New 'Acoustic Metamaterial' Cancels Sound

Reflecting acoustic waves could potentially present a problem by intensifying the waves back to the source reaching a harmonic resonance level and destroying the structure...It's a sonic earthquake machine...Traveling next to a semi on a highway with near perfect acoustic reflecting material on either side would be deafening...literally...It would be like riding in a tunnel all the time....

spiral duct work work?

I agree. It seems that cancelling sound is far diffent from merely sending it back towards the source.

I'm thinking it's not a big problem. Common objects reflect a high percentage of sound in air due to the large difference in acoustic impedance between gases and solids or liquids.

"SOUND ABSORPTION & COEFFICIENTS

Hard, reflective, nonporous interior building surfaces such as glass, wood, plaster, brick and concrete absorb 2% to 5% of the sounds striking the surface to reflect 95% or more of the sound."

http://mbiproducts.com/static/mbi_1/media/misc/RoomAcoustics.pdf

The only thing unique about this device is that it reflects sound (at least at a specific frequency) without blocking the passage of air.

This sounds more like a novel approach for HVAC of performance spaces. Hopefully, the acoustic frequencies of the air handler will reside in the range that this reflects back to the air handler.

I was excited to read about this at phys.org, disappointed to hear it is tuned to a single frequency! I suppose you could assemble a structure of individually tuned components to produce some general effect. The image of the pipe pressed up to the speaker didn't look too promising though... it's a lot less useful if it can't capture sound waves on the fly.

Metamaterials are fascinating, and as 3D printers become more accessible and sophisticated, the prospect of printing your own test pieces is not too far off. Some of the designs I've seen published are quite beautiful in addition to their strange properties. It's the kind of thing I'd love to play with.

I'm thinking that it works similar to a Bragg or dielectric mirror, used in lasers to reflect 99.9% of a particular frequency using multiple layers of dielectric. Of course, sound waves are about 1 million times the wavelength of light waves, so the hole in the middle is not even resolved by the sound waves.

Diagram of a dielectric mirror. Thin layers with a high refractive index n₁are interleaved with thicker layers with a lower refractive index n₂. The path lengths l_A and l_B differ by exactly one wavelength, which leads to constructive interference.

https://en.wikipedia.org/wiki/Dielectric_mirror

OK,now we have an audio band pass filter tuneable to specific frequencies.

This would be great to make silent helicopters or commercial aircraft;I am sure the military already has an interest in this;think stealthy helicopters,tanks,sniper rifles,drones.

Ear protection for hunters or gun enthusiasts.

My personal interest would be to amplify and send back to the source, all of the BOOM BOOM thunderous noise in vehicles that can rattle windows from 1/4 mile away.

One tuned to my wife's voice would be nice too.

She says I never listen to her anyway,and some other stuff.

This message will self destruct if she logs in to this account.

There is active noise cancellation for the applications you have listed (at least some of them). It's a matter of sensing the ambient noise and generating an inverted waveform to cancel it. I have a Bose headset and it's amazing how well it does this job.

https://en.wikipedia.org/wiki/Active_noise_control

I know it works as proof is in the noise cancelling headphones. I am curious where the energy goes. It takes energy to drive the noise cancelling speaker at the out of phase frequency, right? ..and there is energy in the incoming sound wave that is to be cancelled...

So when the incoming sound wave and the out of phase cancelation wave meet and cancel, what happens to the energy? ...does it get extremely hot at that point? ....or maybe there is charge buildup? Some plasma of a hand wavey sort? Does it get dusty? Could the missing energy be going to generate fine dust by conversion to matter?

It's amazing how little energy is involved in most acoustic events. Even a couple of acoustic watts can be 'deafening'.

We tend to think in terms of amplifiers with ratings of 100s or 1000s of watts but very little is turned into sound.

That's a good question.

Sound canceling headphones have a microphone that picks up the outside interfering noise. This noise is canceled so that you only hear the music.

