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Old 27th December 2009, 03:45 PM   #1
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Default Help designing BP4 subwoofer for car use

hi everyone,
I have a Domotec 10'' MS-810 sub and a vw beetle. I want to design a sub for the beetle, with the mentioned driver.

T/S specs for the domotec (published not measured) are as follows:

Fs: 35,35 Hz
Revc: 3,80 Ohm
Zmax: 31,66 Ohm
Sd: 0,028 m2
BL: 7,2 Tm
Qms: 4,19
Qes: 1,22
Qts: 0,94
Vas: 56,97 l
Xmax: 6,86 mm

I've entered those parameters in BassBoxPro 6 and modeled up a BP4 box.

-First of all: i barely understand what DF and Q't in the box properties sheet means, and don't know what values to put in there.

-Secondly: Im unsure about car response. I've read somewhere that car gives a 12db per octave gain going down fro 80Hz.

I'm planning to put the sub in the free space behind the backseat of my bug...

so, any help on those numbers?

box dimensions where not the IDEAL suggested by software.

thanks in advance

gaston_pdu
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File Type: jpg bp4_domotecMS810.jpg (193.4 KB, 103 views)
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Old 27th December 2009, 05:12 PM   #2
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Default Not so much help I'm afraid...

The simulated response looks absolutely horrible with cabin gain. I approximated the cabin gain from an internal cabin length of 1.9 meters. I could have done better, but my own beetle (-68 1500) is buried in a meter and a half of snow, so i cant measure it at the moment.

I think you would be best off with as big of an closed box as you can live with, and stuff it full of that white fluffy stuff that people put inside of pillows. Such a box still needs a bit of boost in the bottom, but most car amps have that option anyway.

Your bandpass:
Click the image to open in full size.

Sealed 60 liters:
Click the image to open in full size.
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Old 27th December 2009, 06:36 PM   #3
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both sealed and bp4 has a huge and ugly peak at 50Hz, maybe i should flat it down with eq. I prefer a bp4 over a sealed for excursion control, and the extra output, so maybe moving the passband up can solve the peak.. or not?

thanks
gaston_pdu

PD: my bug is brazilian, 1980 1600 two carbs..
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File Type: jpg amp_bass_boost_eq.jpg (113.9 KB, 14 views)

Last edited by gaston_pdu; 27th December 2009 at 06:41 PM. Reason: adding image
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Old 27th December 2009, 07:56 PM   #4
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Well, just for clearification, here is the sealed box together with the bandpass. I don't know what kind of power these subs can take, so my program just assumes 100w each, but that is not an issue below about 100Hz. The driver runs out of xmax before that:
Click the image to open in full size.

This really messy graph shows what happens if you raise the port resonance frequency:
Click the image to open in full size.

Oh, and i included a graph of a slightly larger box with two ports that goes all the way down, but still has that huge peak. A driver with a lower Qt is really needed for a good bandpass...
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Old 31st December 2009, 03:18 AM   #5
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Hi gaston_pdu

My understanding for car sub is the box is tuned for high Q. Cabin gain will raise the lower frequencies.

Graph from JBL GT5-12 (12" car sub), shows Q and 12dB cabin gain for sealed box. From your TS parameters, you can't do a bandpass. Your Domotec MS-810 is for a sealed box. Any volume from 25L to 50L will work. Hope it helps.
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File Type: jpg CAR_Q.jpg (33.6 KB, 20 views)
File Type: jpg DOMOTEC_SEALED.jpg (70.4 KB, 23 views)
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Old 2nd January 2010, 09:20 PM   #6
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can you explain why a bandpass design won't work? or it will work but the response will be too ugly?

thanks
gaston
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Old 2nd January 2010, 09:34 PM   #7
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Quote:
Originally Posted by gaston_pdu View Post
can you explain why a bandpass design won't work? or it will work but the response will be too ugly?

thanks
gaston
Front loaded horns and single-reflex bandpass boxes are narrow bandwidth alignments. By juggling the variables, you can trade bandwidth for efficiency.

The problem that you're going to run into is that the high QTS value of your subwoofer will make it very "peaky" in one of these alignments.

It's not impossible to pull off, but you'd need a woofer tester and a microphone to flatten out the response.

If you're willing to invest the time, you'll be rewarded with an efficient box with low distortion.

If you don't have the tools to measure and tweak things, you'll likely end up with a box that's a one note wonder.

Limited bandwidth alignments like these can sound very clean if built properly, but they're very easy to screw up.

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Old 2nd January 2010, 09:43 PM   #8
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For a Bandpass, 150L box. Not very pretty. More important is Bandpass designs demand very accurate Thiele Small Parameters. Unless you can measure your sub, I would advise against it. I measure all my woofers and I can say less than 1% actually is the same as the manufacturers' specs. With a Sealed Box, it is much less of a problem.
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