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Scientists Have Discovered a Shape That Blocks All Sound (fastcompany.com)
Scientists have developed an "acoustic meta-material" that can catch certain frequencies passing through the air and reflect them back toward their source. When a loudspeaker was placed into one end of a PVC pipe with a 3D-printed ring of the metamaterial, the ring "cut 94% of the sound blasting from the speaker, enough to make it inaudible to the human ear," reports Fast Company. From the report: Typical acoustic paneling works differently, absorbing sound and turning the vibrations into heat. But what's particularly trippy is that this muffler is completely open. Air and light can travel through it -- just sound cannot. The implications for architecture and interior design are remarkable, because these metamaterials could be applied to the built environment in many different ways. For instance, they could be stacked to build soundproof yet transparent walls. Cubicles will never be the same.
The researchers also believe that HVAC systems could be fitted with these silencers, and drones could have their turbines muted with such rings. Even in MRI machines, which can be harrowingly loud for patients trapped in a small space, could be quieted. There's really no limit to the possibilities, but it does sound like these silencers will need to be tailored to circumstance. "The idea is that we can now mathematically design an object that can blocks the sounds of anything," says Boston University professor Xin Zhang, in a press release. You can see a demo of the noise cancellation device here.
The researchers also believe that HVAC systems could be fitted with these silencers, and drones could have their turbines muted with such rings. Even in MRI machines, which can be harrowingly loud for patients trapped in a small space, could be quieted. There's really no limit to the possibilities, but it does sound like these silencers will need to be tailored to circumstance. "The idea is that we can now mathematically design an object that can blocks the sounds of anything," says Boston University professor Xin Zhang, in a press release. You can see a demo of the noise cancellation device here.
rtfa: yes it allows air flow.
It's a band reject filter, been used in acoustics for around 100 years. A narrow-band Helmholtz radiator that cuts ~1/30th of an octave in bandwidth. So it's great for a specific tone - but not broadband. And the dimensions of the elements (neck diameter, neck length, size of constrained volume) are proportional to wavelength, so what works in the demo at 1 kHz is massive at 100 Hz - and beyond house-sized at 20 Hz. This is just someone going "oh wow they can notch out a single frequency think about the impact!" when acoustics NVH guys have been doing it for 10 decades...
Browsing at +1 - no ACs, I ignore their posts. So refreshing!
*sigh*
Since the editor and submitter didn't do it,
here is the BU research alert, which includes an image of the new material, and
here is a link to the published paper, from which you can get a DOI number if you want to read about their work.
The acoustic suppressor looks like thick a 3-d printed bushing.
I hope you aren't being snarky back to the AC (who deserves it) so I don't subsequently look like a moron in what follows, but on the off chance you aren't... The exhaust from a piston engine consists of high pressure, very hot bursts of gas at the exhaust ports. Hot, high pressure, and bursty going into relatively cool outside air -- perfect combination to create a lot of sound. Exhaust manifolds and mufflers combine and smooth out the exhaust bursts to create a smoother flowing, more uniform in time exhaust flow resulting in less noise. There are all kinds of pressure and flow reflections in a good exhaust system to help scavenge the exhaust gases during the exhaust stroke and reduce noise. Even a smooth flow is loud if it is fast and hot enough -- as exemplified by jet engine exhaust, which (I think) makes its noise from turbulent mixing in itself and with the outside air.
Here's the paper published in Phys. Rev. B 99 in case anyone is interested: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.99.024302
Only for extremely small values of "All".
Scientists Have Discovered a Shape That Blocks All Sound
No they haven't. At least, not according to the linked video in the article. It quieted it down quite a bit, but I could still hear it EASILY with the ring in place. It wasn't inaudible, not even CLOSE. Since they didn't put a ring of the same dimensions and material but in the shape of a simple, smooth, right, regular cylinder with a right, regular, cylindrical hole through it, centered and along the same axis as the material cylinder, (just like the "magic" ring but without the magic shape on the end,) while the speaker was generating the same sound at the same volume, to COMPARE it with. At least, that is, not in THAT video.
So this really is a non-story. Even if it quiets it quite a bit, it also occupies a LOT of the space, which RESTRICTS airflow, and likely adds a good bit of weight. (Imagine how big, thick, and therefore HEAVY it would have to be to fit the inner diameter of a big turbofan engine on a jet aircraft!) Then the output would need one too, and I understand the exhaust is pretty warm on one of those things, or at least can be. That may cause additional engineering challenges and cost, either of which could easily end up being insurmountable.
By the way, reflecting sound energy doesn't DESTROY it. It's ENERGY. Even if it DID work and DIDN'T add too much weight, and DIDN'T occlude the flow of air into and out of the engine to an extent that makes the plane unable even to take off under its own power, AND heat from the engine's exhaust wasn't a problem, etc., then the SOUND ENERGY being reflected INTO the engine will cause it to heat up even more. What are you going to do with all that extra heat? It's a LOT of noise, (and therefore, likely, a LOT of HEAT). Oh, and you're going to need a pair of these rings for EACH engine. How much weight will the required additional cooling cost? So you obviously will need much bigger, much more powerful (and noisier) engines. That will necessitate even bigger, thicker, sturdier rings, which will cause additional heat buildup... ETC.
If you're wondering how MUCH heat, well, it's actually pretty easy to work it out. Have you ever stood anywhere near a big turbofan jet engine, operating at, or at least near full power? Now imagine having to reproduce that sound, that same volume of noise, the same intensity... and while producing the same note, with a SPEAKER system. How big of an amplifier, in terms of Watts, would you need to do that? THAT'S how much sound power, (assuming you are somehow reflecting ALL of it,) we're talking about here. That much power is going to get trapped at BOTH ends of this hypothetical muffled engine. Where TF is it going to GO?
I see this tech as largely an interesting oddity, and will most likely come to nothing of any particular significance whatsoever in the world. Now maybe it MIGHT have some terrestrial applications, but don't look for this to be on aircraft anytime soon... that's my prediction.
Our reign has gone on long enough. Indeed. Summon the meteors.