Physics - Amplitude, loudness and intensity
Amplitude, loudness and intensity
It is the amplitude of a sound wave that determines its perceived
loudness. However, the term loudness is not a scientific term. Loudness
is really a subjective judgement based on the response of the listener. It
is also severely affected by the pitch of the tone being heard.
Loudness is related to the energy carried by a sound wave. The amount of
energy passing through a square metre area each second is called the
intensity  of the sound wave. It is easiest to remember this definition
by picturing the diagram.
Since the concept of energy per second is really a measurement of power
, the unit of the watt is used. Intensity is therefore the power (in
watts) passing through an area of a square metre. The unit of intensity is
therefore watts per metre squared (W m).
The amplitude of the vibrating sound source determines the intensity of
the sound wave produced. Doubling the amplitude results in a sound wave of
four times the intensity. Tripling the amplitude results in an intensity
that is nine times greater. That is, intensity is proportional to the
square of the amplitude.
A convenient way to work with this relationship is to re-write it in the
If presented with a situation in which the amplitude of a particular
source is altered, the value of (I/A) should always give a constant value.
Each experimental setting would have its own specific value for this
constant. This can be used to determine the resultant intensity.
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