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Old 19th March 2005, 07:45 PM   #1
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Default 4th order bandpass troubles

i built a 4th order bandpass subwoofer with a 10 inch driver from parts express. i also put in a 2nd order passive filter. my problem is the bass seem to stay inside the box. do you have any tips for me to correct this
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Old 19th March 2005, 11:15 PM   #2
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Default Re: 4th order bandpass troubles

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Originally posted by halfdome
i built a 4th order bandpass subwoofer with a 10 inch driver from parts express. i also put in a 2nd order passive filter. my problem is the bass seem to stay inside the box. do you have any tips for me to correct this
It might be helpful if you took us through your design process - how did you decide on volumes, port length, etc?. Also post the driver specs and maybe even a drawing or picture of the box.
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Old 21st March 2005, 08:23 PM   #3
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Here's the spec on the driver:
Specifications: *Power Handling: 80 watts RMS/120 watts max *VCdia: 1.5" *Le: 1.2 mH *Impedance: 8 ohms per coil *Re: 3.10 ohms *Frequency range: 30-1,500 Hz *Fs: 25 Hz *SPL: 92.7 dB 2.83V/1m *Vas: 3.90 cu. ft. *Qms: 3.20 *Qes: .43 *Qts: .38 *Xmax: 6 mm *Dimensions: Overall Diameter: 10-5/8", Cutout Diameter: 9", Mounting Depth: 4-3/4"

I used the program offered for free by. www.ajdesigner.com

The port I used was for a 4" diameter and converted the dimensions to use a rectangular port.
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Old 22nd March 2005, 04:35 PM   #4
Ron E is offline Ron E  United States
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How big are your sealed and vented chambers, what are your port dimensions?.....
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Old 22nd March 2005, 04:37 PM   #5
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Why the additional passive filter? You are aware of the huge problems with trying to use passive filters in that kind of bandwidth?
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Old 22nd March 2005, 10:01 PM   #6
Ron E is offline Ron E  United States
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Other things that would be helpful to know are whether this is a car sub or a home sub, where it is placed, details on your filter.
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Old 23rd March 2005, 05:15 AM   #7
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This is a home subwoofer. I do not know the issues involved with pasive filters. The sealed chamber volume is: 43.5 liters and 31.256 liters for the ported chamber. The port size is 10 mm with a volume of 2.25 liters. I used a 15mH inductor and 100Uf capacitor for the filter. I am new to this so any assiatance is appreciated. Thanks
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Old 23rd March 2005, 06:37 AM   #8
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Well something seems wrong with your filter if you ask me..... Since the sub is 8ohm DVC I would assume you wired in parallel for 4 ohm operation. For a 2nd order low pass filter at 60hz into 4ohms one needs a 1000uf capacitor wired in parallel with the driver, and a 15mH inductor wired in series with the driver and capacitor. You said you used a 100uF..... I think that would be first order up to somewhere around 500hz and then it would begin 2nd order filtering. If this is the only crossover you are using to filter out high frequencys then this is likely why it sounds so bad!

In a bandpass enclosure, above the bandpass the system acts as two sealed boxes(the ports cease to exist functionally) and all the sound created by the driver is absorbed/transmitted through the cabinet walls. Once you get down to the tuning of the front chamber the port loads the driver for additional efficency. Below the tuning of the front chamber, the front chamber doesnt exist(functionally) and the system acts as if it were plainly a sealed box(for 4th order bandpass).

Because your filter is probably inadequate it is letting high levels of out of bandwidth signal go to the sub. The sub is playing those high frequencys, the box is trying to damp them, and probably most of what you're hearing is this
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Old 23rd March 2005, 08:51 AM   #9
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The filter may not necessariliy be wrong in the way you describe there. There are many ways to build a 2nd-order filter because you can change the Q.

There are two issues with the filter. The first is that passive filters need a stable, constant impedance to work into. The impedance of pretty much any driver/box combination in the bass range varies wildly. Secondly the filter is not even required because the box itself performs the filtering function on the mid and high (in principle).

I think you may also be misleading with your comments on the operation of the box, particularly the bit about all the sound being absorbed or transmitted through the cabinet walls. A bandpass box cuts off the sound above the tuning frequency because the vent appears to be a very low acoustic impedance. Through the tuning frequency sound is let out OK and the cone is supported by the vent giving an efficiency gain, then as you say below the tuning frequency the vent is totally open but the characteristics of the sealed box become prevalent.
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Old 23rd March 2005, 05:25 PM   #10
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I presume you have tuned the box to 47Hz or so? Your port dimensions make no sense, you need to supply height and width of the port slot and the length of the port.

Assuming you have it tuned correctly, you should have a fine response from ~30 to ~80Hz. Try running it without the filter and see if it gets a lot louder - if so, you may have wired the filter incorrectly.

Note that the DCR of a 15mH inductor may have a substantial effect on Qts of thr driver, increasing Qts and requiring a larger box. Do you know what teh DC resistance of the inductor is?
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