"acoustic tile" - referencing Master Handbook of Acoustics | John Sayers’ Recording Studio Design Forum archive

Hi long time reader here you don't necessarily need to have read the book to answer this question I've been slowly making my way through the master handbook of acoustics by Everest. I'm sure a lot of you have read it and may even think of it in biblical proportions pages 179 - 181 (sixth edition) shows a great example of reverb times of an untreated room, and then treating it. It talks about targetting the 1 kHz range first and foremost using an "acoustic tile" 3/4 in thick. Figure 11-23 then shows the absorption co-effs and Sb of the acoustic tiles, and it has sabins at 1 kHz of 285, Sb (2 kHz) of 248, Sb (4 Khz) of 217. In other words, the absorption of this tile is most effective at 1 kHz and less so above and below these frequencies. Now my understanding of acoustic absorbers, tiles, panels, etc. is that they will attenuate frequencies similar to a low pass filter, and the frequency around the "LP filter" is determined by how thick the absorptive material is (thickness = 1/4 wavelength of target frequency?). So for a panel that would effectively attenuate 1 kHz, it should (even more effectively) attenuate 4 kHz and above. I'm guessing I have this concept wrong in my mind. If anyone could help me out with my understanding I would greatly appreciate it thankyou!

Hi walleye, and welcome! :) You are viewing acoustic tile as if it were purely a porous absorber, but it isn't. It is partly porous, but also partly reflective. The face of an acoustic tile is actually fairly hard, and therefore reflect high frequencies quite well. The higher the frequency, the better it is reflected. To illustrate this concept: think of a large superchunk bass trap in the corner of a room. It is purely porous, and thus absorbs across the entire spectrum (with varying degrees of efficiency). So it does a decent job of absorbing bass energy, but it also does a stupendous job of absorbing highs, which is not good. It's a BASS trap, so it is not supposed to absorb highs! And since most rooms have too much high frequency absorption anyway, that's not good. The usual solution is to place plastic in front of the superchunk, to reflect the highs back while still allowing the lows through. You can adjust the frequency range that you want reflected back, by adjusting the thickness of the plastic. The thicker the plastic, the lower the cut-off frequency. If you need to go really low, then you replace the plastic with a harder, more rigid membrane, such as thin plywood or MDF. So you end up with a mostly porous "thing" that has a reflective front surface. Just like an acoustic tile! Except on a larger scale. In both cases, if you look at a graph of the coefficients of absorption at various frequencies, the graph will start of with low absorption for very low frequencies, rise to a peak where the absorption is at its maximum, then drop off again in the higher frequencies, where the face is reflective. So in electrical terms it is not like a shelving filter or low-pass filter, but more like a bandpass filter. It is not pure inductance or pure capacitance, but rather a tuned circuit, with both capacitance and also inductance. The front face is like the inductor, in that it allows lows to pass but not highs, and the rest of the unit is like the capacitor in the sense that it allows highs to pass but not lows. (sort of!). Together, they form a tuned band-pass filter. Then you can think of the absorber itself as being resistance. So you basically have a tuned LCRcircuit... Now, with acoustic tile (and also some types of porous absorber devices...) you also have the factor of the air gap behind it: that too has an effect on the absorption frequency...

the frequency around the "LP filter" is determined by how thick the absorptive material is (thickness = 1/4 wavelength of target frequency?)

Sort of, but not really. 1/4 wave gives you maximum effect for a wave arriving at normal incidence to the surface, but not for waves arriving at other angles. You can actually have a very much greater effect on non-normal incident waves, since they have to travel through much greater effective thickness of absorber. So the effect changes with angle, but also at very high angles, there's the problem of reflection again, and also possible refraction, and diffraction, and all those other fun things. At extreme angles, the effect drops back down again, for most porous absorbers. Unless the surface is sculpted, which adds a whole new kettle of fish to the stew...

So for a panel that would effectively attenuate 1 kHz, it should (even more effectively) attenuate 4 kHz and above.

Only if it is purely porous absorption, and only for normally-incident sound. For real-world materials, there's a combination of absorption and reflection at the interface surface, and real-world sound waves are not absolutely always 100% normally incident. Far from it! So the actual response of most materials in typical applications is rather different from the perfect picture painted by simple charts of coefficients of absorption. In fact, it turns out that you can get very good bass trapping from a porous absorber that is only 7% the thickness of the wavelength for normally incident sound, and 3.5% for diffuse sound with random incidence. That's why superchunks work down to very low frequencies, even when they are not 15 feet thick! Two to three feet is plenty... - Stuart -