Rolling off woofer with HF absorbent material in front of it. - diyAudio
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Old 19th June 2002, 08:18 AM   #1
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Default Rolling off woofer with HF absorbent material in front of it.

It just occured to me the other night when I was throwing together a truly awful shed speaker for the upcoming circlotron doomsday amplifier with N-channel hexfets, that instead of using a conventional crossover inductor, perhaps you could put a strategically placed wad of sound absorbent stuff in the hollow region of a woofer cone (not touching it) to roll off the high frequencies. That way there would be no dc resistance of the inductor in series with the voice coil and you may get better damping. Might also be cheaper. Anyone done this?

GP.

P.S. I am not a speaker designer. Does it show?
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Old 19th June 2002, 01:43 PM   #2
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Quote:
Anyone done this?
Yeah, but down the throat of HF metal horns to tame the tinnitus factor.

Eric.
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Old 19th June 2002, 05:24 PM   #3
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I have thought about this a long time ago but never followed through. I can imagine many variants on this idea; down firing woofers on a carpeted floor being an obvious one.

I've wondered how much the various possibilities are going to shift the phase, an important issue for the overall crossover design.

I can imagine a distributed acoustic filter on a mid-range driver that would allow the off axes responses to be the same as the on axis response.

I can also imagine a 3 way system that used acoustical low pass filters and electrical hi pass filters.

This idea is worthy, IMHO, of some serious investigation.
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Old 20th June 2002, 03:25 AM   #4
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Now we're thinking! As far as horns are concerned, I could imagine with a large LF one lining the complete inside surface of the flare. Any unwanted hf's reflecting around inside it would be stopped. Not that I have played with horns, but if you clapped your hands deep inside one it would have a characteristic sound. If you could reduce or elimate that with an absorbent lining it ought to improve things.

GP.
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Old 20th June 2002, 03:56 AM   #5
JoeBob is offline JoeBob  Canada
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Well a big problem it seems to me is getting the right frequency. If it's a two or three way (opposed to a sub) you want the high pass to be relatively simmilar to the low pass filter -3dB points. That just seems like a big problem to me.
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Old 20th June 2002, 06:04 AM   #6
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A parallel thread going on over at the Full Range Forum including a link to an analytical study of which tissue works best on Yamaha NS10 monitors.

dave
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Old 20th June 2002, 08:48 AM   #7
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Quote:
Originally posted by JoeBob
Well a big problem it seems to me is getting the right frequency. If it's a two or three way (opposed to a sub) you want the high pass to be relatively simmilar to the low pass filter -3dB points. That just seems like a big problem to me.
Oh well, at least on a system with a conventional crossover it might be ok for shooshing up any unwanted minor mechanical noises coming from the driver. What about placing a wad of stuff maybe 6 inches in front of the tuning port to reduce any chuffing noises?

Even if it is no good as a crossover, for a LF horn, covering the surface might be worth a try as per the clap experiment. Would probably be especially effective in a folded horn because HF's bounce of shiny surfaces but would be almost completely absorbed by the treated surfaced seeing there is no direct line of sight path from the driver to the outside world.

GP.
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Old 24th June 2002, 09:12 PM   #8
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I read somewhere that the designer John Curl ofton flips a wool sock over his speaker's tweeter for overly harsh CD's. I believe the Deckware ?sp copany had a speaker designe using Dynaudio drivers where the woffer was run full range with some sound asorbing material in front of it.

Just my 2 cent's worth

Bob12345678
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Old 25th June 2002, 02:31 AM   #9
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Default Yamaha Improvements

Hello Dave and All,
I used to have a pair of NS-20 - 8" 2way big brother to the NS-10 and using the same tweeter.
With factory crossover, both these cabinets are HF ear bleeders !.

My soloution was to ditch the factory crossover, and wire the 8" full range, with RC network across the voicecoil.
I did not bother to compensate the resonance impedence hump.
The tweeter was fed via 1.1111 uF, and it too had an RC network across the voice coil.
This cabinet then measured flat impedence to out past 40 kHz.

The network across this metal dome tweeter both smoothed its response and extended response past hearing range.
Highs were then rather good, and no earbleeding.
This setup transformed these cabinets into really fast detailed and efficient loudspeakers.

With this arrangement, phase linearity is excellent, however absoloute polarity of the source music is perfectly revealed, and for complete satisfaction, speaker polarity needs to be reversed accordingly.

It ends up being a PITA having to swap polarity just about every second track, but the result IS worth the 5 seconds of effort.
It is disarming to learn that there really is no standard polarity in the recording industry.
I found several albums where the feature/hit track was recorded in one polarity, and the rest of the tracks reversed.

Regards, Eric.
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Old 25th June 2002, 03:24 AM   #10
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If pressed, I could probably dig up references showing that absolute polarity is not audible.

But back to Circlotron and his original and follow up thoughts.

I did a quick check with the Master Handbook of Acoustics, Chapter 10 and am left with the opinion that pass through absorption for low frequencies is problematical, at best, if one is thinking of replacing the crossover networks for woofers. The slope is not fast enough and the required thickness of material would be prohibitive.

I can see filtering possibilities for the vent chuffing that would at least remove some of the high frequency components of chuffing but to reduce the low frequency components would require a solution, I believe, that results in a resistive vent.

I found myself most exited about the possibility of a fabric hemi-sphere placed over a mid-range and designed in such a way that the off axis response is the same as on axis. You might call it a fabric based acoustic lens with absorption characteristics. Alas, at present, I don't have time to make some preliminary tests let alone come to a solution.

Regarding a sock over the tweeter, John Curl could have come to a more elegant solution.

No, wait, I have an idea! "Tice Clock", "Tice Sock" - what a concept and he's just the guy to do it. Make mine an argyle!
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