Definition of Intensity
The intensity (or energy flow) of a sound wave is the power (in energy/sec)
transmitted through an area of 1m oriented perpendicular
to (normal to) the propagation direction of the wave.[1 ]
Almost everyone knows that if they move away from a constant sound source, they
perceive a decrease in loudness. Consider the following example:
Figure 2.1 Decrease in loudness over distance
Assume that a sound from a source propagates through 1m
of air at 10m from the source. Looking at the diagram we can see that the power
that is concentrated over 1m
at 10m from the source, is
spread over a larger area at the distance of 20m. The same amount of energy is
spread over a larger area, so the intensity has decreased. Specifically, the
area at 20m is 4m
which is 4 times the area at 10m (1m
), making the energy at 20m 1/4 the intensity that it was at
10m. That is:
The sensation of loudness is determined by the intensity. The greater
the intensity the greater is the perceived loudness. It is usual to symbolise
intensity as I expressed in watt/m.
Human hearing falls roughly in the range 1 Watt/m (the
loudest, called the Threshold of Feeling) to 10
Watt/m
(the softest, called the Threshold of Hearing.
10
= 0.000000000001 . (12 decades.)
These two thresholds vary from individual to individual and depend on the
particular frequency under consideration. The size of this range is tremendous
( a trillion values); The unit of watt/m produces very
unwieldy numbers. For these reason and others, the units watt/m
is impractical.