Types of Sound Level Meter

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.