Bandpass Subwoofers

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I am writing a research paper about the advantages, disadvantages, and design variations of the Bandpass subwoofer for second-semester Junior English. Why do I never hear about Bandpass subs in high-end audio (excepting the KEFs that first brought the design to commercial attention)? In my research paper, I describe using the PB subs with a crossover, although I make a mention of the huge disadvantages associated with crossoverless BP subs. This paper assumes that the Bandpass subwoofers are designed, and warns against the so-called "universal bandpass" for car audio.

What I have so far (feel free to report any mistakes):
-The air in the ports should be a much nicer medium to radiate pressure waves into the outside air than a bending, distorting woofer cone.

-In properly-flared ports, a huge amount of port excusion can be like a long-excursion woofer.

-Many BP subs have louder output than a direct-radiating sub with the same driver.

-The only real problem I can see is a response that is not flat.

-I have read that air is linear, so in my mind, using air in a port to pass sound into the outside world should be much freer from distortion than a driver with a non-linear suspension.

-4th-order Bandpass subwoofers have nice, predictable cone motion and pretty good transient decay.

-6th-order bandpass subwoofers have huge output and poorer transient decay.

-8th-order Bandpass subwoofers, aside from being impractical (and just being for shaz and giggles), have extremely low distortion, but terrible transient decay.

Any help you can provide to enrich my paper would be much appreciated, and I plan to put it on my website which is in the works and is intended to be for the benefit of fellow DIY-ers.

Thanks,

BAM
 
diyAudio Moderator Emeritus
Joined 2001
BAM:

Your quote: "The only real problem I can see is a response that is not flat."

Haven't built one. But I have read several papers about them, including the original paper by Laurie, (man's name in England), Fincham. He's the "F" in KEF, and he introduced the bandpass box to HIFi.

Response can be flat. Here is a response chart for the Rockford Fosgate RFP 3408 for two configurations of 4th order bandpass.


Red is: Front chamber, .2 cubic ft, vented to 61 Hz. Rear chamber, .4 cubic ft, sealed.

Green is: Front chamber, .28 cubic ft, vented to 61 Hz. Rear chamber is .4 cubic ft, sealed.

As you can see, both give flat response. The more sensitive box has a bandwidth that is one octave between 3 dB down points, the less sensitive box has a bandwidth of 1.5 octaves.

Try this yourself on WinISD. So the response can be flat, but yes, the box can be more sensitive with a bandpass over a vented or closed box system.
 

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diyAudio Moderator Emeritus
Joined 2001
The Audio Engineering Society has 2 "Preprints" that are useful. One is the original article by Laurie Fincham. It is preprint # 1512.

The other is by Earl Geddes, of the audio division of Ford Motor Company. It is preprint # 2383.

I don't know if the articles are downloadable or if you have to wait for them in the mail. They are 5 bucks each from the Journal of the Audio Engineering Society. Here is the page:
http://www.aes.org/publications/preprints/search.html

Here is the contact page of the Journal of the Audio Engineering Society if you want to see if these articles are downloadable:
http://www.aes.org/contact/hq.cfm

If you have nay more questions, let me know. I have both articles, but alas, no scanner, (when am I ever going to break down and buy one?) A couple of pages are missing, but I think I know the articles pretty well.
 
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Joined 2001
"What is the biggest turnoff to building a Bandpass subwoofer?"

Why SHOULD a bandpass subwoofer be desirable?

When considering possible designs for production, costs and market share must be considered, as in any product to be marketed.

-A bandpass cabinet requires signifanctly more complex construction, which requires more mdf, better and larger ports, and more veneer, etc. than a comparable sealed, pr or ported variant. Read: SIGNIFICANT cost increase!

-Larger size is frowned upon. Size of the subwoofer is a primary concern for many consumers when shopping for a subwoofer.

-No superiority as compared to sealed OR ported designs, excpet when a significant amount of gain is designed in, to raise efficiency significantly. However, this narrows the already limited bandwidth and creates steeper cutoff rates, which in turn causes poor transient response, and peaking. Read: One-Noted bass!

Now, of course their are applications where a bandpass alignment can be used succesfully.....such as low price mini subwoofers for computers, etc. that can use a very cheap high q woofer, slap it in bandpass box and get ALOT of low quality bass output to impress gamers on a budget. Another example IS car audio, where the bandpass box sells itself, as a novelty or feature with 'cool' looking plexi glass windows, etc even though most of these are generic boxes, with horrible sound quality since they cabinets are not even designed with the paramaters of the woofers in mind, but as an ALL-IN-ONE solution to sell at Best Buy and Circuit City.

