urves. Hearing 8/7 120 phons 100 200 1 10k Frequency (Hz) Figure 8.4 Response of the ear A further unit of loudness is the Sone, although this seems to have levels. However it has been found that, regardless of the sound level, found limited favour. It is intended to reflect the fact that a 10dB the A network gives measurements which correlate reasonably well change in level produces in general an approximate doubling or with subjective assessments of loudness. Measurements made in halving of loudness, as mentioned above. For P phons the loudness this way are given in dBA. This is sometimes written dB(A) but the in sones (S) is given by Equation 8.13. former unit, dBA, is generally preferred. Table 8.4 lists some typical measurements. S 2 (P-40)/10 (8.13) Thus a loudness of 40 phons corresponds to 1 sone, 50 phons to 2 Table 8.4 Typical dBA measurements sones, etc. 8.3.4 Loudness measurement Sound Jet aircraft at 150m As has been stated above this is not strictly possible because loud- Chain saw at 1m ness is a subjective phenomenon. Further, the use of a microphone, amplifier and suitably calibrated meter would give erroneous indi- Loud motor horn at 5m cations because such a system would make no allowance for the Busy street variation in the ear's response with frequency. However the use of appropriate weighting networks having frequency responses ap- Fairly busy road (at roadside) proximating to the response of the ear means that fairly good Radio or TV set (set to loud) at 2m correlations exist between such observations and perceived loud- ness. The most commonly used frequency weighting network is Speech at 1m known as the A network. This matches roughly the 40 phon curve Inside fairly quite car and was originally intended to be used only with fairly quiet sounds, other networks (B and C) being designed to match higher sound Inside quite house 30 40 8/8 Sound, speech and hearing 8.3.5 Hearing impairment 1. L90; the noise level in dBA which is exceeded for 90% of the time. It is well known that exposure to ex