are 34,68, 102 Hz etc., and this can result in a 'boominess' Measurement of T with suitable apparatus is almost certainly which may, in extreme cases, impair intelligibility. going to give better results with much less tedious calculation or With larger spacings, say I = 40m, the lowest frequencies of observation. A usual technique is to produce a warble tone from a resonance are approximately 4.25, 8.5, 12.75, 17 Hz, etc. The lowest loudspeaker, the warble being used to minimise excitation of par- frequencies are below the audible range and the higher ones are ticular acoustic resonances, and a microphone picks up the decaying sufficiently close together for their effect to be probably less objec- sound when the warble tone is cut. A microprocessor device can be tionable. used to calculate T Measurements are usually made at a number A formula due to Lord Rayleigh (Equation 8.20) takes account of of points in the room, and at several centre frequencies of the the fact that a room will normally have at least three pairs of parallel warble. surfaces. SO that not only are there potential resonances between T60 is at its most crucial in concert halls and recording and each pair but there will also be diagonal resonances between oppo- broadcasting studios. In other less critical circumstances it is still site edges and between opposite corners, where p, q and r are important as a major parameter in deciding intelligibility of speech. integers and I, W and h are the room dimensions. Typical values are given in Table 8.8. Table 8.8 Typical values of T60 8.5.2 Acoustic resonances Resonances can occur between two parallel surfaces when their Location Typical T60 (seconds) spacing is equal to an odd number of half-wavelengths. In these conditions there is a pressure antinode (maximum) at the surface Lecture rooms and similar auditoria 0.5 - 1.0 with one or more nodes in the region between, as in Figure 8.10. Theatres 1.0 (approx.) Such maxima and minima are known as standing waves. Average domestic sitting room The frequen