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Sound and Vibration Instrument Hire and Calibration

Background Noise Measurement

Bruel & Kjaer 2250 Sound Level Meter

Background Noise is the sound level at a given location and time, measured in the absence of intermittent noises, any other extraneous or alleged noise nuisance sources. It is also referred to as the Ambient or Residual Noise.

Measuring background noise is straight forward in busy urban locations, general office spaces and factories. However measurements in quiet locations, for example a studio, a library or in the countryside, late at night, can be a problem without the right equipment i.e. a Statistical Sound Level Meter.

Statistically speaking the LA90 value is commonly used to describe background noise levels and is defined as the noise level just exceeded for 90% of the time. The 'A' indicating the measurement was A-weighted.

In other words the measured level was lower than the stated LA90 level for 90% of the time, and is written as L90 = 30 dBA for example.

Some organisation, perhaps for historical reasons use the L95 or 95% values. However LA90 is specified in, the long established, BS 4142 - Method for rating and assessing industrial and commercial sound.

Measuring background noise levels as low as 30 dBA should not be a problem for Class 1 meters as the lower limit, set by the 'noise floor' of the individual meter's electronic circuitry, is typically < 20 dBA for modern precision meters.

However, say you needed to measure the ventilation noise in a new office building and the design specification was 25 dBA, to be measured at 45° and 1 meter from each grille. Then you would need to know the 'noise floor' of your meter, which should, ideally be 10 dBA lower than the ventilation design criteria. If the meter 'noise floor' was also 25 dBA then the sum of the levels would be 28 dBA and the grilles would be deemed not to have met the design criteria.

The manufacturers published figures, for the meters we hire, are listed under each meter's specifications, for example, so you can check before you hire. In critical cases the 'noise floor' of individual instruments should be validated just before and after measurements are made.

To measure a meter's 'noise floor' replace the microphone with the manufacturers equivalent capacitor 'input shorting plug', also available for hire from us. The A-weighted and/or filtered 'noise floor' for the actual meter/analyser may then be measured, on site and the results included in any report.

See also our range of certified Sound Level Meters for Hire