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New ideas in acoustic treatment

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New ideas in acoustic treatment

Postby Watchmaker » Sun Mar 10, 2019 1:31 pm

I'll just put this here in case anyone's curious about "advances" in materials sciences. Curious to hear any thoughts on this. I'm not convinced it has practical studio implications other than making quiet ducts much easier to build.

https://phys.org/news/2019-03-acoustic-metamaterial-cancels.html
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Re: New ideas in acoustic treatment

Postby blinddrew » Sun Mar 10, 2019 1:39 pm

Interesting, will be curious as to how it scales up in practice.
I'm thinking air-movement systems primarily but potentially also as porous room dividers.
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Re: New ideas in acoustic treatment

Postby ef37a » Sun Mar 10, 2019 1:44 pm

Maybe a silent but still zippy computer fan?

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Re: New ideas in acoustic treatment

Postby Martin Walker » Sun Mar 10, 2019 1:55 pm

The associated video IS impressive! :clap:

https://www.youtube.com/watch?v=Fd1D42dVxS0


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Re: New ideas in acoustic treatment

Postby Wonks » Sun Mar 10, 2019 2:38 pm

Not sure how technical the reporter's background is: "Standing in the room, based on your sense of hearing alone, you'd never know that the loudspeaker was blasting an irritatingly high-pitched note. If, however, you peered into the PVC pipe, you would see the loudspeaker's subwoofers thrumming away" (my italics). :headbang:
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Re: New ideas in acoustic treatment

Postby Martin Walker » Sun Mar 10, 2019 2:44 pm

Wonks wrote:Not sure how technical the reporter's background is: "Standing in the room, based on your sense of hearing alone, you'd never know that the loudspeaker was blasting an irritatingly high-pitched note. If, however, you peered into the PVC pipe, you would see the loudspeaker's subwoofers thrumming away" (my italics). :headbang:

Yep, that made my eyebrows go up and down as well ;)

I suspect that sentence may have been written by some marketing bloke who didn't really understand the mechanics of loudspeaker reproduction :beamup:


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Re: New ideas in acoustic treatment

Postby Wonks » Sun Mar 10, 2019 3:25 pm

Of course, one could postulate that speaker was producing a frequency matching the resonant frequency of the open tube, and that the bung interfered with that resonance.

I'm a born sceptic. A demo using a noise source would have been far more impressive and practical - but maybe it would have shown up significant weaknesses. Does it only work on one frequency? Quite a few of the 'noise cancelling' products I've seen

Also, there was no indication that there was any airflow through the duct - so once you use the duct as it was supposed to be used, what happens then? How much noise does the device itself generate (undoubtedly more broadband than the test noise)? Does moving air carry noise through with it?

There are already many good ways to reduce noise in HVAC systems. One of which is to correctly balance the air supplies - which we do (generally fairly well) in the UK but is only just starting to catch on in the US. There are also very effective in-line silencers.

Most HVAC noise is generated on ductwork bends or on terminal devices/outlet grills where turbulence is created, so it's not simply a case of fitting one device by a fan and all your problems are solved.

Also, that ring was significantly smaller than the circular duct. With airflow, that means both increased resistance and creation of turbulence/noise. As you'd have to have many of these added to a standard system, that would either mean you need increase duct sizes to reduce air velocities and overall pressure loss (both far more expensive and often not practical or physically possible) or else use a bigger fan (which uses a lot more energy and create more noise in the process!).

There is also the fact that in offices, a reasonable level of background noise is required to help mask conversations by others. If offices are too quiet, it becomes hard for people to concentrate and so white noise generators get fitted or a low level of noise is mixed in to any PA or voice alarm system fitted.

Obviously recording studios and performance halls benefit from low noise, but there are existing ways to deal with those ( I've worked on the HVAC control systems of quite a few performing venues, so I know how quiet they can be made).

There may be some applications where it could possibly be used (maybe alongside roads to reduce noise), but it would still depend hugely on cost. There's huge resistance to putting up conventional noise-reducing fences alongside motorways (the M4 near us is getting two hundred metres of fence but it really needs to be a few km long to benefit us).

