A question about 6th order bandpass design

[simon5]

I want to know why not much people here use the 6th order bandpass subwoofer?

I modeled for fun a big Tumult based 6th order bandpass subwoofer.

It's flat ± 0.5 dB from 9.5 Hz to 95 Hz at 121 dB SPL with a 700W signal on WinISD Pro. (Considering room gain being approximated by a parametric equalizer with a fc of 10 Hz and a Q of 0,22)

450 liters rear chamber tuned to 10 Hz with a 15,25 liters front chamber tuned to 58 Hz.

Something is wrong with this 6th order bandpass design?

Cone excursion is under control down to 8 Hz before going into overexcursion...

The only thing that is nasty I think. is the group delays start to go over 20 ms under 15 Hz to peak at a very ugly 109 ms at 9 Hz.

[Jim York]

It's seldom used because room lft supplies the necessary sensitivety below 30 Hz in the average room; flat to lower than that just isn't necessary, so few bother with the added size and complexity required.

[BassAwdyO]

Yes group delay is horid, the only way you can avoid the horrid group delay using a bandpass is tuning the smaller chamber to a frequency out of the bandwidth, because this resonance only effects a small segment of the frequency response. There are problems to this however. The hump above the crossover in sensitivity could cause out of spectrum distortion which could lead to an easily localized sub. The second problem is the fact that tuning out of the bandwidth will not effect the FR in the bandwidth significantly. I'll take my 6 cubic feet tuned to 10hz and forget about the 47ms delay @ 10hz, because I know that above 25hz (the audible frequencys) I'm less than 8ms and nearly identical to the group delay of a critically dampened sealed enclosure. A 6th order bandpass is merely two ported enclosures of different size/tuning with a single speaker mounted in both simultaniously such that the front of the cone faces in one box and the magnet faces into the other. The effects of the two boxes add to one another. The box tuned higher will decouple below it's tuning causing it to act as if it were not an enclosure at all. When this happens the response becomes that of the driver and rear chamber alone. Above it's tuning frequncy it acts as a filter with a rolloff of about 12db/octave. When frequencys significantly higher than the high F3 are played they are "trapped" inside the enclosure only to dissapear into acoustic losses in the enclosure. Playing out of spetcrum frequencys through a 6th order bandpass might not only be damaging to imaging, but also to the enclosure itself.

[simon5]

Thank you for the explanation.

I would also like to know if there's an advantage to do a 4th order bandpass design.

I tried this kind of enclosure in WinISD Pro and I don't see any advantages over a sealed enclosure of the same size.

[BassAwdyO]

there's no real advantage to either, except the boost you can achieve from the port gain of the higher tuned port. This is of course achieved at the cost of group delay. Another advatage might be the fact that the driver is not directly exposed to the acoustic environment. This protects the driver from sunlight, and other things that could cause damage to it(something a grille cloth or metal grille can do quite well anyhow). A number of people like the sound of certian enclosues regardless of the fact that they aren not very accurate in reproducing the original signal.

Bose has used quite a number of bandpass configurations in it's subwoofers. Most of the designs that I've seen used a 3 chamber bandpass system where two chambers were used as a 6th order bandpass where the drivers are mounted between them. The ports from those chambers both exit into the third chamber which is ported into the lisnening area. That would be a even higher order bandpass which would have even worse group delay, so I have no idea why they would use such a setup. You ought to drop them a note and see what they have to say about it. Maybe they know some magic advantage that the rest of us dont.

[BillFitzmaurice]

4th order has the advantage of allowing a smaller box, sometimes much smaller, depending on the driver specs. Its especially helpful with high Qts drivers in that regard. The lowpass function of the front chamber narrows the bandwidth and lowers system resonance, which results in either the same first octave response from a smaller box or a higher SPL from the same size box. Group delay would be problematic if you tried this in the directional frequencies, but since you aren't doing so with a sub anyway it matters little.