to go no matter what", but when you step back and look at what's really going on you notice that a lot of the differences are within / less than 1 dB. And as much as we would all love to consider ourselves audiophiles, not very many of us (if any of us) can hear within 1 dB.

My first awakening to the .707 isn't king of everything was back in the day of US Ground Zero. I thought a Nuke 12 would work well in 0.3 cubes sealed because the .707 was less than 0.3 cubes. Come to find out it sounded like a tin can in anything other than a medium sized, mid-tuned, ported enclosure.

[Electrodynamic]

Oh crap, I almost forgot about your question: WinISD is a good program for approximations of F3's for a large living room / theater / anechoic chamber, port tuning, power handling, and monitoring cone excursion. You're correct though, Qtc is calculated automatically.

But your findings / results are not in-accurate at all. Those programs cannot account for room gain (let alone cabin gain) or any other factors involved with the subwoofer setup. That is one of the main reasons why WinISD completely sucks balls for SPL estimates in car audio. Hell, the spot that you place the driver in makes a difference and there's no way for a program like WinISD to take that into consideration, let alone other variables.

[pKelly]

Nice write up; it's always better to actually know why your subwoofer sounds good/bad instead of just assuming, as 95% of the audio world does.

I salute you, good sir. Back to EE homework :-/

[hobie1dog]

The ears are the best sq meter of all

[M-Special]

Originally Posted by hobie1dog

The ears are the best sq meter of all

I agree, however it can get time consuming, expensive, and dusty building different sized boxes trying to find the sound you're/I was looking for. Having guideline or a rough estimate of what to expect is very useful. I'm just saying that I've had mixed results using company recommended enclosures as well as what winisd has led me to build.

Mids/tweets are easy, seal door as best you can, aim, and good to go. Sub boxes I just can't seem to get what I want. Sealed yielded very little output and wanted more power than I had. Ported was loud but not as accurate as I would like. Maybe its the equipment/cars that I've used or I have unrealistic expectations....maybe both. This is why a speaker that is only recommended in a 1 cube box is appealing to me, I get trunk space and I don't have to think too hard about what kind of box(es) to build

[pKelly]

Originally Posted by M-Special

I agree, however it can get time consuming, expensive, and dusty building different sized boxes trying to find the sound you're/I was looking for. Having guideline or a rough estimate of what to expect is very useful. I'm just saying that I've had mixed results using company recommended enclosures as well as what winisd has led me to build.

Mids/tweets are easy, seal door as best you can, aim, and good to go. Sub boxes I just can't seem to get what I want. Sealed yielded very little output and wanted more power than I had. Ported was loud but not as accurate as I would like. Maybe its the equipment/cars that I've used or I have unrealistic expectations....maybe both. This is why a speaker that is only recommended in a 1 cube box is appealing to me, I get trunk space and I don't have to think to hard about what kind of box(es) to build

What I like to do is to model the system using WinISD and various tools like that, as well as applying the model to various self-imposed constraints (ie Qtc), then before building apply common sense and logic to the system as it applies to my situation. This way, you get what would be "perfect" in an ideal system, then you apply tweaks to make it conform to your environment (cabin size, sub placement in relation to you, etc.)

[bromo]

Great write up. It is much appreciated.

[sundownz]

Regardless of Qtc the new Mag sounds amazing in 1 cube I think that is the main point Nick is making, that the Qtc is not the end all specification.