Posted By Kevin Walker
This generally depends on the quality of results required, if its just for screening purposes then it doesn't really matter and quiet room is sufficent. If this is conducted with noise excluding ear muffs the levels can be fairly high as they reduce the ambient noise anyway.

OSHA recommmends

TABLE D-1 - MAXIMUM ALLOWABLE OCTAVE-BAND SOUND PRESSURE LEVELS
FOR AUDIOMETRIC TEST ROOMS

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Octave-band center
freq (Hz). 500 1000 2000 4000 8000
SPL(dB) 40 40 47 57 62
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hope this is of use

kevin