Hearing measurements in an audiometric test
room may be conducted for different audiometric
purposes over different test frequency ranges.
However, if the ambient noise level is excessively
high, some hearing threshold levels measured in
that environment will be inaccurate. This occurs
because excessively high ambient noise will create
an elevation of hearing threshold levels. This is
a psychoacoustic phenomenon called masking.
Exclusion of all ambient noise from an audiometric
test room is not feasible nor practical from structural and cost considerations. However, to ensure that hearing tests are not influenced by excessive ambient noise masking, there is a need to specify maximum permissible ambient noise levels
(MPANLs) that can be allowed in an audiometric
test room. This is true particularly when testing is
done at sound pressure levels for pure tones corresponding to 0 dB hearing level. Further, there is a need to specify MPANLs for different audiometric conditions, test frequency ranges, and earphone types that will permit testing at reference
equivalent threshold levels specified in ANSI S3.6-
1996 American National Standard Specification for
Audiometers.
This Standard includes the specification and measurement of MPANLs in an audiometric test room.
Within this Standard, MPANLs are specified for octave and one-third octave band intervals from 125
to 8000 Hz for the audiometric conditions of testing
with ears covered using a supra-aural and insert
earphone and ears not covered for the test frequency
ranges 125 to 8000 Hz, 250 to 8000 Hz, and 500 to 8000 Hz.