Measurement
of ambient noise levels
ANSI
S3.1 -1999
Measurement
instrumentation
HTL measurements
above/below 0 dB
Ambient
noise levels should be measured annually or whenever a new
noise source (i.e. new equipment, new ventilation etc)
is introduced in the testing environment.
When
conducting ambient noise measures, it is important to do so with all the test
equipment turned on. This way,
internal noise that would normally be present during testing will be captured in
the measurements. Conducting
measurements without the equipment and lights turned on may deliver low
ambient noise levels that are not reflective of the test situation.
The
measurement needs to be conducted in the test room with
the microphone of sound level meter located at the place
where the listener’s head would normally be situated.
Measurements need to be taken at octave and
one-third octave bands (125 – 8000 Hz) at the intervals
indicated on table II and III.
If a
sound level meter is not available, sound measurement can
be done using a psychoacoustic check method. This should only be used when the tests conducted in the
sound booth use ears covered.
In a psychoacoustic check, two normal listeners are
needed. Each
listener will be placed in the test position with
earphones on, as if he was going to be tested.
The audiometer dial is then set to 0 dB for each
test frequency and the listener is required to respond to
each test frequency at this level.
What each listener is required to do is, listen for
beats, ambient noise masking and direct acoustic radiation
from the audiometer or any other instrument present in the
environment. The
listener is also required to check for the presence of
intense low frequency vibrations that may be present in
the test room. This
is to ensure that low frequency vibrations are not
transmitted to the ear through bone conduction.
Low frequency vibration can be transmitted if the
listener touches a table or headrest or some other fixed
object in the test room. Low frequency checks are performed with the audiometer turned
off. According
to ANSI S3.1 – 1999, the test room is satisfactory for
testing hearing at RETSPL if each of the normal listeners
responds to each test frequency pure tone at 0 dB without
reporting these above mentioned problems.
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