5. Strength of Sound

A sound source radiates out sound waves. These sound waves travel through a medium and arrive at receptors. So there are three distinct elements in a system of sound:

(i) Source, which produces the acoustic energy, (e.g loudspeaker).

(ii) Conveying medium, which transmits the sound, (e.g. air).

(iii) Receptor, which senses the sound, (e.g. human ear).

 

pressure amplitude.

sound power in watts.

 

The strength of a sound can be described by two methods :

(i) The pressure amplitude, Prms in Pascals or N/m2, which is the root-mean-square value of the pressure differences.

(for sine wave) (9)

(ii) The intensity, I, in W/m2, which is the sound power passing normally through a unit area of space (see Fig. 4).

(10)

where Wa is the total acoustic power passing through the area S.

 

Figure 4 Definition of Sound Intensity

 

Sound intensity at a point is a vector, and therefore is direction dependent. When more than one sound source is present, the intensity at a point in any direction is related to the net Prms in that direction by the following formula :

(11)

where r = density of conveying medium, kg/m3

c = velocity of sound in the medium, m/s