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Old 12th April 2005, 05:29 AM   #1
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Location: Delaware
Default How to choose Qtc for new car sub enclosure?

I'm looking for some help on choosing the "best" Qtc for my new subwoofer enclosure in my pickup truck.

I built my existing enclosure about 12 years ago (back in my car audio competition days ) but recently the foam woofer surrounds deteriorated. Upon removal of the box (very large--dual 6 cu ft sealed boxes), I discovered a water leak in the cab and realized that the box had been ruined.

So, I've decided to start again and build a similar box using new drivers, and this is my chance to do it better than before. I'm interested in sound quality and accuracy since I spend about 2 hours per day commuting. I'm not planning to enter any SPL contests, but if it can also go loud that's OK!

I already purchased the new drivers before I realized I was going to build a new box. Two 15" MB Quart Premium PWE354--here are the T/S parameters:

Dual VC 4-ohm
Qms = 6.24
Qes = 0.541
Qts = 0.498
Fs = 21 Hz
Vas = 157 L (5.54 cu ft)

WinISD calculates a sealed enclosure of 5.5 cu ft for Qtc = 0.707 and 3dB cutoff at 30Hz. I managed to find the parameters for my old 15" Pyle drivers and plotted them for my existing box and came up with Qtc = 0.85.

My existing setup is slightly boomy at higher bass frequencies (guessing 60Hz and up), and I'm assuming it's because of the high Qtc. But I also had no problem shaking the mirrors with a 20Hz test tone. Overall I was very happy with the sound.

So for the new enclosure, I'm having trouble deciding on what Qtc I should be targeting--initially I was thinking 0.7.

I've read many posts here about car and home subs, and it seems like it's very important to take the room (interior) into consideration. And from that, it seems like I should build for a Qtc lower than 0.7 to offset the high Q of the "room".

I don't want the box to be much bigger since I'm already at 11 cu ft for Qtc = 0.7. I started to consider isobaric configuration but I like the simplicity of dual sealed enclosures and the higher efficiency.

I'm looking for recommendations or suggestions. I have the lumber and I'm ready to build as soon as I can answer this question.

Thanks very much!

Bryan
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Old 12th April 2005, 01:10 PM   #2
simon5 is offline simon5  Canada
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The best thing to do would be to measure one of your sealed subwoofers outdoors to check the frequency response curve. Then you place it in your car and you'll be able to know how much gain you have at each frequency in your car. Then, you know which Q you should go for.

If you think it's boomy at 60 Hz and up, then yes Qtc 0.707 will be way better than Qtc 0.85.

Remember also that your new subwoofer probably have a lower Fs than your old Pyle. You must be careful because maybe the frequencies under 30 Hz will be too much amplified now.

If you have the parameters of your old Pyle, you can check in WinISD to compare with your new subwoofer, to check how much more output you'll have under 30 Hz.

Good luck!
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Old 12th April 2005, 01:41 PM   #3
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Generally inline with what Simon said. I think the cause of your boominess was not the Qtc, which at 0.85 is not that bad at all, but caused by the cabin gain coming into play well before your drivers started rolling off. You are going to have this same issue with your new drivers I think. What was the F3 of your old box?

At what frequency are you rolling off at the top end? This can also cause boominess if not chosen correctly with regard to frequency and slope.

In short, I think you need not concern yourself with Q (at least not if it's in the 0.7-0.9 range), but more with making F3 match up with when cabin gain kicks in.
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Old 12th April 2005, 02:36 PM   #4
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I've attached the WinISD graph. The F3 is 30Hz for the new enclosure and driver, and was around 41Hz with the old driver. Also the old driver has a bit of a peak around 80 or 90Hz (higher Qtc causing different curve).

The new driver definitely has a much better looking response to me and that's at slightly less volume than the original box.

Would damping material (i.e. Dynamat) in the cabin help to reduce the "cabin gain"? I'm already planning to install some, to reduce some rattles and road noise.

Bryan
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Old 12th April 2005, 02:50 PM   #5
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That amount of peaking is fairly benign. The issue as I see it is that your new speaker has much lower F3. Neither Dynamat nor box stuffing will help because cabin/room gain is when the driving mode of the listening space changes from wavelength reproduction to pressure mode. However the Dynamat will help reduce vehicle resonances and will help to tighten the sound up, but it can only reduce things back to what they should be -- which will still be a peaky response where the box output is still flat and the cabin gain is coming in.

Cabin gain frequency is roughly 344 / (longest dimension in metres * 2) and rises with an approximate 12dB/oct slope in a vehicle.
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