Appendix 4 – Instruments for assessing noise levels
A sound level meter (SLM) is an instrument consisting of a microphone, amplifier and indicating device. The meter has a declared performance, and is designed to measure a frequency - weighted and time-weighted value of the sound pressure level.
Sound level meters and integrating averaging sound level meters should comply with the specifications of Australian Standards AS IEC 61672.1 and AS IEC 61672.2 respectively. Sound exposure meters should comply with the specifications of Australian New Zealand AS/NZS 2399 Acoustics - Specifications for personal sound exposure meters (non-Queensland Government link).
SLMs have four principal grades of precision:
|
Type/Description |
Tolerance |
|
0-Laboratory reference meter |
± 0.4dB |
|
1-Precision |
± 0.7dB |
|
2-General Purpose |
± 1.0dB |
|
3-Survey |
± 1.5dB |
Noise assessments should be performed with Type 2 general purpose meters, or better. Type 3 survey meters are usually inexpensive but may have wide precision tolerances and some models cannot be calibrated. Type 3 survey meters are only suitable for preliminary noise checks to find out whether more accurate assessments are needed.
The SLM may have an integrating function that enables the meter to process a continuous, variable, intermittent or impulsive signal to give a single integrated level or Leq for the sampling period. A meter with this function is an integrating sound level meter (ISLM).
The SLM may also have a peak detector indicating characteristic. This is necessary to measure the C-weighted peak sound pressure level. The term, peak sound pressure level, should not be confused with the maximum sound pressure level.
A sound exposure meter (SEM) or noise dose meter is an instrument for measuring noise exposure by automatically integrating sound energy over a measurement period and displaying the result. The instrument may be worn by the person concerned, or placed at a suitable location to estimate the noise exposure received by one or more persons, stationary or otherwise for a given period, for example, a working day.
The SEM records the personal noise exposure of the worker. Some SEMs can record a time history of a worker’s noise exposure for the measurement period. A typical time history report will provide a histogram of minute by minute noise exposure levels. This is an advantage in identifying major contributors to the average daily noise exposure that can then be further investigated with a hand-held meter.
The following points should be considered when using a SEM:
- Reflection of sound from the clothes and body can cause an increase of about 1 to 3 dB.
- The microphone should be attached as close as possible to the ear. For example, if the microphone is attached to the lower part of the collar or pocket, it may be much closer to a noise source than the ear and an unduly high result will be recorded. Also, the body may shield a noise source.
- The assessment of exposure over just one day may not give an representative sample.
- If possible, it is best to take measurements over a few days.
It is advisable to check the SEM results with a hand held SLM. Some SEMs do not measure impulse sound adequately.
Meters should be checked with an acoustic calibrator24 immediately before and after measurements are taken. A full calibration of acoustic measuring systems should be performed at regular intervals not exceeding two years by a laboratory that produces test reports recognised by the National Association of Testing Authorities, Australia, covering the relevant accredited tests.
24 'acoustic calibrator' means a device for applying a sound pressure of known level to the microphone of a sound measuring system, for the purpose of calibration.
Last updated 22 July 2008