uare. Isolated troublesome resonances, particularly in the low frequen- In any room or enclosure a sound created within it will undergo cies, can sometimes be treated with some form of Helmholtz reson- repeated reflections at the various surfaces, wall, floor, ceiling, etc., ator, represented in Figure 8.8. losing energy at each reflection. Some energy will also be lost in travelling through the air but this is not usually significant below a frequency of about 4kHz and even then is often not important in small rooms. Such multiple-reflected sound energy is known as the reverberant sound. Reverberation Time (T60) is defined as the time taken for the reverberant sound in a room to decay through 60dB. In practice this can be regarded as being roughly the time taken for a fairly loud sound to decay to inaudibility. T can be calculated to a reasonable accuracy by a formula devised by WC Sabine, as in Equation 8.15, where V is the volume of the room and S is the total absorption in the room in Sabines. Too = 0.16 V (8.15) The Sabine is an unit of sound absorption and is equivalent to 1m2 2 Figure 8.8 Helmholtz resonator of 100% sound absorber. S is found from Equation 8.16, where S is the area of a material having an absorption coefficient The action, briefly, is that the mass of air in the neck m vibrates (8.16) against the compliance, Co, provided by the air in the main body of the device. The device may be compared with a weight oscillating Absorption coefficient is defined as the fraction (or percentage) of vertically on the end of a coiled spring. The frequency of resonance incident sound energy absorbed by the material. Some typical is given approximately by Equation 8.17, where S the cross-sec- values of absorption coefficient are given in Table 8.7. tional area of the neck, 1 is its length and V is the volume of the body. Table 8.7 Examples of absorption coefficient 1-22(iv)" (8.17) Material 125Hz 500Hz 4000Hz It should be noted that a Helmholtz resonator absorbs or re- Brickwork radiates sound depending on th