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Help me design a tri-chamber 4th order bandpass enclosure

 
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Lee in Montreal
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PostPosted: Wed Sep 01, 2010 9:36 pm    Post subject: Help me design a tri-chamber 4th order bandpass enclosure Reply with quote

Anyone can give me some cues in regard of a tri-chamber bandpass enclosure? Same principle as the 104/2 or 107, but with B300. My understanding is that the center section's volume and ports will dictate the depth of the bass as well as the cut-off. But I don't understand the sealed enclosures. Should they be full volume as a regular sealed enclosure (about 85-90L) or half the volume to limit the driver's excursion. I want to go no higher than 90Hz, and as low as possible.

What are the basic princples?

http://www.diysubwoofers.org/bnd/trichamberbp.htm


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Kef Calinda - since 1979
Kef Cantata - since 2009
Kef 105/2 - since 2009
Evo 105/2 - in the build
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speakerguru
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PostPosted: Thu Sep 02, 2010 9:01 am    Post subject: Reply with quote

It's all in Laurie Fincham's paper here http://www.hifiloudspeakers.info/Anatomy/Articles/ArticlessIndex.html

Design the single driver system first, then combine the outer two volumes using a common port. The centre volume is 2X the single outer volume.

Your understanding is incomplete. Don't try to do it intuitively. You'll twist your brain Laughing
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speakerguru
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PostPosted: Thu Sep 02, 2010 2:06 pm    Post subject: Reply with quote

Have thought a bit more about this. 1975 was along time ago. Smile

The ratio of front volume to back volume determines the bandwidth of the system. As ever, the wider the bandwidth the lower the sensitivity. The back volume must take into account the non-infinite compliance of the woofer; hence alpha and alphaT in LRF's paper.

The woofer in the back box must have the same resonance frequency as the port in the front box, otherwise the bandpass response will be lop-sided. i.e. if you want to lower the centre frequency of the bandpass you have to lower the resonance of the woofer in the rear box as well as the port in the front box. The usual stuff about box volume and efficiency applies. Same old laws of physics .........

It's pretty well impossible to get there just by calculation, because it's so difficult to allow the correct amount for all the system losses. There will be woofer, port, box and lining losses.

Best way is to make a test system, measure the nearfield acoustic response and the electrical impedance. Use these data to make a model in your simulator of choice and then nudge the design closer to what you want. That's what we did for Model 1, 2, 3 and 4, some 20 years after the original paper.
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Lee in Montreal
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PostPosted: Thu Sep 02, 2010 4:33 pm    Post subject: Reply with quote

I read Laurie's paper a while ago and it made my head dizzy. I applauded the ingenious principle of acoustic being interpreted as an LCR system, but it lead me nowhere...

My perspective is that the ported center box simply is an acoustic filter. But before the soundwave is filtered, it must be generated. This part of the subject is easy. Make it purely acoustical, the good old way with a big sealed enclosure. Or reduce it substancially and correct the Q with electronics (AKA Kube). Small sealed boxes for better membrane control, and convenient cabinet size. Correct the Q with electronics, then filter the output with the long upward tunnel.

In my case, I want to keep away from the electronics, except eventually for a Zobel or notch filter to correct the drivers' impedance at resonance. Therefore I will probably end up with two 90L sealed enclosure (volume for which the B300 was initially designed for, before being reduced to 75L for convenience) and tuned to the woofers' free air resonance frequency of 23Hz. If I reduce the size of each "backbox" in order to end up with an enclosure "that fits well in my living room", then I will have to use a Kube to correct the very low frequencies. Isn't it actually what was done for the 104/2 and 107?
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speakerguru
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PostPosted: Thu Sep 02, 2010 7:41 pm    Post subject: Reply with quote

Making something you can live with and eq. it electronically is a perfectly reasonable way to go as long as the acoustic component values are in the right ball park.
I didn't have a lot to do with either the 104|2 or 107 bass sections. I think that may have clashed with my KEF Professional products days. Speaking of which, the reflex version of the KM1 was the original 4 section closed box volume with 4 ports tuned for minimum excursion at 70Hz (the centre of the kick drum range) and then equ'd with a 4th order integral Kube type circuit.
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Lee in Montreal
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PostPosted: Mon Sep 06, 2010 9:45 pm    Post subject: Reply with quote

Currently giving a few thoughts at horn-loaded bass bins.


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Kef 105/2 - since 2009
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