Room tuning voodoo
Incidentally, the absorption coefficient tables/graphs are the standard way for acoustics people to compare the efficiency of various absorption methods (not scattering, absorption) according to thickness and frequency. This data is widely, internationally used when designing homes, public places, concert halls and studios. Even if you don't speak English, you can read .... 125Hz, 0.85 .... and that tells you everything you need, except for cost and cosmetics.
From time to time various 'room tuning' voodoo gadgets appear on the market. They usually have a romantic sales story. They include mystical bells the size of an egg, magic crystals, cosmic ray neutralisers and the rest. The fact is that the absorption coefficient of these (hard) devices is certain to be zero so they cannot have any meaningful influence on the sound in the room. Why do people throw themselves at these products when inspection of just two numbers - absorption coefficient (or percentage absorption, the same thing) and frequency can comprehensively describe their entire acoustic (non-) performance?
Professional acousticians think that the audiophile who is suckered into believing in such self-evidently non-functional products is a complete idiot. If these tweaky products actually worked, acousticians would be able to save their architect clients the cost and weight (and cosmetic impact) of Rockwool and could transform the acoustics of a public lavatory into that of a great concert hall for very little time, money and effort.
Simple, common sense rule of thumb:
- To absorb sound the absorber must be fibrous, or a combination of a (semi-rigid) skin + fibrous + air gap
- The thicker it is the more sound it will absorb.
- The more dense it is the more low frequencies it will absorb (but it may be more reflective at higher frequencies)
Below is an article from a studio magazine which gives a hands-on guide to DIY absorbers. NOTE! In this article the absorber panels do not cover much of the total surface area of the walls. This means that their absorptive effectiveness will not be great in the lower frequencies. Also note that the Rockwool used is only 30mm thick. If you look-up the absorption coefficient of 30mm Rockwool again here, the second line in the first table (703 plain 25mm is the nearest to 30mm), then following along to the 125Hz column gives a figure of 0.11 - virtually useless - compared with the previous 0.85 for the 100mm thick material (previous post) at 125Hz. With thin absorber material you just cannot absorb low frequencies - that's physics.
Alan A. Shaw
Harbeth Audio UK