nd separate sounds but an awareness of a time gap nevertheless can delay for the Haas Effect to occur. manifest itself in a different way: a mental assessment of the proximity of reflecting surfaces. Thus a person with normal hearing can generally judge the approximate size of a room, even without 8.6.4 visual clues, simply by speaking in it. In such cases the nearest wall may be no more than two or three metres away SO that the time Brief mention may be made of the subjective effect of repetition of intervals are of the order of 10 to 15mS or even less. a sound. Echo is defined as a repetition when there is a perceptible time interval between the first and second sound, it being assumed that the repeat has essentially the same characteristics as the first 8.6.5 Masking sound. For the brain to recognise the existence of an echo there must be a time gap of more than 30 to 40mS between the first arrival and the If a steady sound is detected and then another, at a different fre- echo. Time intervals less than this are not perceived as two distinct quency, is started from a low level and increased, there may come a point where the first sound is no longer detected. It is said to be masked. In general a high frequency sound can be easily masked by Table 8.11 Time delay and level difference relationship for a low frequency but low frequency masking by a high frequency is Haas effect much more difficult. Figure 8.16 shows the effect of an 800Hz masking tone on other frequencies. The numbers by each curve give the level of the 800Hz tone above Time delay (ms) Level difference (dB) the threshold of audibility while the vertical axis shows the thre- shold shift, the difference in level between the normal threshold of audibility and the level at which the otherwise masked tone becomes audible. It will be noticed that the effect on frequencies greater than, in this case 800Hz, is considerable but there is little or no effect on lower frequencies. In contrast Figures 8.17 and 8.18 show the effect of masking tones of frequenci