I'm in the process of building a sealed sub enclosure, with my calculations I will come out with a Qtc of 0.764 and a F3 of 48Hz. Enen though a Qtc of 0.707 will yeild the flattest curve and the lowset F3, I've heard that most people prefer the sound of a Qtc=.9 to 1.1. I am looking for SQ 1st and SPL 2nd. Newayz, How do you guys prefer your Qtc, what value is ideal for music in an automotive environment?

 

you should not target a QTC, but a -3 db point. in a sealed enclosure, when you keep the QTC higher than 0.5 and lower than 1, you're moste likely to get an in-car flat response from 20hz (or 10 or 15hz , consider from as low as you can hear) up to the -3 db point. this is the range from what you would use your subwoofer in a SQ setup. because of the car cabin gain, as long as your subwoofer response curve follow a slope around 12 db/octave, it will have a more or less flat response in your car. then it will roll-off naturaly, and then even more quicly because of the low-pass filter (crossover) you apply. You probably understand that a Qtc of 0.707 is ideal in a home or large room because it have no peak of response before rolling off. In a car, since the roll-off slope become the part of the response that is used to have a fidel reproduction of bass, a QTC of 0.707 will turn out to have a smoother response, but if the F3 is too low, it will over-emphasis on the extremely low frequency, and be somewhat not punchy in the critical "bass drum kick" frequency. So a Qtc alone don't tell much, and a F3 point alone either, you need to watch them both. when you have a higher QTC, you can have a lower F3 and still have a pretty large band of flat in-car response. when you have a low QTC, you need a higher F3 to achieve the same bandwith, but the bass will be overally smoother. so for SQ, I would prefer a lower Qtc with higher F3, but something totally different in term of qtc and F3 could sound similar if both are crossed over low enough to eleminate high Q resonance.

considering your situation, If I plot something similar to what you tell (QTC of 0,764, F3 arounf 48), a JL 10W6v2 in a 0.75 cu.ft enclosure is what is the nearest to that. It give a -3 db point at 44.5hz. IMO, this is somewhat low, considering the Qtc. For this particular woofer, I would probably try at first a 0.55 cuft box, would turn out to have a Qtc of 0.833 and a F3 of 52,5 hz. It is slihtly less efficient at 20 hz, but should be quicker in the upper range, and turn into a tigher bass. it also have an higher mechanical power handling (500 vs 350Wrms)

if you use a woofer totaly different than I named, well, be happy, it will sound like a JL audio

 

Torkguy thanks for your reply.

The Qtc and F3 were numbers that I ended up with for a Phoenix Gold 12" sub in what they call their "reccomended" enclosure. If I decide to go with this sub, then I'll just change the Vb for a higher Qtc. The only reason I tried for the "optimum" enclosure size was that I wanted that extreme low end extension that I could get from a tuned ported enclosure. I also have a JL Audio sub laying around that I may plot some numbers for. You mentiond that an F3 can be too low, I always thaught the lower the better.

 

If you shoot for High Fidelity, the lower the F3 is not the better in a car because of the cabin gain. in a home or a large room, it would. Even if you go ported, try to find an enclosure that will give you a "diagonal" response as much as possible, big bass, but still keeping thing linear. I've been told by SQ guys that the bass should be as flat as possible from 20 to 60 hz, then could drop to reach the midrange level around 250hz. that is about what an SQ sub in a sealed enclosure will give (idmax 12, for example)

 

when building a box, don't look at qtc. it's almost irrelevent because the graph of the qtc is basically anehoic. Put the box in the car and your frequency response will be much different. As you know, each car has its own resonant frequency so what you try to aim will be a bit skewed.

I'm not sure how people are able to say they like 0.9qtc better than 1.1. 0.9 and 1.1 has no definite sound. then again, i don't even pay attention to qtc. lol.

 

the Qtc alone won't say much, but the higher the Qtc, the higher the tendencie that the sound get out of control at resonant frequencie. This is because of the quick impedance rise at this particular frequency, and how the amplifier will "react" to it. You can't say for fact that the same woofer will sound better in an enclosure that result in a Qtc or 0.9 than if it is in a Qtc of 1.1 the fact is that a woofer who end with a lower Qtc while having the same response in the lower 1½ octave is more likely to sound nice and smooth than the one with an higher Qtc, because the higher Qtc (like around 1) tend to show a peak in the 50-60hz range, addionnated to the car resonant frequency that is in the same range,turn out into the typical "boominess" of car subwoofer.

 

of course it's going to be smoother in the lower QTC range. but that's only because it's impossible to physically keep the size of the box while changing the qtc. lower qtc will require a larger box, and we know larger box tends to have smoother FR. We can't say one is better than the other simply by a magical Qtc value.


which leads to the main point of the question:

For me, i build my boxes accordingly to what i hear is best for me. I own many subs so each sub box that comes out perfectly for me has different qtc value.


none. There are no exact value that is ideal. They all depends on various things. If there were to be one exact value, then all the competitors will be using the same qtc. People are emphasizing too much one computer calculations and graph. More than likely, they are assuming the environment is perfect, which we know is not.

Again, this is not to put down anyone else's view on qtc, but it takes trials, trials, and trials to get what you want, not what the computer thinks.