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

Sonia concerns the “power of sound” in the physiological sense (impression of power of sound).
The sensitivity of the ear is a variable of the frequency; therefore, whilst at 4000 Hz a sound pressure of less than 2.10-5 Pa is sufficient in order to cause an auditory sensation, at 50 Hz, a pressure of 2.10-2 Pa is necessary, i.e. 1000 times more.

Likewise, the pain threshold is variable as a function of the frequencies, according to a similar variation with much less pronounced slopes. It is therefore possible to plot isosensation curves for given sound levels at 1000 Hz known as phones; these curves were drawn in 1918 by FLETCHER and MUNSON. It is noticed for example that the sounds appear much louder for frequencies ranging from 1000 to 6000 Hz, whereas the sound pressure levels are equal.

• The sound level
• The overall noise level