Wikipedia:Reference desk/Archives/Science/2019 May 11

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May 11

How do those sound absorbers with small holes or slots work?

I've found quite a bit of material on sound absorbers, but very little on how they actually work. The kind of absorbers that use pyramids or triangular wedges make intuitive sense. They can either be thought of reflecting sound repeatedly towards the deepest part of each v-shaped valley, losing energy along the way. Or for low frequencies, perhaps can be thought of as creating a gradual impedance gradient, promoting absorption rather than reflection.

But the kind with a regular grid of small holes (wood absorber panels) or irregular slots (like bass traps) don't make much sense. With those, it seems as if most of the surface (90%+) would still cause reflection and whatever happens in the holes or slots would only have a small effect. But they somehow seem to still be effective. What's going on and how is the sound actually being dampened with these?

Are there situations where one kind of absorber is more effective than the other kind of design?

See Absorption (acoustics). When a panel is perforated with holes spaced much less than a sound wavelength, the Acoustic impedance of the panel is intermediate between that of the panel material and air. Sizing of the perforations thus allows the impedance of the panel (that varies with frequency and direction) to be controlled by design. DroneB (talk) 13:19, 11 May 2019 (UTC)

Is it true for sound as it is for light that simulating the geometric mean of acoustic impedances in an intermediate "layer" is the optimal strategy to mitigate reflections? The characteristic acoustic specific impedance of air is around 410 at room temperature. The acoustic impedance of pressed wood I estimate at around 1.1 million based on density and speed of sound from tables I found (could easily be off by a factor 10 here). If the idea was to simulate a material with impedance of the geometric mean of these two numbers then it would be closer to leaving 2% of the material (small flat spikes protruding from the panels) than taking away a few % of the material (small holes in the panel). My math or assumptions could be flawed, but it seems like the small percentage of material removal one sees with typical panels don't make much sense for optimal acoustic absorption. Could anyone point me to some resources to help clarify? It's a rather practical problem and knowing how to optimally build simple absorbers would be super useful. 196.210.82.111 (talk) 18:54, 11 May 2019 (UTC) Original Poster

Check out Helmholtz resonator.Greglocock (talk) 05:17, 12 May 2019 (UTC)

Global warming and geologic activity

I was musing about the known effect of global warming on the day length due to angular momentum. I'm thinking that if the top of an ocean increases in height by 1 m and the bottom of the crust is 10 km down (actually that doesn't matter), and the water is 1/5 as dense as the rock, then assuming currents eventually get in equilibrium with the ground under but that the crust moves around separately over the mantle the weighted average position of the crust goes out by 0.1 m of 6000 km, and to keep a constant rotation * distance the rotation rate of the crust should decrease by 1.6E-8. 60*60*24*365*100 = 3.1E9, so this would be 98 seconds a century, which is much more than the 0.5 ms/century given here [1] - I assume they took the Earth as a rigid object??? I don't know that's false, but it would be surprising, given plate tectonics.

Anyway, if you can alter the position of the Earth by something near a second per year with global warming (and the odd practice of leap seconds makes me think it's not so implausible) then every year a plate on the equator would tend to move, relative to the mantle, by 40000 km /(24*60*60) = 460 m. But ... not everywhere can the crust move relative to the mantle - in places there are subduction zones linking parts of the crust to deep roots in the mantle. Maybe the subduction zones are good tethers and the crust doesn't change rotation by that outlandish amount. But then there is stress on those sections, which might change the pressure, might break something, might make a volcano go off? Is that plausible or just crazy? I'm no geologist! Wnt (talk) 20:42, 11 May 2019 (UTC)