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Discussion Starter · #1 ·
first some background:

http://www.avsforum.com/avs-vb/showthread.php?t=1164457

http://www.diy-av.net/forum/viewtopic.php?f=28&t=408

http://www.geocities.com/f4ier/newsubsim.htm


basically when i tried to model a bandpass box i realized it just wasn't going to work. the boxes get too huge and the SPL advantage is minimal. time for plan B.


and plan B is to use a non-resonant acoustical lowpass filter. this should make for a more compact enclosure, smoother frequency response and perhaps even a more effective filtering.


only question that remains is - just how do you build one ?


LOL !


ideas ?


i need a nonresonant 50 hz acoustical lowpass filter ...
 

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I think you have realized why electronics have some advantages. What I understand you are looking for is a friction based muffler. Going back to Smith and Morrison, This would be a very large passage stuffed with friction material. ( fiber). They did not discuss filter slopes or Q. I seem to remember a old Speaker Builder piece on "SAFE" enclosures. Friction based. Not sure I buy what they said, but I did not build one, so who knows.

"Big" seems to be a common thread when talking bass. My suspicion is you have not seen one built as it is impractical. ( consider 2 op amps, 8 resistors, 3 caps and a 9V battery)
 

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Ah, completely valid requirement. I forgot you mentioned that.


Thinking in the box, so to speak. How about a BP with two drivers mounted in push-pull. This will provide some advantages in even order harmonics.

Build the chambers with odd angles to slightly reduce HF reflections through the cones.

Stuff the primary chamber to the Q you prefer.

Very thick ( 6 inches maybe?) 703 lining in the secondary chamber.

Stuff the secondary box and maybe the port.

All this stuffing would take a lot of trial and error. It would lower the efficiency a bit, widen the Q's a bit. Might work.


Another idea. What if the secondary exit port was a very long piece of pipe insulation (compressed FG) If it was under 1/2 Lambda long and several inches thick, hard outer shell, it would be a LP acoustic filter. I guess it could be folded.


What would happen if the secondary chamber was a transmission line rather than a port-tuned chamber? Getting really big again I suspect.
 

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Discussion Starter · #5 ·

Quote:
Originally Posted by tvrgeek /forum/post/16929659


Another idea. What if the secondary exit port was a very long piece of pipe insulation (compressed FG) If it was under 1/2 Lambda long and several inches thick, hard outer shell, it would be a LP acoustic filter. I guess it could be folded.

that's what i was thinking. i was thinking something like a port but made out of a leaky material. and the front chamber itself also made out of leaky material. and the chamber not just lined but stuffed completely.


the problem is - this is BS.


because why do we need a port ? a port is inherently resonant so if we're trying to build a nonresonant system then why are we using one ?
 

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Discussion Starter · #6 ·

Quote:
Originally Posted by tvrgeek /forum/post/16929659


What would happen if the secondary chamber was a transmission line rather than a port-tuned chamber?

well a port is a more effective low pass filter. stuffed transmission line does attenuate high frequencies but not as aggressively.


also i am wondering whether placing stuffing directly in the path of airflow would create distortion.
 

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Discussion Starter · #8 ·
ok i know !


build a solid front chamber out of plywood.


put a rectangular ( plywood ) port in there with 12" x 12" cross section.


put a slab of auralex acoustial foam into it ( it comes in 12 x 12 x 24" size ) so its completely plugged


now cut a 8" diameter cylindrical hole in that slab


now line the sides of that port with 1.5" of acousta stuff and use a 5" aluminum mesh pipe to keep the acousta stuff in place.


so we have 3 concentric ports - air inside stuffing inside foam inside plywood.


and then the chamber itself stuff densely with acousta stuff.


that should kill both the port resonances as well as the system resonance yet preserve the lowpass behavior as well as allow direct path for airflow.


what do you think ?
 

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Discussion Starter · #9 ·

Quote:
Originally Posted by Fatawan /forum/post/16933653


Use PR's like the ill-fated BMF designed by Mark Seaton. Read the info starting here:

http://www.***************.com/fusio...d/131768/tp/2/


Drawing attached

no a PR would contribute its own distortion, plus i am not looking for a resonant system because then the front chamber would have to be modeled and built with a certain volume. to save space, i would rather have it SMALLER than such a model might ask for.
 

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Quote:
Originally Posted by vasyachkin /forum/post/16933818


ok i know !


build a solid front chamber out of plywood.


put a rectangular ( plywood ) port in there with 12" x 12" cross section.


put a slab of auralex acoustial foam into it ( it comes in 12 x 12 x 24" size ) so its completely plugged


now cut a 8" diameter cylindrical hole in that slab


now line the sides of that port with 1.5" of acousta stuff and use a 5" aluminum mesh pipe to keep the acousta stuff in place.


so we have 3 concentric ports - air inside stuffing inside foam inside plywood.


and then the chamber itself stuff densely with acousta stuff.


that should kill both the port resonances as well as the system resonance yet preserve the lowpass behavior as well as allow direct path for airflow.


what do you think ?

Maybe. Getting large again. It would be total trial and error for the port tuning. Sounds like a long wintertime project.
 
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