the isobarik 6th order bandpass transmission line folded horn subwoofer idea - diyAudio
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Old 22nd October 2001, 02:06 AM   #1
diyAudio Member
Join Date: Jul 2001
I am designing a subwoofer for PA use for a friend. A prime factor is cost, so arrays of 18"ers just wont do. The sub doesn't really have to reach too low, about 40hz will do. Also, it has to be effiecient, since he want to run it off his 100w amp that he already has. So i started thinking, what type of enclosue has good effeciency? I came up with this idea:
the bandpass enclosue has to ported sections: one port fires into a transmission line (tuned for 40hz) and the end of that line joins with the other port at the throat of a horn.
ALso, the bandpass enclosure will have two subs in an isobarik arrangement, so that both channels of a stereo amp can be used (with a mono signal), and so the bandpass enclosure can be smaller. this should work because all of the output of a 6th order bandpass is via the ports, and the T-line inverts the phase of one port, which then reinforces the output from the first port. The horn then directs the sound, so none is wasted behind it where there are no listeners. Of course this would be a ridicuoulsly large system, i think the horn mouth has to be 9m^2 or close to it, and the T-line has to be long too (i think 10-30m) ... i have all the calculations done, just not handy. My main concern is if the t-line will act as a port, making neither of them work as predicted. I think this may be compensated for by making the t-line crossection much larger than the port's. is this type of system feasible or not? according to some very rough calulations, from various sources, this may have up to a 110db 1w/1m sensitivity when using 93db drivers...that could make your ears bleed running off a 100w amp...

P.S. it doesn't have to sound "great" either, as long as its really loud and really cheap, it will maily be used for small outdoor concert/parties

I am probably going to use 2 12" woofers, any suggestions?

DO you think that this would actually hit 110db sensitivity, or is that misleading?

[Edited by Yoda on 10-22-2001 at 06:46 PM]
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Old 23rd October 2001, 04:34 PM   #2
Wizard of Kelts
diyAudio Moderator Emeritus
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
I cannot speak for you bandpass into a transmission line idea. That is unique.

If you really want increased sensitivity, try a bandpass box with the front chamber larger than the back. Also, start with high Q woofers-.7 or above-when you do that. These would be free air woofers.

I have not actually built a bandpass box but I have read papers on how they work by Geddes and Fincham. Do you have speaker modeling software? I have a box worked out in BoxModel-a freeware application-that increases the sensitivity of the subwoofer by 6 db.

If you are interested, I can give you the parameters.

Again, I have never built one. So by all means, anyone who has built a bandpass box feel entirely free to contribute.

[Edited by kelticwizard on 10-23-2001 at 11:37 AM]
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Old 23rd October 2001, 05:10 PM   #3
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Join Date: Oct 2001
Location: 5280'
You'll end up with a mess due to impedance miss matches between the various alignments. Band passes are designed to fire into a room, not another alignment. And TL's don't mix with horns.

Build a either a standard horn loaded or bass reflex enclosure with a couple of high efficiency 15"s, and let it go at that..
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Old 24th October 2001, 03:00 PM   #4
Wizard of Kelts
diyAudio Moderator Emeritus
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State

Just for the heck of it, I am curious: suppose your bandpass into a transmission line idea works and you are able to produce a subwoof that is 110 dB @ 1meter/1watt sensitive. What were you planning on crossing into?

Horns have super high efficiency, but they are expensive, even for one that goes down to 800 Hz. I assume your subwoof was going to cross into something at 150 Hz or so. You mentioned cost was a factor, so I don't think you had a horn loaded bass bin capable of going down to 150 Hz or below handy.

You are quite correct in saying that high efficiency is a premium consideration in PA applications. That is why Eminence and other manufacturers make their PA speakers at 100 dB @ 1 meter/1watt or so. English manufacturer Fane, (available US) has a couple of 15 inch models rated at 102 dB @ 1meter/1watt. These might not be 110 dB efficient, but they sure are better than 93 dB or so.

A bandpass box has the possibility of raising sensivity 6 db or so, for a passband of an octave or octave and a half. After that, you must cross into something.

Overall, for simplicity's sake, you are better off grabbing one of those 100 dB efficient PA speakers and building a reflex box to suit.
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