e amount of damping (i.e. acoustic Rough friction) in the neck. Gauze across the opening has been found to be concrete effective. 0.009 The Sabine formula given above is reasonably accurate provided that: (typically) Breeze 1. The absorptive material is fairly evenly dispersed. blocks 2. The enclosure does not differ greatly in the ratio of its dimen- (unplastered) sions from the conventional (corridors are thus excluded). Heavy 3. The mean absorption coefficient does not approach unity. curtains Where there is ver heavy absorption a more accurate result is Thick carpet given by a formula due to Eyring, as in Equation 8.18, where a' is the mean absorption coefficient. mineral wool (8.18) mineral wool A further refinement is to make allowance for the absorption by the air which, as stated above, varies with frequency and also with Proprietary materials, such as the mineral wool quoted above, relative humidity. Figure 8.9 gives the relationship between T60' may be retained behind perforated hardboard for protection and also volume and absorption units (Sabines), based upon the Eyring to make them visually acceptable. There can be significant vari- formula. ations in the absorption characteristics depending upon the percent- Calculation of Two even with the aid of diagrams like Figure 8.9 age of perforation. is tedious and apt to be at best only approximate, if only because Acoustics 8/11 ABSORPTION units 5 91012141618 25.10 10.5% 60. 08090100 110 4.136, 100 500 40.000 VOLUME ROOM Figure 8.9 Determination of T60 (Courtesy Klark-Teknik plc) there can be considerable uncertainties about the actual absorption It will be quickly appreciated that with relatively small spacings coefficients of materials present. Such uncertainties are likely to be (1 of the order of 2 to 10m) the lowest resonance will occur well into made worse because diffraction effects can make the effective area the audible range. For example, taking I as 5m the resonant frequen- of a material greater than its physical dimensions would indicate. cies 