Bandpass boxes could be used, with small size and high quaity sound if they were done using an isobaric woofer configuration internally, but again...this would raise cost. The famous KEF 7.2 with it's bandpass subwoofer integrated, used isobaric setup internally i believe. Of course, the cost of an additonal driver was not too much of a factor on a $7-8k set of speakers, and was probably offest considering the extra woodwork and veneer/paint it would have required to use a single woofer internally.

-Chris
 
Bandpass

Humm if i had my choice of a subwoofer i would so go for a band pass you need like nill power to run them and they use sometimes a small box i had a kicker c10 in a band pass box and it kicked *** it was louder than my 15" usd sub.. till i ported it but the size difrence AHA bandpass was smaller..
 
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"Humm if i had my choice of a subwoofer i would so go for a band pass you need like nill power to run them"

True, IF you design high gain into the alignment, resulting in narrow bandwidth and POOR transient response.


"and they use sometimes a small box i had a kicker c10 in a band pass box and it kicked ***"

When speaking of size of a cabinet, reference is to the driver being discussed, as varying t/s paramaters, such as compliance(VAS) determine the actual volumes of specific drivers. When for example I say, "Driver A requires a 2.5ft^3 ported enclosure to reach F3 of 25 Hz, as compared to a 1.8ft^3 front chamber plus 2.2ft^3 chamber in the rear of a bandpass alignment to reach the same lower cutoff point.

" it was louder than my 15" usd sub.. till i ported it but the size difrence AHA bandpass was smaller.."

Well this is hardly relevant. Again, discussion is being made of the SAME driver in different alignments, and their relative sound quality vs. efficiency, which is a compromise that must be decided upon.

At a friends request, a few years ago I designed and built a 4th order bandpass enclosure with a high level of gain, achieving approx. 4 db better efficiency than a ported equivalent, using a jl audio 12w1 as a specific request from someone who only wanted relatively low bass output at high spl for the given dollar amount available. Roughly, this was almost as loud as 2 of these woofers in a ported box. The cost of this? Sound quality and bandwidth. Of couse, THAT WAS THE GOAL in this circumstance.

-Chris
 
diyAudio Moderator Emeritus
Joined 2001
Quote Chris8 on bandpass boxes: "No superiority as compared to sealed OR ported designs, excpet when a significant amount of gain is designed in, to raise efficiency significantly. However, this narrows the already limited bandwidth and creates steeper cutoff rates, which in turn causes poor transient response, and peaking. Read: One-Noted bass! "

Beg to differ. First, a ported design is more efficient than a sealed one, so if you are of equal efficiency to a ported design, then you are ahead of the sealed.

The bandpass box consists of a sealed chamber feeding a ported chamber. If the sealed part of the bandpass has a Q of 1 or above, it will give superior bandwidth, at equal sensitivity, to a ported box.

The 3 dB down point of a ported box is given by the following equation:
F3 = [The square root of (Vas/Vb)] times Fs.

So in a ported speaker, when placed into a box equal to it's Vas, will be 3 dB down at Fs.

Here is an Audax woofer placed in a bandpass enclosure that is equal to it's Vas, but it's F3 is half an octave BELOW Fs.

Don't let those two humps fool you. This woofer is plus or minus 1.5 dB between cutoff points-a smooth response.

The woofer is the Audax AP210MO.
Qts = .72
Vas = 2.25 cu ft
Fs = 46 Hz.
SPL = 90 dB.

When the sealed box portion of a bandpass has a Qtc of 1 or above, it seems to outperform the ported box in terms of cutoff frequency. When the Qtc is lower, it does not.
 

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Disabled Account
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"Beg to differ. First, a ported design is more efficient than a sealed one, so if you are of equal efficiency to a ported design, then you are ahead of the sealed. "

Oh...your idea of 'superiority' is efficiency, as I would have to automaticly assume from your statement. I would not think that you actually believe that to be true though. So, of course a ported box is always preferred to a sealed, bandpass always preferred to a ported. Well then, hands down the bandpass box will win. My reference, is accounting for transient decay behaviour and LF extension. You MUST compromise. You wish to compare tone bursts responses anywhere near rolloff? The excessive ringing you just noticed is not a desirable attribute. The VERY high corner Q of the rolloff slope of the bandpass example you provide would sound and measure horribly, as far as transient decay is concerned. But if things such as sound quality are not an issue, then why bother with anything else. If you wish to reduce the gain of the bandpass box to a point of where it approaches the transient response of well designed ported alignment, then you have all of a sudden lost that efficiency that was so desirabe in the first place. Oops. Now you have a more complex, larger and more expensive box to simulate a ported box. Hey..didn't I already summarize all of this previously?

