Sound Level Meters

Sound level meters should be calibrated before and after every use to ensure that they are working as they should.  The method used, then, must be amenable to field use.

One way would be to expose the meter to a known sound pressure level, ie., to use an acoustical calibrator.  These are designed to fit closely over the microphone and can be divided into two groups:

• Pistonphones

• Calibrated source oscillators

Pistonphones are precision calibrators which should be accurate to ±0.1 dB.  Their use is generally confined to laboratories.

Alternatively a built-in or insert voltage calibration can be made.  This injects a known, precise electrical voltage into the microphone circuit which produces a known response.  The drawback of this approach is that it would not detect any change in the microphone sensitivity.

There are other, more precise, laboratory methods of calibration, but these are used to calibrate test laboratory instruments rather than normal measurement instruments for use in the field or in the laboratory.

All calibrators are used in the same way.  The calibrator is placed in position over the microphone and the SLM reading is noted; if it is not the same as the known output level of the calibrator, then adjustments are made to the meter until it displays the correct reading.

In older instruments, this adjustment is usually made using a screw in a recess on the instrument, the recessed position preventing it from being accidentally knocked.  A large variation (ie., more than 1 dB) from the previous calibration indicates a fault in the instrument which needs further examination.  Modern equipment incorporates electronic adjustment systems.

It is important that the calibration should produce a sufficiently high output as to be unaffected by the ambient noise level.

Background noise may affect the calibration.  It could either produce an unsteady reading on the SLM or add to the indicated level, with the result that the meter would be adjusted to a lower level than is correct.  It is best to choose as quiet a place as possible for the calibration, but in any case the calibrator signal must be at least 10 dB higher than the ambient background level.

The meter should also incorporate a ‘battery check’ setting and a ‘low battery’ indicator, eg., a flashing light or digital display warning of low battery levels.  Battery power is consumed rapidly; leaving the equipment switched on overnight in its case will usually drain the batteries.  It is essential to keep a spare set of batteries with the instrument.

In addition to these routine tests, the instrument should periodically be more thoroughly tested in a specialist laboratory.  Individual equipment suppliers should be consulted as to how frequently they recommend a full laboratory calibration of their equipment.  The HSE suggest (in Noise Guide 3) that the interval between calibrations should not normally be longer than 2 years).

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