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The Shape of Silence - Acoustic Meta-materials make sounds humanly imperceptible.

rcmaehl

Source:
Phys.org

Released Paper
 

Summary:

Researchers at Boston University have developed a sound barrier that doesn't restrict airflow, potential usage include jet engines and other airflow dependent devices.

Media:
BU researchers develop 'acoustic metamaterial' that cancels sound

 

Quotes/Excerpts:

Quote

Boston University researchers...released a paper in Physical Review B demonstrating it's possible to silence noise using an open, ringlike structure, created to mathematically perfect specifications, for cutting out sounds while maintaining airflow. "Today's sound barriers are literally thick heavy walls." Although noise-mitigating barricades, called sound baffles, can help drown out the whoosh of rush hour traffic or contain the symphony of music within concert hall walls, they are a clunky approach not well suited to situations where airflow is also critical. Imagine barricading a jet engine's exhaust vent. They calculated the dimensions and specifications that the metamaterial would need to have in order to interfere with the transmitted sound waves, preventing sound—but not air—from being radiated through the open structure. The metamaterial needs to be shaped in such a way that it sends incoming sounds back to where they came from. They decided to create a structure that could silence sound from a loudspeaker. They modeled the physical dimensions that would most effectively silence noises. They used 3-D printing to materialize an open, noise-canceling structure made of plastic. The researchers sealed the loudspeaker into one end of a PVC pipe. On the other end, the tailor-made acoustic metamaterial was fastened into the opening. 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. The metamaterial, ringing around the internal perimeter of the pipe's mouth, worked like a mute button incarnate until the moment when Ghaffarivardavagh reached down and pulled it free. The lab suddenly echoed with the screeching of the loudspeaker's tune. By comparing sound levels with and without the metamaterial fastened in place, the team found that they could silence nearly all—94 percent to be exact—of the noise, making the sounds emanating from the loudspeaker imperceptible to the human ear. Zhang says the possibilities are endless, since the noise mitigation method can be customized to suit nearly any environment: "The idea is that we can now mathematically design an object that can block the sounds of anything," she says.

 

My Thoughts:

This is definitely interesting to see. I can imagine several usages from the ones listed such as HVAC and drones, to tech uses like Naturally noise cancelling headphones. Granted they don't list the limitations of the designs within the news articles, however even if limited in size or shape, there'll still be tons of use for such non-electrical noise cancellation.

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Is the subwoofer playing the high-pitched noise?

 

Otherwise cool idea. I would imagine constructive interference and vibration to be a problem however. 

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16 minutes ago, rcmaehl said:

Researchers at Boston University have developed a sound barrier that doesn't restrict airflow, potential usage include jet engines and other airflow dependent devices.

Inb4 9x Noctua industrial 3000rpm fans totaling 3db of noise

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3 minutes ago, TVwazhere said:

Inb4 9x Noctua industrial 3000rpm fans totaling 3db of noise

9x 12,000 RPM fans, 3db noise

 

Case airflow will never be the same /s

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This doesn't look like it accounts for noise (or alternately, sound of a broad range of frequencies), only an isolated frequency...

 

if they showed it in action at, say, a concert or an MRI machine as the article talks about, I'd be more impressed. Not sure of the usefulness of a single frequency sound eliminator.

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49 minutes ago, rcmaehl said:

9x 12,000 RPM fans, 3db noise

 

Case airflow will never be the same /s

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not sure if i understand it, does it just change the frequency to one that humans cant hear? if thats the case you can damage your hearing without even knowing some sound is blasting full volume near you?

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13 minutes ago, Neftex said:

does it just change the frequency to one that humans cant hear?

 

Looks like it doesn't.

1 hour ago, rcmaehl said:

The metamaterial needs to be shaped in such a way that it sends incoming sounds back to where they came from.

If the sound waves are reflected back to the source, they probably cancel out some of those that are heading for the "silencer".

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How to make sound reduction sound better than it is: express it as a percentage. 94% reduction is equivalent to 24 dB reduction.

 

Don't understand the mechanism claimed, but my first thought is does it work over broad frequency ranges? If not, that'll massively limit its application in itself. Does it have to go on the end of a tube? 

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1 minute ago, Captain Chaos said:

 

Looks like it doesn't.

If the sound waves are reflected back to the source, they probably cancel out some of those that are heading for the "silencer".

yea they mentioned reflecting it back, what got me thinking was the part saying:

2 hours ago, rcmaehl said:

making the sounds emanating from the loudspeaker imperceptible to the human ear.

it didnt say the sound "wasnt there", just that humans didnt hear it

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1 minute ago, porina said:

How to make sound reduction sound better than it is: express it as a percentage. 94% reduction is equivalent to 24 dB reduction.

 

Don't understand the mechanism claimed, but my first thought is does it work over broad frequency ranges? If not, that'll massively limit its application in itself. Does it have to go on the end of a tube? 

ye it has to go at the end of the tube, if sound can bounce around the silencing thing, it cant function

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Wait a second, wouldn't the sound wave resonance/reflection vary quite substantially based on frequency? 

 

Also 94% sounds like a big reduction, but that is only a 12 decibel reduction. 12 decibel reduction is not anything close to that impressive by itself compared to the sound proofing (or NC) efforts already done.

 

But it would potentially be useful in airplanes at least.

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8 minutes ago, porina said:

How to make sound reduction sound better than it is: express it as a percentage. 94% reduction is equivalent to 24 dB reduction.

 

Don't understand the mechanism claimed, but my first thought is does it work over broad frequency ranges? If not, that'll massively limit its application in itself. Does it have to go on the end of a tube? 

If they mean 94% reduction in sound power (which I'm 99% sure they do), it's only a 12 dB drop not 24.

