Noise levels are not difficult to measure, just use a meter and take a reading. But expressing exposure to noise, or any high sound level, over a period of time is more tricky. Real-world noise fluctuates and long periods of low ambient sound levels may be punctuated by brief bursts of high intensity noise.
One way of expressing noise levels over a period of time is the Leq. This is the sound level which, integrated - 'evened out' - over a period of time, is equivalent to a steady sound pressure level. In fact, the expression should ideally be Leq,T - where T is the time over which the exposure is integrated. Sometimes also you will see LAeq,T - this signifies that an 'A weighting' filter has been used to mimic the frequency response of the human ear.
Legislation in the UK, which is similar in intent and spirit to legislation elsewhere, specifies that the daily duration of exposure to noise should be no longer than a stated maximum over a range of sound levels.
|LAeq||Permissable duration of exposure|
This makes interesting reading. Music that we would consider to be loud is regularly over the 100 dB SPL level. The point however is that the level is integrated over a period of time. The fact is that a typical recording session involves a lot of discussion in and around takes and playbacks. So if levels peaked at 111 dB SPL on one single occasion that lasted two seconds, it may not make all that much of a difference.
Legally, whoever is in charge of a session is responsible for controlling levels. The studio that you hired does not have that responsibility (Standard legal disclaimer - get your own legal advice on these matters!). So if you wanted to ensure that you don't get a lawsuit from a session musician in twenty years time when he finds his hearing is failing, consult your local legislation and get a Leq meter. Use it on all sessions and keep a log of the readings.
Factory managers take noise levels very seriously. In music this is something that is often ignored. An awareness of the potential for hearing damage wouldn't go amiss though.
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