Central to a discussion of room design is the different effects of sound absorption. Basically, sound energy is 'absorbed' when it is converted to another form of energy. In most cases, this takes the form of conversion to heat and, to a much lesser extent, kinetic energy. Conversion to heat results from the actions of friction as air molecules interact with other materials and the resistance of various materials to movement and deformation.

Obviously the amount of heat generated is minimal as the amount of sound energy is also quite small if you consider it in Watts (see the section on typical sound power levels).

NOTE: It is important to point out that this use of the word absorption refers to the amount of sound energy actually converted to heat when sound energy impacts a boundary. The absorption coefficient (a), however, refers to all the sound energy that is not reflected from a boundary.

This means that the absorption coefficient also includes the transmitted as well as the absorbed component.

Different Types of Absorption and Absorbers

Porous Absorbers

Porous absorbers, such as mineral wool, fibreboard or plastic foams, have an open pore structure. Conversion to heat is produced by friction when vibrating air molecules are forced through the pores and interact with the pore walls. These are effective primarily for high frequencies with short wavelengths. In a sound wave which is incident on a rigid wall, the maximum particle velocities (amplitudes) occur at 1/4 and 3/4 of the wavelength. If the thickness of the absorber is much less than one quarter of the wavelength, they will have little effect.

Figure 1 - The relationship between absorber depth and frequency response. You can drag the frequency selector in the graph or the actual absorber in the material cross-section.

A porous sheet placed some distance away from a solid wall will have almost the same effect as a thicker absorber. The maximum effect is achieved when the distance to the wall surface from the centre of the absorber equals 1/4 the wavelength and is restricted to a comparatively narrow frequency band. This is because the maximum particle velocities of both the incident and reflected waves will occur within the porous material.

Membrane Absorbers

Membrane absorbers may be either flexible sheets stretched over supports or rigid panels, both mounted at some distance from the front of a solid wall. Conversion to heat takes place through the resistance of the membrane to rapid flexing and to the resistance of the enclosed air to compression.

They will be most effective at their resonant frequency which depends upon their surface density (M) and the width of the enclosed space (b).

where

F_r = The resonant frequency (Hz),
M = The mass of the panel per m² (kg/m²) and
b = width or depth of the air gap (m).

Figure 2a and b - The frequency response of membrane absorbers.

In practice, the method of fixing and the stiffness of the panels will also have some effect as the panel itself will tend to vibrate. This means that it will act as a sound radiator so it is rare to find such a system with an effective absorption coefficient greater than 0.5. Membrane absorbers are most effective at low frequencies which is really why they are used.

Cavity Absorbers

Also known as Helmholtz absorbers, these are simply air containers with a narrow neck. The air within the cavity has a spring-like effect at the particular resonant frequency of the enclosed air volume.

where

S = the cross sectional area of the neck(m²),
V = the volume of the cavity (m³) and
L = the length of the neck (m).

Figure 3 - The frequency response of cavity absorbers.

These absorbers give a very high absorption coefficient in a very narrow frequency band. This can be broadened slightly by placing a porous material lining the inside of the cavity.

Perforated Panel Absorbers

All three of these mechanisms are combined. The panel itself may be plywood, hardboard, plasterboard or metal, and may also act as a membrane absorber. The perforations, holes or slots with the air space behind them will act as multiple cavity resonators, improved with some porous absorber. Most of the broad spectrum commercially available acoustic materials fall into this category.

Figure 4 - The frequency response of perforated panel absorbers.

NOTE: From wave theory, it can be shown that absorbing material is most effective when positioned in the corners of a room because it is there that every normal mode has a pressure maximum.