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How to measure noise levels

Three different measures are used to determine the level of noise. For long term effects an equivalent sound pressure level is used. This measure describes the corresponding level for a varying noise level if it would have been of constant level. It is often expressed as the equivalent level over an 8-hour working day.

To describe the variation of a noise level over for example a day the sound pressure level is measured using the time constant ‘fast’ on the sound level meter. This means that level is measured for each of a number of short time periods. The maximum sound pressure level is then determined as the largest value of these measurements.

The third measure is the peak value of impulse noise. If the sound has very short duration only a few pressure waves will occur. The pressure of the largest peak in such impulse is measured using a very fast reaction time of the instrument often indicated as ‘peak’. The result is usually presented as impulse peak level.

Two different filters are commonly used for noise measurements to weight the frequency content of the noise. This results in A-weighted or C-weighted sound pressure level. A-weighted sound mimics the way the ear naturally picks up sounds at relatively low level. It is measured using a filter that suppresses some of the lower frequencies. Since A-weighted sound corresponds to the risk for hearing damage, this is what is usually indicated.

C-weighted sound mimics how the ear perceives significantly louder sounds, and is measured using more of the low frequencies. The difference between the C- and A-weighted sound levels gives crucial infor­mation about the character of the noise (if it’s low-frequency or high-frequency dominated).

 



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© Magnus Johansson, Peltor AB







 THE AUDITORY SYSTEM

 » How does sound occur
 » The auditory system 
 » The outer ear 
 » The middle ear
 » The inner ear
 » Hearing
 » Exposure to noise 
 » Hearing impairment
 » The concept of equal energy
 » How to measure noise levels
 » Choose right hearing protection
 » Do not over attenuate
 » Noise regulations