If you have two sources of sound, there will be locations where there is cancellation (zero amplitude) and other locations where there will be reinforcement (twice amplitude), depending on the relative distances and phase relationship of the two sound sources. So the null in sound intensity of the interfering outside sound source is achieved at the expense of an amplification elsewhere.

The electronics in the headphones senses the outside noise and provides the proper amplification and phase shift to create a null amplitude at your ear. The sound absorbing material in the headphones absorbs this energy where there is not cancellation.

The energy does not disappear, it just is moved elsewhere.

Great explanation. Thanks. I should have remembered the other parts of the interference.

It does lead me to a slightly related question...and since your answer to the previous was so concise and effective, maybe you'll have a stab at it....

Would it be possible (given the ability to make a beam of a laser conform arbitrary cross sections and effect small divergence) to use one beam to destructively interfere with effectively all of another beam?

If so, once again, where does the energy go?

If not(and in the case of the accoustic waves as well) since regions are reduced in energy and other regions increased in energy via interference, how quickly upon overlaying as interfering beam does the energy move away from the destructive interference areas to the constructive? Does it move in a prefered direction or both? If the wave is destructively brought to zero energy at a node, how does it continue to propagate away from the source after that node? Seems like there must still be energy flowing down beam...yet it is zero at that node?

Excellent! Thanks! GA.

That is a great video. Thank you. GA

OK, now we have an audio bandpass filter tuneable to specific frequencies.

So, you can make a unique kind of whistle...

As I interpret what I read on this, we have a band stop or notch filter, not a band pass filter.

I saw nothing about what kind of frequency it can reflect. I can imagine a use for such a filter if it were able to stop the passage of noise from the blower through the ducts of an HVAC system, but I seriously doubt if the device could work at the low frequencies of such noise.

Three things can happen to the sound: it can be reflected, absorbed, or transmitted. In this case, the sound is mostly reflected at the resonant frequency and mostly transmitted at other frequencies.

Here is my understanding of how this device works:

The device consists of a ring, open through the center. The outside part of the ring consists of spiral channels. The travel distance around the outside spiral path, in this case, is 10 times the distance through the center. At the resonant frequency, there is cancellation between the sound that goes through the hole and sound that takes the longer, spiral path.

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

In the diagram above, the acoustic refractive index of air in the hole, n₁ = 1, and the 'effective' refractive index of the outer spiral path is n₂ = 10. At the resonant frequency, f, n₂*t = λ/2, or λ = c/f = 2*n₂*t, where t is the thickness of the ring, c is the velocity of sound in air, and λ is the wavelength.

In the referenced paper, the resonant frequency was about 460 Hz.

OK, bonus question...Can you put a number of these in series tuned to different resonant frequencies to create multiple nulls in the spectrum and create a more broadband sound cancellation?

That certainly sounds reasonable!

Undoubtedly, there must be some comparison/correlation between these devices for sound waves and antennas for radio waves, yet significant differences, since sound waves are mostly longitudinal, while radio waves are mostly transverse.

When you think about it, this device is just a delay and sum, or physical analog of a 2 tap Finite Impulse Response (FIR) digital filter. Stacking a number of them in series would work like a multitap FIR, although with much less precision than an electronic FIR filter.

https://en.wikipedia.org/wiki/Finite_impulse_response

I see no reason why a series of these couldn't widen the stop band notch or make multiple notches. Also since these are passive devices I expect a resonant oscillation would not be a worry, well except for possible loose mounting hardware.

RetroReflection Applications including "Holy Baptism" for your Clothes

"My personal interest would be to amplify and send back to the source, all of the BOOM BOOM thunderous noise in vehicles that can rattle windows from 1/4 mile away." HiTekRedNek

To send waves back to their source technologists use "retroreflection". One simple, but effective form of retroreflection uses "corner retroreflectors". Corner Retroreflectors are three reflective flat surfaces set at 90 degrees to each other. They are meat and potatoes to surveyors in combination with theodolites using light waves instead of sound. There is an array of corner optical retroreflectors sitting on the moon which astronomers use to get precise measurements of the distance from the Earth based lasers to the retroreflector and back and measuring the time it took to make the round trip.

