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Noise Floor
This is the lowest sound pressure level that the meter can display on its
operating range.
The instrumentation itself by its very nature of operation has some internal
electrical noise which can be minimised only. The electrical noise therefore
contributes to the noise floor on the lower ranges of the instrumentation.
Generally sound meters which are capable of reading very low sound pressure
levels of 25 to 30dBA levels are of higher specification using specially designed
low noise circuitry, guard rings on the pre-amplifier and higher sensitivity
microphones.
See also ‘Overload’, ‘Pre Amplifier’ and ‘Under Range’.
Overload
When the peak sound pressure level starts to exceed the signal handling
capability of the pre-amplifier circuitry then an overload condition occurs and a
warning is displayed on the instrumentation.
This value is generally specified by the manufacturer and usually just above the
top of linearity range of the meter. When an overload condition occurs your
instrumentation is likely to be operating out of specification with any results
becoming invalid for noise assessments.
Where appropriate change to a higher range on your instrumentation. See also
‘Noise Floor’, ‘Pre-Amplifier’ and ‘Under Range’.
Pre-Amplifier
This is a special low noise electronic amplifier to amplify the low level signals
produced by the microphone.
Some pre-amplifiers are fitted with a special guard ring which surrounds the
signal connector of the microphone and encapsulates the signal until it reaches
the input stage of the amplifier. The guard ring is used to reduce the
capacitance effects of inserting the pre-amplifier and in doing so reduces the
instruments electrical noise which allows lower level signals to be measured.
Sound meter instruments are supplied with fixed or removable pre-amplifiers,
where the pre-amplifier is removable it should have should have a unique serial
number which is identified on the calibration certificate along with microphone
and instrument serial numbers.
See also ‘Noise Floor’, Overload’ and ‘Under Range’.