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seeing as its round this could be great for fans, maybe we can finally have those damn deltas in our computers whilst also being able to talk to another human being in the same building

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33 minutes ago, Curufinwe_wins said:

If they mean 94% reduction in sound power (which I'm 99% sure they do), it's only a 12 dB drop not 24.

They used a 4" sub and i assume they are all typical and output 86 db by default so to be as they state "making the sounds emanating from the loudspeaker imperceptible to the human ear" it does make sense also that 94% of Atypical 86 db is 5 db which make sense that it's imperceptible too. That would make the sound 11.5 times quieter.

 

 

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Finally something that blocks the sound of people eating in the library 

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25 minutes ago, Franck said:

They used a 4" sub and i assume they are all typical and output 86 db by default so to be as they state "making the sounds emanating from the loudspeaker imperceptible to the human ear" it does make sense also that 94% of Atypical 86 db is 5 db which make sense that it's imperceptible too. That would make the sound 11.5 times quieter.

Decibels are exponential however, so "11.5 times" doesn't really cover it

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1 hour ago, Curufinwe_wins said:

If they mean 94% reduction in sound power (which I'm 99% sure they do), it's only a 12 dB drop not 24.

I had to go back to the article to check. It doesn't help...

Quote

By comparing sound levels with and without the metamaterial fastened in place, the team found that they could silence nearly all—94 percent to be exact—of the noise, making the sounds emanating from the loudspeaker imperceptible to the human ear.

So, it's unclear if it is level or power they're talking about. I'm used to talking about level, hence going for the 24 dB value.

 

24 minutes ago, Franck said:

94% of Atypical 86 db is 5 db which make sense that it's imperceptible too. That would make the sound 11.5 times quieter.

dB values don't work that way.

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2 minutes ago, porina said:

I had to go back to the article to check. It doesn't help...

So, it's unclear if it is level or power they're talking about. I'm used to talking about level, hence going for the 24 dB value.

 

dB values don't work that way.

Here's the paper

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.99.024302

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Inb4 this gets used for Gun Silencers.

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1 hour ago, Franck said:

They used a 4" sub and i assume they are all typical and output 86 db by default so to be as they state "making the sounds emanating from the loudspeaker imperceptible to the human ear" it does make sense also that 94% of Atypical 86 db is 5 db which make sense that it's imperceptible too. That would make the sound 11.5 times quieter.

 

 

That is 100% not how it works. But moving from 52 dB to 40 dB with a noise floor of 45 dB is 'imperceptible'.

 

1 hour ago, porina said:

I had to go back to the article to check. It doesn't help...

So, it's unclear if it is level or power they're talking about. I'm used to talking about level, hence going for the 24 dB value.

 

dB values don't work that way.

Level as in voltage difference? Since it's in a physics paper talking about post release dampening, talking about voltage drop doesn't really make sense in this context, so I'm assuming they mean power output (since that is what is actually measurable after the fact).

 

also this quote from the paper indicates power to me

Quote

reduction in the transmitted acoustic energy of up to 94%.

 

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1 hour ago, Curufinwe_wins said:

Level as in voltage difference? Since it's in a physics paper talking about post release dampening, talking about voltage drop doesn't really make sense in this context, so I'm assuming they mean power output (since that is what is actually measurable after the fact).

Level as in sound pressure level. What people experience. I see what you mean about measuring the resulting power. I have no experience on the research side.

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4 hours ago, porina said:

 

Level as in sound pressure level. What people experience. I see what you mean about measuring the resulting power. I have no experience on the research side.

Ahh SPL. Forgot that's a thing lol. Obviously in retrospect it is (a thing), I forgot about that though. 

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I remember seeing an article on this device with a very provocative headline (what isn't these days?), but they also included a video demonstration that told a very different story.  While the title - much like the one on this thread - makes it sound like this is a miracle product that just kills sound completely, the fact was it only worked on specific frequencies for which it was designed - not wide band - and the result, though much quieter, was very much still noticeable and in no way going to take anything from loud to inaudible - more like loud to quiet, or just "average".  Beyond that, I would imagine that a lot of the noise of an airplane engine, at least when inside the cabin, is conducted/created through vibration and not sound through the air, and in theory this would do nothing for that.

 

11 hours ago, porina said:

How to make sound reduction sound better than it is: express it as a percentage. 94% reduction is equivalent to 24 dB reduction.

 

11 hours ago, Curufinwe_wins said:

Also 94% sounds like a big reduction, but that is only a 12 decibel reduction.

 

And yeah that was my other thought.  I know from having calculated this in the past that when you start talking significant volume differences it's factors of 1000, or maybe even 1M, so 94% isn't much.  We would need to start looking at 99.9%+

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44 minutes ago, Ryan_Vickers said:

I remember seeing an article on this device with a very provocative headline (what isn't these days?), but they also included a video demonstration that told a very different story.  While the title - much like the one on this thread - makes it sound like this is a miracle product that just kills sound completely, the fact was it only worked on specific frequencies for which it was designed - not wide band - and the result, though much quieter, was very much still noticeable and in no way going to take anything from loud to inaudible - more like loud to quiet, or just "average".  Beyond that, I would imagine that a lot of the noise of an airplane engine, at least when inside the cabin, is conducted/created through vibration and not sound through the air, and in theory this would do nothing for that.

 

 

 

And yeah that was my other thought.  I know from having calculated this in the past that when you start talking significant volume differences it's factors of 1000, or maybe even 1M, so 94% isn't much.  We would need to start looking at 99.9%+

Agreed.

 

 

In fairness, it might still be useful for airliners that are under pressure from governments to reduce their noise pollution, and that's an application where these could see a lot of use if the narrow resonances are usefully diminished.

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