cies of resonance are then given by Equation 8.19, where 1 is the spacing between the parallel surfaces, C is the velocity Large cathedral (low frequencies) 10-15 of sound and n =1, 2, 3 etc. Radio talks studio Large television studio f = nc 21 (8.19) Large concert hall 8/12 Sound, speech and hearing the whole structure. The problem is often compounded because of many parallel (flanking) paths. Figure 8.11 illustrates this. noise N = NODE A = ANTINODE Figure 8.10 Standing waves Figure 8.11 Transmission of vibrations in a building Reduction of structure-borne sound is generally very difficult and/or expensive. The easiest solution is to reduce the noise at source, even by stopping the offending equipment if this is possible. Fitting flexible anti-vibration mounts between the machine and the ground may be very effective if properly done. Broadcast and recording studios, unless well away from any sources of vibration, 8.5.3 Background noise criteria are frequently isolated from the structure of the building by resilient supports below the studio floor, but to be effective this also means Table 8.9 lists typical acceptable background noise levels. that the walls and ceiling must be similarly isolated from the main structure. Flexible tie-rods, for example, are needed in cavity walls. Table 8.9 Acceptable background noise levels Airborne sound is generally easier to control. In the case of walls the most effective treatment is mass; the heavier the partition the better the sound isolation. Figure 8.12 shows what is often called the 'Mass Law', relating (approximately) sound reduction with mass Location per unit area of partition. It will be noted that a single leaf brick wall Radio studio (drama) will provide a sound reduction of about 45 dB, assuming that there TV studio are no flanking paths. It should also be noted that doubling the mass Hospital operating theatre Hotel corridor Open plan office Public library Lecture room 8.5.4 Sound isolation In deciding how much noise needs to be reduced it is clearly necessary 