New muffler tech?

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New muffler tech?

Postby MitchC635 » Sat Mar 16, 2019 7:59 pm

Boston University researchers just came up with a new shape capable of reducing sound by up to 94%. The mathematically designed, 3D-printed acoustic metamaterial is shaped in such a way that it sends incoming sounds back to where they came from.

Details at: https://www.fastcompany.com/90316833/sc ... co-workers?

Because the basic concept is a specially shaped annular ring that is digitally printed, it appears that it could be turned into some amazing mufflers tuned to sound exactly as desired. High temp capable materials and carbon build up would be challenges, but with 94% reduction as the baseline there appears to be significant room to accommodate some losses in efficiency.

GWR's skunkworks for exhaust could be a perfect place to develop exhaust applications of this tech. Much wider frequency range possible than with a Helmholtz chamber. An easy test might be as a tip insert for a race single. Potential is there to make it very streetable with a simple insert.
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Re: New muffler tech?

Postby Chuck H » Wed Mar 20, 2019 7:31 am

I really wish there was a little more technical information in that article. It seems really cool, but they're not at all specific on whether it's a broad band noise cancellation, or narrow band. And the graph doesn't really show anything useful without knowing the spectral content of the noise that the speaker was generating. If it was a pure tone that the muffler was tuned for, it's not really a breakthrough. If it was broadband noise, then it's amazing.

Looking at the plastic insert they show at the top of the article, I suspect that it works a lot like a Corsa muffler, in that it's basically a quarter wave resonator packaged in a really small space. If those spiral channels in the insert are open at the top and sealed at the bottom, then they will function as quarter wave resonators, reflecting some of the sound waves back to the source. And at the right tuned frequency, the waves will be 180 degrees out of phase and will cancel. If it really is a quarter wave resonator, it's going to be fairly narrow band, just like the J-pipe resonators some of the truck guys use to kill droning on their V8s.

The real trick would be if you can print something like that out of a high enough temperature material to handle exhaust heat. You could make a bigger version that looks more like a conventional round muffler, and then you could make each of the spiral channels slightly different in length (stagger the plugs at the bottoms of the channels). By doing that, you could probably match or even exceed the bandwidth of a Helmholtz resonator. You would probably want to have the openings into the channels be holes in the thru-tube rather than on the ends, as well. But it would definitely have potential to custom tune the sound to be almost anything you want.

EDIT: Watched the embedded video with the sound on, finally. It's definitely not working on broad band noise like exhaust noise. And it was high enough frequency that the quarter wave resonator could be fairly short. Still a cool new take on the resonator geometry, though, that packages better than something like a J-pipe.
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Re: New muffler tech?

Postby njaremka » Wed Mar 20, 2019 8:46 am

My take on that device is that the technology isn't suited well for an exhaust system. Seems like air flow through the device would alter the characteristics dramatically. In a typical speaker system, you have sound reverberation, but not much air flow. In an exhaust system, you have much more air flow along with that sound reverberation.

Of course, I'm not an acoustic engineer...
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