Measurement of sound

In order to measure sound levels, a “sound level meter” is used, which transforms the acoustic energy into electrical voltage. If sound sources are doubled, this amounts to increasing the intensity by 3 dB. In contrast, the “sensation” of doubling of the intensity only occurs with an increase of 10 dB. However, acoustics does not content itself with “physically” measuring sounds; indeed as we have seen, sound is both an objective and subjective notion and one must use criteria that take account of these two characteristics.

Criteria chosen for measurement of sounds:

• Sonia
• The sound level

As we have seen, the ear does not have the same sensitivity for all audible frequencies: therefore, a sound of 50 dB with a frequency of 1000 Hz produces a much more intense auditory sensation than a sound of 50 dB at a frequency of 100 Hz.

In order to take account of this particularity of the auditory system, filters are used which weight the levels as a function of the frequencies, based on the isosonia curves.
Several filters are used, the most common being the filter A. The new level, taking account of this weighting, is expressed in dB(A) in this case. A physiological magnitude for the sound level is thereby obtained, no longer merely a physical magnitude.

The significance of this weighting is considerable. Indeed, very low-frequency noises, which may reach more than 140 dB in a High Speed Train entering a tunnel, do not appear dangerous to hearing (which does not mean that they do not have any effect on health). Likewise, the "beat" (low-frequency) emitted by the bass drum of a drum kit which passes through walls and is heard by the neighbours of discotheques is not dangerous to the ears, in spite of the nuisance that it represents.

• The overall noise level