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Old 26th June 2008, 08:19 PM   #1
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Default Bandpass not so bad?

So, I noticed with some interest that Audio Intelligence sells bandpass subwoofers.

I have a great deal of respect for Dr. Geddes based on his writings and participation in various online communities. On the other hand, my limited experience with bandpass subwoofers has been that they are by and large inferior to vented enclosures.

So I'm wondering what design principles can be used for high quality bandpass enclosures, and what experience people here have with them. I'd prefer to keep this to objective metrics where possible, since I'm trying to identify to what degree my own subjective impressions are misguided.
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Old 26th June 2008, 08:51 PM   #2
awedio is offline awedio  United States
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I have no "objective metrics" for you, but you would be wrong to think that either your subjective impressions are misguided or that a respected audio designer bringing a certain topology to market means it is suddenly better than you thought it was. Or really is.

I suspect that Dr. Geddes is making bandpass subwoofers due to their improved sensitivity, and that's about it. If crossed over low enough, the performance may be satisfactory, but it is my belief that BP subs lack transient response to a degree that makes them plainly unsuitable for music reproduction. Even in a car.
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Old 26th June 2008, 09:58 PM   #3
GM is online now GM  United States
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Compression horns are BP4s with one heck of a big vent for bass frequencies, which last time I checked was the goal for a truly serious 'you are there' rig, so seems like a properly designed BP4 sans big vent would be the average size room's version, though to go low it will still be big by most folk's standards. IIRC EG mentioned his were around 9 ft^3.

That said, proper TLs, TQWTs have the best impulse response after the theoretically 'perfect' BLH. Bottom line is that we don't hear well down low, so it's the harmonics we have to get right all the way up to perceive a heart attack 'fast'/'tight' transient since the lower the fundamental, the wider its BW.

As always though, YMMV.

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Old 26th June 2008, 10:43 PM   #4
Ron E is offline Ron E  United States
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There is nothing about a properly designed bandpass that is inferior.

You have a lot of tradeoff options, you can design for bandwidth, efficiency and response flatness. The best designs have no ripple and rather large enclosures. In BP4 systems, they can have the same box size and F3 as a vented box using that particular woofer, but the rolloff is 12dB/octave instead of 24. As a tradeoff, the power handling near F3 is a bit less for a bandpass than a vented box because they are tuned higher. On the other hand 4th order bandpass are somewhat protected from overexcursion at very low frequencies, where vented are not. Transient response can be better than a vented box.

6th order (and higher) bandpass have pluses and minuses, but the extra amount of performance you get does not merit the complexity, IMO. They overload easily with frequencies below the low tuning.

Another issue is people think a bandpass needs no crossover, well, it needs a carefully designed notch filter(s) at the very least, and preferably a crossover.
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Old 27th June 2008, 02:43 PM   #5
GM is online now GM  United States
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Why the notch filter(s)? I never felt the need and while out of BW vent pipe harmonics can warrant a XO, damping it seemed to work fine, though I never had the kind of accurate measuring gear that folks take for granted these days, so it wouldn't surprise me if there's some subtle gains to be had.

TIA,

GM
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Old 27th June 2008, 09:24 PM   #6
ScottG is offline ScottG  United States
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Quote:
Originally posted by Ron E
There is nothing about a properly designed bandpass that is inferior.
Thats not strictly true, group delay is worse than a sealed system and at best near resonance the same as a "normal" vented system (same driver same tuning freq. comparable volume enclosure).

Still, like you say it is a series of trade-offs.

Note to others: group delay is more audible as you near the source, so it is WORSE in car audio than in home audio. Its also LESS audible as freq.s decrease.

..and one other thing:

The linked page (prompting this thread) is listing *PRO* "subwoofers", note the low freq. response. ONLY if the driver's qts is very low (and it has a large enclosure volume), will it have an extended low freq. response similar to a sealed system. Practically speaking this means that while the PRO designs seen here have high eff. and high power handling, it will also have a rapidly decreasing low freq. response. This means its likely to have little or no infrasonic freq.s anywhere near the systems operating average. Strictly speaking in terms of Home Audio - this is a bass unit, NOT a subwoofer.
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Old 27th June 2008, 09:44 PM   #7
Ron E is offline Ron E  United States
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Quote:
Originally posted by ScottG

Thats not strictly true, group delay is worse than a sealed system and at best near resonance the same as a "normal" vented system (same driver same tuning freq. comparable volume enclosure).
Group delay of a bandpass is difficult to interpret in terms of usual highpass alignments. You will find that if a typical highpass (vented, sealed) is lowpassed at the same frequency as the bandpass woofer , it will have nearly equivalent step response and delay. In the case of a 4th order bandpass, that means sealed box equivalent.
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Old 27th June 2008, 10:23 PM   #8
Eva is offline Eva  Spain
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The actual problem with bandpass enclosures for bass is that they exhibit a low-pass effect with its characteristic phase shift, unlike other systems, and this makes crossing them over with another system much more tricky. Some form of phase correction is always required. A generic crossover with symmetric slopes does not work, it always results in cancellation. Group delay by itself is neither audible nor a problem, as long as it is matched between both systems across the crossover region.

Even 45 degrees of phase mismatch between the output of the bandpass box and the main system already translates into 3dB of efficiency loss in the crossover region, which is half the power wasted. A 90 degree errror translates into -6dB and 75% of power wasted. The crossover region is nearly 2 octaves wide for LR24 filters, and much wider for lower slope crossovers.

Then again, does all those people blaming bandpass enclosures or any other particular system have a clue about crossovers and phase alignment? They don't for sure.
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Old 30th June 2008, 09:25 AM   #9
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There was an interesting article in Australian Hifi a number of years ago about a local company called Whise. It raved about their reinvention of how to model speakers, and their large high output low distortion subwoofers, apparently very musical. It was claimed they weren't quite like any existing alignment.

Recently I had a chance to talk with Colin Whatmough. He used to sell Whise subwoofers. Apparently they are bandpass. He now has a bandpass design of his own. One of the benefits is that distortion products are filtered.

The sub of his that I heard was a 10" driver with 18mm xmax and a 200w amp. Although the concrete floor and masonry walls would have had an impact, that little subwoofer packed a punch. It was also very musical and at around $2k it seems very good value.

It challenged my ideas about bandpass designs.
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Old 30th June 2008, 10:47 AM   #10
Ron E is offline Ron E  United States
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IMO and IIRC, Whise's design was nothing other than an attempt to circumvent Bose's patent on high order bandpass designs. Nothing magic there.
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