Sound Level Meters

Even if a sound level meter is well maintained, properly calibrated and used in the ideal position, there may still be some error in the result because of inherent deficiencies in the instrument itself.  Most instruments are expected to conform to a certain quality and these are categorised in Bs EN 60651:1994.  This standard lays out the tolerances of measurement accuracy not only for the basic instrument but for most of the functions to be found on the SLM, the weighting networks and the detector circuits. 

According to BS EN 60651, instruments are classified into four types which differ as to how accurately they can measure noise:

Type 0:  these have the lowest measurement tolerance, ±0.4 dB, and are used as laboratory standard or reference instruments.  They would not actually be used to measure noise in the field but are used in test laboratories under controlled conditions.  They do not have to satisfy the same set of stringent environmental conditions that are applied to instruments used in the field (Types 1,2,3)

Type 1:  the tolerance on measurements made using these instruments is ±0.7 dB.  They are precision instruments which might be used for carrying out detailed surveys where very accurate readings are needed.

Type 2:  these are general purpose survey instruments with a measurement tolerance of ±1.0 dB.

Type 3:  these are fairly basic instruments with a measurement tolerance of ±1.5 dB.  These instruments would be used in general noise surveys to ensure that noise criteria are not exceeded.  Borderline measurements would be referred back for a more detailed survey with Type 2 or Type 1 instruments.

The following table (similar to Table 1 in Noise Guide 3) summarises current descriptions and quotes typical accuracies in use; the latter are generally a bit worse than the specified accuracy of the equipment, as other inaccuracies creep in during the course of measurement in the field.

There is a separate standard covering integrating sound level meters – BS EN 60804: 1994.  Like BS EN 60651, this classifies meters according to their overall accuracy.  It relates their precision to the frequency content of the noise being measures eg., Type 2 meters (general field) will be suitable for most field surveys except where there is a significant high-frequency component (above 10 kHz).  With Type 3 meters, allowances for reduced accuracy need to be made where a significant component of the noise exceeds 6 kHz.

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