In Part 5 of this series, I discussed how the sound absorption of a material or product is measured, so this time, I thought it might be interesting to see how absorption coefficients vary between materials and what absorbs what.
There are essentially three ways sound is absorbed. However, in the final analysis, it is the conversion of acoustic energy into heat that reduces the energy in a sound wave and so absorbs the sound. The first type of absorber, and probably what we all think of as a sound-absorbing material, is the porous or dissipative absorber. These tend to be soft or fibrous materials that reduce the energy of a sound wave as it passes through them, by providing a resistance to the passage of the sound wave. Typical materials include carpets, drapes, padded seats, glass fiber/mineral wool and acoustic foams. These materials generally absorb sound at mid to high frequencies, from typically around 500Hz and upwards.
The second type of absorber is the panel or membrane absorber. This vibrates or moves under the influence of a sound wave. The energy taken to move the mass of the membrane reduces the energy of the sound wave. This type of absorption is provided by many building elements and structures, such as plasterboard ceilings and walls, window panes and glazing, and almost any thin paneling (particularly if it has an airspace behind it). Even larger building elements can and do absorb some sound. Panel absorbers naturally absorb sound at low frequencies, e.g., from 125Hz to 250Hz, although some limited absorption may occur up to 500Hz. As compared to porous or resistive absorption, membrane absorption is pretty inefficient, as shown by their much lower absorption coefficients. Figure 1 compares a number of common materials that are typical of these two types of absorption mechanism.
As can be seen from Figure 1, most materials do not absorb well at low frequencies, which is why many rooms and buildings have an uneven Reverberation Time (RT) characteristic that is often greater at low frequencies than it is at high frequencies. To absorb more sound at low frequencies requires either thicker materials or the use of specially tuned absorbers. These can include Helmholtz resonators, which are a member of the family of reactive absorbers…the third type of absorption mechanism.
The effect that the thickness of a material can have on a material’s sound-absorbing properties is shown in Figure 2, where I have plotted the sound absorption for four thicknesses of glass wool/mineral wool (16kg/m3). As can be seen, the thicker the material, the better it absorbs, not only in terms of its overall value (efficiency), but also in terms of its low-frequency performance.
Altering the density of a material can also be used to change its absorption properties. This is shown in Figure 3, where I have plotted the absorption for two materials having the same thickness but different densities.
Most reference information and many product data sheets provide sound absorption data in terms of octave band values, usually over the six-octave range of 125Hz to 4kHz that, for most applications, is absolutely fine. However, when building rooms where a more precise assessment is required (e.g., recording studios or sound control rooms), more precision and 1/3 octave information is required. Furthermore, a number of the available acoustic prediction programs require 1/3 octave data. Although the current absorption standards (see last month’s column) require absorption data to be measured on a…