The strength of sound is expressed in decibels (dB). The higher the number of decibels, the louder – and more damaging – the sound. Instead of dB, in noise measurements you often see the A-weighted sound pressure value or dB(A). What is dB(A)? And why do we weigh sound? You can read the answers to these questions in this article.
Strength of Sound (dB)
Sound is actually a vibration, a variation in air pressure. We can measure and display these air pressure variations in the unit of pressure: Pascal. The louder a sound, the higher the air pressure. But the range of the human ear is large, and the air pressure we hear as sound varies between 2.10-5 and 20 pa, a factor of 1 million. Therefore, the representation of the loudness in Pascal is impractical and is converted to a logarithmic scale: the decibel.
Weight of Sound
Normally, we measure a linear or unweighted value of the sound in dB. That is an accurate representation of the air pressure differences, but not a good measure of the nuisance that someone experiences from the noise. The eardrum in combination with the hammer, stirrup and the round window typically behaves like a mechanical filter with a certain sensitivity in the different octave or third bands. That is why sound measurements often use an electronic filter that attenuates the sound just as much as the human ear does.
A fairly exact representation of the filter of the ear is given by the phon. However, because this is a difficult unit to handle, the A filter is almost always used in practice. The A filter is based on the equal sound perception curves of 40 phon.
In addition to the A filter, there are also the B, C and D filters. However, in practice, it is almost always the A filter that is being used. It has been found that precisely this filter provides a good obstacle measure over a large area of levels. The D-filter is still used for air traffic noise measurements.
So, the A filter corrects the linear sound pressure level based on the air pressure variation for the sensitivity of the human ear. That is why you see dB(A) being used in the standards for noise in the workplace.
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