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Old 3rd August 2004, 11:44 PM   #1
beady is offline beady  United States
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Default Help on Dual Chamber Reflex box & car audio response Q's please (LONG)

Sorry, perhaps I should have posted this topic in this forum instead. Don't want to double post, so please take a look in the loudspeaker forum for my original post, linked below.

Help on Dual Chamber Reflex box & car audio response Q's please (LONG)

Thanks,

Mike
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Old 4th August 2004, 11:02 AM   #2
Did it Himself
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As a rough guide, cabin gain begins at the frequency where the wavelength is equal to the longest dimension of the cabin, and rises with an approx 12db/octave curve.

F = 344 / length

Thus if you had a sealed box (12db/oct roll-off) such that it was 3dB down at the cabin gain frequency, it would match up to the cabin gain to provide a fairly flat response.

A vented box has a sharper roll-off so wouldn't match up as well. Generally, a vented box has worse transient response that an sealed box and, a double-chamber type is even worse I believe, so if that's a key requirement then maybe a vented box isn't best for you. Also, I think you would also be struggling to fit the box inside the vehicle without taking up too much room or tuning it low enough/accomodating the vent dimensions.

If I was in your position I'd be looking to high-pass filter the bass-mids at around 60Hz to take some of the strain off them. If you try to run your sub up higher than 80Hz to make up for lack of bass from your bass-mids, it will muddy the sound, IMO. Maybe a HPF as suggested on the bass-mids and a little careful bass boost on them will help them out.
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Old 4th August 2004, 07:37 PM   #3
beady is offline beady  United States
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Thanks for your reply. When you say the longest dimension in the cabin, I assume you mean from the lower outer corner of the back hatch to the lower front corner of the windshield, correct? My Z is about 9.5 ft in that dimension.

The dual chamber ported box I am referring to is described here:

http://www.diysubwoofers.org/prt/dual_chamber.htm

From discussions Iíve read on the box on this and other sites, it seems that it is tuned to two frequencies, the lower overall volume frequency, and again at half that frequency. So, I figured I could tune the overall box to 35-40 Hz and still have it efficient at 70-80 Hz, plus since the speaker faces outward, could still play up a bit higher. Since cone movement is limited by the design I thought there would be less distortion than a ďregularĒ ported enclosure? Since the cone movement is less, I was thinking that the transient response should be faster than a regular ported box? Please correct me if Iím wrong though.

Could you post a gain response plot with a lot of low end extension that is well suited for an automotive interior environment, or point me to where I could find one? Not asking for something to fit my car exactly, just looking for ballpark curves so I know what Iím shooting for.

I used to have the 6.5's crossed over lower, but they wanted to distort at higher volume levels. I *think* I have them set at 100 or higher right now.

Thanks,

Mike
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Old 5th August 2004, 07:41 AM   #4
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Your dimension seems OK. It's not a clear-cut rule anyway, more a rule of thumb/starting point. BTW the formula I gave was for metric units. The 344 is the speed of sound in metres/sec.

Your interpretation of the double-chamber tuning and distortion performance is correct. I think the transient response is worse than a normal vented box because there are two resonant frequencies rather than one. I could be wrong though.

With the double-chamber box, there is also a very small dip in output at the higher tuning frequency, which may take a little bit of punch out of the sound, as around 80Hz is where punch sound comes from.

For matching the cabin gain, basically you use a sealed box and design it such that your box F3 is at the cabin gain start frequency. You end up with a roughly flat response. If you were using a vented box you would need to apply a boost of 12dB/octave, which would soon run into problems as you go below Fb.
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Old 5th August 2004, 12:41 PM   #5
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Quote:
Originally posted by richie00boy
Your interpretation of the double-chamber tuning and distortion performance is correct. I think the transient response is worse than a normal vented box because there are two resonant frequencies rather than one. I could be wrong though.
I don't know for sure, but a friend of mine who has built quite a few and even managed to sucessfully write a program in Mathematica to calculate all the parameters, tells me that the transient response of a dual chamber box is identical to a sealed box fed via a low-pass filter that has the same HF rollof as the dual chamber box. Certainly a tone burst has a different spectra than a continuous tone.
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Old 5th August 2004, 01:04 PM   #6
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Circlotron, I think you are talking about a bandpass box, whereas we are talking about a 'Weems' double-chamber vented box as in the link posted by beady. The 'Weems' box does not have an upper roll-off.

If you are talking about bandpass boxes, then I agree with what you say re the parallel with sealed+LPF.
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Old 5th August 2004, 06:12 PM   #7
beady is offline beady  United States
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Can someone explain to me how to model the dual chamber ported (Weems) enclosure usinf WinISD please? I can model the overall ported enclosure just fine, but don't know how to model the second, higher tuned frequency also so the graph has two dips.

Thanks,

Mike
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Old 6th August 2004, 11:57 AM   #8
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Quote:
Originally posted by richie00boy
Circlotron, I think you are talking about a bandpass box, whereas we are talking about a 'Weems' double-chamber vented box as in the link posted by beady.
Whoops! So I was.
Oh well, a Weems double-chamber vented box on each side of a bandpass box (i.e. 4 chambers total) would be interesting.
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