Introduction
This topic covers how workplace noise and occupational vibration are measured, how the results are compared with hygiene standards, and how the health risk is judged. Assessment is the basis of every prevention programme: without measured exposure there is no way to decide who needs control measures, hearing protection or medical surveillance.
Assessment of noise
Physical quantities
- Sound is a pressure wave. Its level is expressed in decibels (dB), a logarithmic scale: a rise of 3 dB doubles the sound energy, and a rise of about 10 dB is heard as roughly twice as loud.
- The ear is not equally sensitive at all frequencies, so measurements use a frequency weighting. dB(A) imitates the response of the ear at moderate levels and is used for the risk of hearing damage; dB(C) is used for very high peak levels.
- Human hearing covers 20 Hz to 20 kHz. Noise is described by its level, its frequency spectrum and its time pattern (continuous, intermittent, impulse).
Instruments
- Sound level meter (integrating): measures the instantaneous level and the equivalent continuous level. It must be calibrated before and after the measurement.
- Octave-band (or third-octave) analyser: splits the noise into frequency bands. This is needed to choose the right control measure and hearing protector, and to identify tonal noise.
- Personal noise dosimeter: worn by the worker, it integrates exposure over the whole shift. It is the method of choice for people who move between places or whose noise varies.
Measurement strategy
- Map the workplace and the work tasks, identify the noise sources and the groups of exposed workers, and record the duration of each task.
- Measure at the position of the worker's ear (head position, without the worker present for a fixed-point measurement), in representative conditions: machines running normally, typical production.
- The result is expressed as the equivalent continuous sound level (Leq) and standardised to an 8-hour working day as the daily noise exposure level (LEX,8h). Peak sound pressure is measured separately for impulse noise.
Evaluation against limits
- The safe exposure limit is 85 dB(A) for 8 hours a day; exposure above 85 dB(A) requires hearing protection.
- In the European Union directive on noise at work there are action values (lower 80 dB(A), upper 85 dB(A)) and an exposure limit value of 87 dB(A) that takes the hearing protector's attenuation into account; the peak action values are expressed in dB(C).