In your example, the bandpass may very well be desirable over the sealed, since with that particularly very high Q motor on the audax, it would not have very much usable LF extension in a closed box, and is not usable in a ported box(less you want the poorest transient response possible). Sort of a 'some or nothing' kind of deal. Of course, 'some' is better than 'nothing'. LOL. BTW, a differing bandpass alignment would yield better response and lower cutoff with that speaker than you demonstrate.

Now, on my statement you could claim that then a ported box can never be superior to a closed box in any design....uh-hu. In many cases, an extended low frequency response can be around 1 octave lower than the sealed box alignment. In all, transient decay will remain the same as a low Q design in the same bandwidth that the sealed box would operate within, with only small penalties approaching even 1/2 octave from tuning, then, rather low q rolloff(for a vented) can be implemented to have minimal penalites near cutoff(as little as only 1/2 cycle overshoot compared to a low Q sealed box, if done right).

Have a nice day.

-Chris
 
ok I got a dumb question

ok this may sound like a dumb question but I'm willing to bet a whole bunch of you are asking yourself the same thing and just don't want to sound dumb so you pretend you know. What exactly is transient response besides a catch word for good sound. If I knew what it was I could better decide if I was willing to sacrifice it for frequency extension, efficency, and box size.
 
listen here pebbles

oh I'm sorry it's bam bam. I was under the impession the purpose of this forum was so inexperienced people could ask questions of more knowledgable people. Not get insulted by them because they don't know everything. I wasn't implying that you don't know what transient response is I was just saying that I really don't and I've been intensly studying speaker design for about five years now so I don't think I was stepping out of line in assuming that a few others don't either. so don't get your knickers in a twist. and if you don't have the patience to explain things than just go away and leave the forum to those who actually want to be helpful.
 
Transient response defines the speaker's ability to accurately reproduce a signal WRT its rise (attack)/decay time. An overdamped system will keep the signal from reaching its peak and decay too quickly (with the amp's slew rate being the limiting factor), conversely an underdamped one will overshoot it and decay slower (over a wider BW) than the signal calls for (ringing), ergo critically damped means it's spot on.

Note that the lower the frequency, the wider its BW, so a signal with a fundamental in the woofer will have its harmonics span all the way up into the tweeter's BW (and why we hear 'bass' through a 2" clock radio driver though there's no fundamental, or even its first few harmonics with some notes), making getting the whole speaker transient perfect is no trivial pursuit and why some folks (like me ;)) prefer to keep the XO point/slopes well away from our high hearing acuity BW by using either 'fullrange' drivers or horns to span this wide gap in the response.

HTH,

GM
 
Bandpass is basically a fancy acoustic filter.

I've built a bandpass, 6 order, you can search forum for piccies and stuff.

Main advantage - you can make smaller driver go lower trading efficiency or vice versa. Mine 8" bandpass plays about the same as my friends 10" polk audio, but mine goes lower, but his enlosure is smaller than mine. Bandpasses arent siuted for PA or hi-power operation - amount of air pushed through the port becomes enormous, so fitting BIG ports is a problem.

Main reason why we dont see them much is that manufacturing cost is bigger that any normal enclosure, manufacturers count their money too.

Ports with no damping material on them RING, and those resonances are narrow and high. I heard that, sounds not too good but a simple XO kills that once and for all.

Pre-made bandpass enclosures are very wrong, sometimes you just CANT make driver play properly in a bandpass enclosure, generally you'll have 2 humps in the responce graph and at those humps enclosure will boom, and produce 1 or 2 note bass.

Group delay can be kept at normal 10-15ms if you dont push it too low with a too small driver.

Transient responce is good(I have a 6th order bandpass built for 70$, and comparing it to a 500$ Polk sub they are just about the same)

Bandpasses are kinda tricky to design - liter here or there makes group delay jump like crazy - you have 4 parameters to fiddle with, whereas on sealed you have 1 and on ported you have 2.

Yes, it can makes things rattle of a 10W amp, I was like Wow, is it possible ?

Thats all I think.
 
>Bandpasses arent siuted for PA or hi-power operation - amount of air pushed through the port becomes enormous, so fitting BIG ports is a problem.
====
Actually, most hi-power PA speakers are BPs, and fitting big ports is no big deal, though they're usually referred to as horns. ;)

Sorry, couldn't resist. :)

GM
 
Mission Bandpass subs??

I picked up two Mission Bandpass subs at a Pawnshop for $100 Cdn. They are model #73PI. Anybody know anything about them? I have done searches on the net and not found any info. They sound allright...better for movies than music. They seem kind of 'one note' and am not sure about their quality. I actually find low bass kind of annoying. It givs me a headache.
 
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