Here is some good reading for you:

http://www.diymobileaudio.com/forum...dio.com/forum/technical-advanced-car-audio-discussion/117575-why-qtc-707-a.html

Enclosure Dilemma

 

A general Qtc can not really be applied to all subs as each will differ some in response. For instance, some of your shallow subs are designed to be in a fairly high Qtc enclosure (.9-1.2) while other conventional subs may be optimized in a Qtc of .6 to give a shallower roll-off. There's trade-offs of course, but that's where you as the user come in.

First it helps to be familiar with what your vehicle will do to the response of any sub placed in it which could make being concerned with actual Q of the box somewhat useless. Secondly, knowing ahead of what you desire from the sub may not necessary align with the manufacturer's recommendation, especially if you're trying to manipulate the response to tailor it with the vehicle. They simply give you a basis for a mix of power handling and extension. Best thing to do if you're not going to stick with the manufacturer's suggestion is to model it to see what changes will occur and what limitations or diminishing returns will be placed upon the design.

 

QTC is irrelevant in a car.
The cabin will change the response in a huge way.
Aiming for a specific QTC is a waste of space and time.

 

Cabin gain will effect the frequency response, yes, but it will only effect what you have to begin with. A QTC=0.5 for example will have a lower frequency extension than a QTC of 1.0, no matter what your cabin does.

QTC also effects phase, group delay, power handling, and excursion as well.

 

Not so much of aiming for a specific qtc per say, just inquiring about what is considered more desirable in a sense. But thanks for the input thus far.

 

Basically all of the number specs of a sub matters, not just Q values. Once you make a "perfect" enclosure, car cabin's internal volume will greatly affect how the sub perform. A car boot with 400l of space won't sound the same as a SUV with open internal volume of ~800l.

 

The only thing one should worry about when building a sealed enclosure is not creating a knee in the response unless it is desired which I haven't seen since the early 90's. Other than that, if you're worried about trying to gain gobs more extension over a manufacturer's recommendation, chances are you may need to choose a different driver to begin with.

 

Q is essentially a measure of the amount of overshoot allowed at resonance. A Qtc of .707 is a compromise between flat response and low frequency extension. A Qtc of .5 is optimum transient response. A Qtc of greater than .707 provides a small boost above resonance at the expense of transient accuracy.

The fact that the car changes the frequency response doesn't affect this relationship.

 

Thank you for your input Andy....I know you are quite knowledgeable and I have a better grasp of what I was trying to think out.