jorsh.xyz wiki

acoustic metamaterials

tags: [geometry] [3d-printing]

background/motivation

Acoustic metamaterials are like optical lenses, but for sound. But also more different than that:

Acoustic metamaterials can do various things, like reflect sound, block sound, amplify sound (I think?), but I am pretty much only interested in blocking sound, because I like peace and quiet. So the rest of this page is written assuming that’s all that acoustic metamaterials are for.

theory (as I understand it)

I have seen roughly three types of acoustic metamaterials: pockets, resonators, and half-wavelength pipes.

half-wavelength pipes

From the paper “Ultra-open acoustic metamaterial silencer based on Fano-like interference”, among others.

I think this design can only be used to make a notch filter? (to block a specific frequency of sound) The basic idea is that the sound has two paths through the material, and the difference between the two paths is half of the wavelength of the sound that you want to block. Thus, destructive interference happens.

pro:

two issues with this approach:

resonators

“A lightweight vibro-acoustic metamaterial demonstrator: Numerical and experimental investigation” discusses a design of this type.

I think the idea is that it is anti-resonant at the frequencies that you want to block.

pro:

con:

pockets

Such as in “3D Single-port Labyrinthine Acoustic Metamaterial” and “Broadband fractal acoustic metamaterials for low-frequency sound attenuation”

Seems to be a similar concept to the half-wavelength pipes, but this time the pipes have only one end open. So they’re like long, deep holes, or channels, or something. (It’s got to be much longer than it is wide, because the width must be much smaller than the sound wavelength in order for it to function as a metamaterial)

mostly the sam pros/cons as the half-wavelength pipes concept, except this design is generally non-ventilated, so you don’t have to worry about airflow I guess.

my attempts at 3d printing acoustic metamaterials

OnShape cad link

Motivation: I’ve got some 24V fans that I run at 5V in some projects, for very quiet ventilation. But they still make a little bit of noise, around 400Hz and 8-10kHz. here’s a spectrum graph of the noise

So, of course, I want to block this noise. I based my approach on the design presented in “Ultra-open acoustic metamaterial silencer based on Fano-like interference”. I made some designs that looked quite impressive, but nothing really worked.

probable problems:

further reading

AKA link dump of various papers and other reference material that I have found