cessively loud sounds can lead 2. L50; the noise level which is exceeded for 50% of the time. to hearing damage. The subject is a complicated one and can be the 3. L10; the noise level which is exceeded for 10% of the time. matter of considerable debate. The following is a summary of what is generally accepted in the UK and most of Europe. (In the UK the Health and Safety Executive (HSE) issue specific recommenda- Speech and music tions.) Below 90dBA there is probably little serious risk of hearing damage, although in industrial situations some type of ear defenders 8.4.1 Components of sounds should be worn for levels above 85dBA. Above 90dBA permissible exposures depend on the sound level and the duration per day as A sound is characterised, at least in the human ear/brain system, by given in Table 8.5. various parameters. One, loudness, has been outlined above. Two others are pitch and timbre. Table 8.5 Permissible exposure times above 90dBA Pitch is generally described, rather unsatisfactorily, as the position of a note in the musical scale. It is, like loudness, a subjective phenomenon, but in technical terms it is permissible to regard pitch Hours as being very similar to frequency. The difference arises because the pitch of a sound can vary with loudness. If a pure tone from an oscillator is fed into a loudspeaker and the level is abruptly changed by about 20dB, most listeners will experience a change in the pitch, upwards or downwards depending on the original frequency and also depending on the individual person. The higher the frequency of the sound the higher the pitch. A portion of a keyboard, as shown in Figure 8.5 can be used to aid explanation. The letters given to different notes correspond, at least ideally, to the frequencies given in Table 8.6. The note A is adopted as a standard and is known as International A. Note that the exposure time halves for each 3dBA increase in The subject of musical scales and pitch is beyond the scope of this level. There is a complication in that few noises are con