Types of Sound Level
2.1 Detector Response
Most sounds that need to be
measured fluctuate in level. To measure the sound properly we want to be able to measure
these variations as accurately as possible. For this reason, two detector response
characteristics were standardized. These are known as "F" (for Fast) and
"S" (for Slow) (See Figure A4).
(a) "F" Characteristic
provides a fast reacting display response enabling us to follow and measure not too
rapidly fluctuating sound levels.
(b) "S" Characteristic
provides a slower response which helps average-out the display fluctuations on an analogue
meter, which would otherwise be impossible to read using the "F" characteristic.
Figure A4 "F" and
"S" Detector Response Characteristics
2.2 Impulse Sound Level Meter
The impulse sound level meter
(Figure A5) should be used to measure isolated impulses or sound containing a high
proportion of impact noise as the normal "F" and "S" time responses of
the simple sound level meter are not sufficiently short to give a measurement which is
representative of the subjective human response. (See Figure A6)
Since sound level meters include a
circuit for measuring the peak value of the impulse sound, independent of it's duration as
the peak value may cause the risk of damage to hearing. It is known as a Hold
Circuit which stores either the peak value or the maximum RMS value.
2.3 Noise Dose Meters
The need to ascertain, for the
purpose of occupational hearing conservation, the noise exposure of employees during their
normal working day, has lead to the development of a new type of specialised integrating
sound level meter from which the noise dose can be determined directly.
Figure A6 "I" Impulse
Characteristics of Impulse Sound Level Meter
The noise dose is a measure of the
total A-weighted sound energy received by an employee, and is expressed as a proportion of
the allowed daily noise dose. It therefore depends not only on the level of the noise but
also on the length of time that the employee is exposed to it. This instrument is portable
and can be carried in a person's pocket. The microphone can be separated from the dose
meter body and should preferably be mounted close to the individuals more noise exposed
ear. Figure A5 shows a typical application of the dose meter.