Reading the linked article, it looks like it's a fairly high-frequency noise cancelling system, with the emphasis on making it go higher, not lower - and we know that it's low frequency sound that causes most noise issues.

It's also currently made from plastic - something we obviously need to use a lot more of!

To me it does rather look like a proof-of-concept device where they're looking for funding for research to take it further, or else use it for a start-up proposal, but will probably take many years and a lot more development to see anything worthwhile. It also needs input from applications people working in the real world, not just research, if it is to have any practical applications at all.
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Re: New ideas in acoustic treatment

Postby Hugh Robjohns » Sun Mar 10, 2019 4:00 pm

It's a clever technology, but its sound transmission characteristics appear essentially to be an interference-based notch filter as shown in the graphs in the papers (see below). According to the paper, the prototype device reflects back frequencies between 1 and 2.5kHz.

acoustic reflection filter.jpg


So it's potentially useful in applications where the unwanted source noise is narrow-band and with stable pitch -- such as drone rotors or small constant-speed fans -- but of no general use in typical audio applications. It also appears that it would need to be insanely large to deal with the LF noise from typical HVAC systems.

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Re: New ideas in acoustic treatment

Postby Sam Spoons » Sun Mar 10, 2019 10:01 pm

Not sure how placing one beneath the rotors on a drone would affect lift generated by said rotors........ It certainly wouldn't be improved :headbang:
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Re: New ideas in acoustic treatment

Postby Hugh Robjohns » Sun Mar 10, 2019 10:15 pm

Lift is about airflow close around the prop, so provided it doesn't affect the local air flow too much it should be fine... And it's selling point is that it doesn't impede air flow, so the application would seem a reasonably practical one in theory.
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Re: New ideas in acoustic treatment

Postby Folderol » Sun Mar 10, 2019 10:53 pm

Don't think that would work. You'd need two per rotor, one above one below, and the rotor would have to be contained which would dramatically reduce the sideways maneuverability as you wouldn't be able to use the slip action with just a slight tilt.
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Re: New ideas in acoustic treatment

Postby Wonks » Sun Mar 10, 2019 11:24 pm

It will certainly impede air flow. Axial fans don't produce a huge amount of differential pressure, and even a small increase in the pressure loss across it can have a huge impact on the air volume passed by the fans.

And those rings will also add weight and sideways air resistance. Meaning you then need bigger fans and bigger batteries, which adds more weight, or you suffer greatly reduced range performance, and battery life. And I really don't know how directional the sound from a drone's prop is as to whether there would be any real benefit.
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Re: New ideas in acoustic treatment

Postby Watchmaker » Mon Mar 11, 2019 11:24 am

Wonks, you summed up my initial reaction and went well beyond it in scope. HVAC noise is pervasive here in the US and the few studios I've been to with adequate handling have spend small fortunes on duct work.

Thanks all for the discussion. I'm still working out how practical this feat of engineering will be. Maybe watching the subwoofer go back and forth will become more popular than watching the wash go round and round.
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Re: New ideas in acoustic treatment

Postby ef37a » Mon Mar 11, 2019 2:59 pm

Wonks, indulge an old bottle jockey a while?
Very OT but this brain just thought, there are storage HEATERS but I have never heard of storage coolers?
Same principle but instead of hotness you have a big cold lump. I know of course that "cold" does not radiate but the lump could be positioned such that convection would carry cool air around or some very quiet internal fans?

I am sure you are ahead of my thinking that the cold could be stored with off peak juice and then no noisy AC running in studio time. I also understand that if "I" can think of this, others must have and either kept quiet or found it nonsense. Or of course, maybe thermo-dynamics just doesn't work that way?

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Re: New ideas in acoustic treatment

Postby Hugh Robjohns » Mon Mar 11, 2019 3:29 pm

ef37a wrote:Very OT but this brain just thought, there are storage HEATERS but I have never heard of storage coolers?

Not perhaps called 'storage coolers' but they definitely exist. That's why so much marble is used on the floors and walls of buildings in hot countries, and why they traditionally have large vats of water in food storage cellars...

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