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[Art Welter]

Testing a new HF horn today, occurred to me that I had never actually seen what plastic sheet does to sound when placed over a speaker, though I have heard how bad it can sound.

Having done hundreds of outdoor shows with tarps or plastic sheet over the speakers for extended periods of time, it was obvious that the HF was reduced, but not as obvious how bad the comb filtering was.

A "tarp EQ"of a 6-10 dB HF shelf boost helps correct the general downhill trend, but no way can the comb filtering be corrected, explaining the nasty sound.



The thicker the plastic, the worse the effect.
The depth of the HF horn will also affect the location of the dips.
The white line is with no plastic, the light blue is a 1.1 mil thick garbage bag, the orange is around 5 mil thick.

Yikes.

Art Welter

[Moby (Mike Diack)]

Interesting...apart from the response issue, If you use a tight garbage bag you end up with the worlds biggest kazoo. There must be something that lets noise through but not wetness.
M

[TJ (Tom) Cornish]

I sense a market for a new recording plugin:

Outdoor Festival - Transforms your music from the boring pristine-ness of standard recording techniques to the dynamic, warm (dry?) live outdoor concert sound we all crave.

There should also be an app for that, so I can play my ITunes library through this filter.

P.S. - Version 2 forthcoming - will include Gunness De-Focusing

[James Feenstra]

what about the effect of a blow through or scrim?

[Geoff Doane]

Art Welter wrote on Tue, 18 January 2011 02:04

, it was obvious that the HF was reduced, but not as obvious how bad the comb filtering was.

That's what a tester found back in the '80s when he wanted to know what the popular "tissue paper over the tweeters" of studio monitors was really doing to the sound of the speakers.  The tissue paper didn't really reduce the overall level very much, but did create serious comb filtering.

Why it was a desirable technique is anybody's guess.

GTD

[Art Welter]

James Feenstra wrote on Tue, 18 January 2011 08:38

what about the effect of a blow through or scrim?

It would be interesting to test, as would perforated metal screen of various percentage of openness.

If anyone wants to send me some material pieces, I'll give them a shot, a 15 inch by 10 inch piece will fit over the small horn.

I tried the Acoustone grill cloth I normally use, it had almost no effect that could be detected.

Throwing a towel over the horn (as drummers often do with their monitor )  muffled the HF pretty evenly, without a lot of comb filtering.

[Art Welter]

Geoff Doane wrote on Tue, 18 January 2011 09:21

Art Welter wrote on Tue, 18 January 2011 02:04 , it was obvious that the HF was reduced, but not as obvious how bad the comb filtering was. That's what a tester found back in the '80s when he wanted to know what the popular "tissue paper over the tweeters" of studio monitors was really doing to the sound of the speakers.  The tissue paper didn't really reduce the overall level very much, but did create serious comb filtering. Why it was a desirable technique is anybody's guess. GTD

It allowed your mixes on NS 10s to sound like they went through an Aphex Aural Exciter without having to rent one...

A pair of NS 10s sitting on the meter bridge of a console have so many diffraction issues that the addition of some tissue paper makes little comparative difference.

[Tom Reid]

Moby (Mike Diack) wrote on Tue, 18 January 2011 01:04

Interesting...apart from the response issue, If you use a tight garbage bag you end up with the worlds biggest kazoo. There must be something that lets noise through but not wetness. M

Cheesecloth?

Or a wall of fire.

[Greg Cameron]

TJ (Tom) Cornish wrote on Tue, 18 January 2011 05:44

I sense a market for a new recording plugin: P.S. - Version 2 forthcoming - will include Gunness De-Focusing Cameron Pro Audio. "Procrastinators of the world - contemplate uniting!" Art Welter Hero Member Offline Posts: 4874 Re: The Sucking Sound of Tarps over Speakers Moby (Mike Diack) wrote on Tue, 18 January 2011 00:04 Interesting...apart from the response issue, If you use a tight garbage bag you end up with the worlds biggest kazoo. There must be something that lets noise through but not wetness. M Did some more testing after Gian P. Portanova asked me in a PM : “Would the affect be minimized if you add some thin cloth over the speaker first, then the bag?” I replied that a thin organic cloth like cotton would attenuate (absorb) more highs, but the comb filtering would remain. The comb filter effect is primarily from cancellation due to reflection (seen in the even spacing of nulls), but the (plastic) bag also seems to vibrate in "breakup mode" (or kazoo modes), adding an additional random effect. Below are the results of some more tests, done using FPPO (fixed point per octave, 24 points per) which gives a less accurate, but easier to read picture than the 1.5 Hz frequency resolution shown previously. For overall HF attenuation of about 3 dB, a cotton T shirt does the trick. A bath towel lowers the level another 3 dB or so, though with larger peaks. Good for those who have noise induced hearing loss, still lets 4K through   . A high thread count bed sheet (tough to breath through) is obviously reflective, with deep nulls at regular frequency intervals. It does not seem to have the extra random kazoo effect of the plastic membranes. The lower curves show the response of the cotton T shirt with thin and thick plastic sheet over. Putting the cotton on the inside of the plastic slightly lowered the frequency of the nulls due to longer path length. Response still sucked, and lost more HF. Time to do some real work.. Art Welter ProSoundWeb Community Re: The Sucking Sound of Tarps over Speakers Print Pages: [1] 2 3  All   Go Up « previous next » ProSoundWeb Community > Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > SR Forum Archives > LAB: The Classic Live Audio Board FUD Forum Archive > The Sucking Sound of Tarps over Speakers

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[Art Welter]

Moby (Mike Diack) wrote on Tue, 18 January 2011 00:04

Interesting...apart from the response issue, If you use a tight garbage bag you end up with the worlds biggest kazoo. There must be something that lets noise through but not wetness. M

Did some more testing after Gian P. Portanova asked me in a PM :
“Would the affect be minimized if you add some thin cloth over the speaker first, then the bag?”

I replied that a thin organic cloth like cotton would attenuate (absorb) more highs, but the comb filtering would remain.

The comb filter effect is primarily from cancellation due to reflection (seen in the even spacing of nulls), but the (plastic) bag also seems to vibrate in "breakup mode" (or kazoo modes), adding an additional random effect.

Below are the results of some more tests, done using FPPO (fixed point per octave, 24 points per) which gives a less accurate, but easier to read picture than the 1.5 Hz frequency resolution shown previously.


For overall HF attenuation of about 3 dB, a cotton T shirt does the trick.
A bath towel lowers the level another 3 dB or so, though with larger peaks. Good for those who have noise induced hearing loss, still lets 4K through   .

A high thread count bed sheet (tough to breath through) is obviously reflective, with deep nulls at regular frequency intervals.
It does not seem to have the extra random kazoo effect of the plastic membranes.

The lower curves show the response of the cotton T shirt with thin and thick plastic sheet over.

Putting the cotton on the inside of the plastic slightly lowered the frequency of the nulls due to longer path length.

Response still sucked, and lost more HF.

Time to do some real work..

Art Welter

[ProSoundWeb Community]