A low frequency sound retroreflector would be need to be quite large in order to be effective. Hard, massive reflectors for sound present significant wind load which could be minimized using this new technology. Sound is far slower than light so returning sonic booms to flying aircraft would blast trailing aircraft rather than the originator of the sound. Blasting sound back at lightning might be interesting since it is mildly plausible that one could implement a lightning oscillator under some conditions. Retroreflected booming music from cars could potentially shred the speaker cones if they are already operating at the margins of their structural strength. One might conceptually contemplate sonic lasers where the two end mirrors have been implemented with this technology. With plastic 3D printing such a laser would need to be operated within the structural strength of the reflectors or it would destroy itself.

One possible application which seems fertile to me is a large closet sized clothes washer in which a small amount of water sprayed onto the clothes on plastic hangars would be agitated within the closet acoustic resonant chamber and fans could carry dirty water droplets away from the clothes through the holy walls(a baptism allusion, all rights reserved) to be collected and discarded just beyond the closet. Your clothes could be slowly washed and dried on a timed basis, ready for you to wear when you wake up in the morning with essentially zero human effort beyond hanging them up the night before. Your nightly cleaned clothes would be pasta sauce sin free without the usual laborious laundrymat exorcism process, HiTekRedLessNek.

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thewildotter

If this could make a 'stealthy helicopter' then it could make a silent wind turbine too.... But I'm finding it hard to imagine where the "pipe" should go on a moving assembly of 'blades'....

I suppose it would require a housing or "cage" around the rotary blades, as we have on some portable household fans but designed for sound capture instead.

Silent helicopters and drones may end up looking like UFO's.

The winning invention will be the acoustic equivalent of a flashlight that shines darkness!

It would be neat to mount one of those on the side of my house, that way when the neighbors are yelling at each other or blasting their boom box, the sound can be reflected back to them, only twice as loud.

I think we need to avoid the tunnel vision. Many manufacturing processes could use a little noise reduction. As far as reflecting it back, why not 180 out of phase? We all have had experience with waves reflecting in phase and how they tend to increase in amplitude. Let's consider this a step to be followed by another. Vibration frequency and energy.

Light Pumped Shock Wave Generator and Gas Analysis Machine

Consider a spherical shell made of these with every axis pointed radially. Pump it with somewhat collimated light sources (LED's or xenon flashtubes) mounted just outside the center of each hole and pointed at the center of the sphere. Blacken the inside surface of the sphere to allow rapid heating of the air just inside the spherical shell. Now pump the resonant chamber with your light sources at some acoustic wavelength matching the radius * N and matching the optimum acoustical reflector frequency.

The spherical shell should reflect the sound with only small losses. If the energy added to the oscillation during each cycle exceeds the losses, a net accumulation of sonic energy should happen. At some point a shock wave will get created at the center of the sphere. Realize that the acoustic energy density near the center of the sphere is amplified by the ratio of the area on the inner surface of the shell over the surface area of an imaginary sphere at the center. This little sphere will manifest with a radius at which there is enough energy density to create a shock wave.

Shock waves travel at speeds different than the acoustic wave creating them and will likely not get reflected very much at the metamaterial at the spherical shell. Measuring their characteristics beyond the sphere can probably tell you things about the makeup of the gasses near the center of the sphere. This Shock Wave Generator might therefore be the basis for a highly sensitive gas analysis machine.

It might also be interesting to attempt to reach energy densities high enough to do water-less sonoluminescence. Using a shell which can also reflect enough shock wave energy back to the center(perhaps a reinforced concrete matrix for the metamaterial acoustic reflectors) might be one way to reflect enough shock wave to get to the higher energy densities required by sonoluminescence.

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thewildotter

Tunable metamaterials in the news today. This design is pretty amazing!

https://phys.org/news/2019-03-electronically-tunable-metasurface-rotates-polarization.html#nRlv