on of them by the brain. Only a few aspects will be outlined here. water and electricity enter the area. Windows can be major points of noise ingress and the best 8.6.1 Location of sounds solution is to have double or even triple-glazing. It is important to realise here that the few millimetres which are appropriate for Several factors are involved in locating a sound, including visual thermal insulation are very little use for acoustic purposes. A mini- clues. However the single most important one is the time of arrival mum of about 20mm is needed for any real effect, while for obser- difference (t) at the two ears, as illustrated in Figure 8.15. vation windows in broadcasting and recording studios the spacing of the sheets of glass should be not less than about 200mm. A further point is that such windows should have the two sheets of glass of Source different thickness, 6mm and 10mm for example. This is to prevent both sheets having the same acoustic resonant frequencies. Good sealing of gaps round doors is important. Some form of magnetic seal, somewhat after the fashion of the seals around refrigerator doors, is generally desirable. 50 mm Rockwool RW3 slabs 50x50 mm timber stud at 600 mm centres 10 mm chipboard or plasterboard Figure 8.13 Typical lightweight partition. (Courtesy Rockwool Ltd.) Figure 8.15 Time of arrival differences 8/14 Sound, speech and hearing Threshold Shift, dB Figure 8.16 Noise masking Clearly there is no time-of-arrival difference for sounds arriving 8.6.2 The Cocktail Party Effect from directly in front of the person. The maximum value of t, for sounds arriving from the extreme left or right, is of the order of a millisecond or less. Most people, in reasonable listening conditions, This term is given to the ability of the brain to concentrate on sounds can detect aurally the shift of a sound source through about 1° from from a particular direction and largely ignore those from other the centre line and is then in the region of 10us. The time-of-arrival directions. The effect is easily