Sound Level Meters

The dynamic range of the instrument determines the answers to such questions are:  how well does the meter respond to different sound levels?  Does sensitivity deteriorate at high or low levels?  It was suggested earlier that a meter should be able to measure noise levels in the range 0 – 140dB.  In practice, its dynamic range may be less than this. 

Weighting Networks

When weighting network (e.g., A, B, C, Lin) is used to modify the way the instrument responds to the noise being measured, by introducing selective filters.  For example, to emulate the response of the human ear the A-weighting scale is used, producing dB(A) measurements.  Other circumstances may require, B-, C- or D-weightings of a linear response.  Some instruments also include a frequency analyser which enables particular regions of the frequency spectrum to be examined.

Response Time

Most instruments have fast or slow options available.  This essentially defines the speed of sampling of a varying noise.  The time periods involved are 0.2 and 0.5 s respectively.  To measure impulse or impact noises, eg., a hammer hitting a metal plate, then much faster response is required, typically 35 ms (milliseconds) or less with a peak hold facility to freeze the reading.  A typical peak measured using an ‘impulse’ setting will be about 10-20 dB higher than the measured value using a fast response. 

Integrating sound level meters (for measuring L_eq)

Most meters now have integrating facilities, enabling them to calculate electronically the average noise level over a period of time.  This is useful, if not essential, when exposure levels are to be measured under conditions where the noise level is constantly changing.  The big difference between the integrating and the basic sound level meter is that the former operates over a much wider dynamic range (about 60dB).  This can be switched to a particular range, eg., 40-100 dB or 80-140dB.  Such meters measure and display the average energy expressed as a dB L_eq level over the measurement period (either preset or manually set) and weighted as required. 

The response time for an integrating meter is generally similar to that of a basic sound level meter.  Integrating sound level meters conforming to the fast response time criteria for impulse meters are also available.

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