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11. Sound eLearn.Punjab

While standing outside a room, we can distinguish between the For your information
notes of a piano and a flute being played inside the room. This is
due to the difference in the quality of these notes.
Fig. 11.9 shows the waveform of the sound produced by a tuning
fork, flute and clarinet. The loudness and the pitch of these three
sounds are the same but their waveforms are different. So their
quality is different and they can be distinguished from each
other.

Intensity Fig 11.9: Sound waveforms produced
The sound waves transfer energy from the sounding body to by (a) a tuning fork, (b) a flute, and (c)
the listener. The intensity of sound depends on the amplitude of
sound wave and is defined as: a
clarinet, are all at approximately the
Sound energy passing per second through a unit area same frequency. Pressure is plotted
held perpendicular to the direction of propagation of
vertically, time horizontally
sound waves is called intensity of sound.

Intensity is a physical quantity and can be measured Quick Quiz
accurately. The unit of intensity of sound is watt per
square meter (Wm-2). 1. Why the voice of women is more shrill than
that of men?
Sound Intensity Level 2. Which property of sound wave determines its
(a) loudness, (b) pitch?
3. What would happen to the loudness of sound
with increase in its frequency?

The human ear responds to the intensities ranging from

10-12 Wm-2 to more than 1 Wm-2 (which is loud enough to be painful). Because the range is so wide,

intensities are scaled by factors of ten. The barely audible and the faintest intensity of sound i.e.,

10-12 Wm-2 is taken as reference intensity, called zero bel (a unit named after Alexander Graham

Bell).

The loudness of a sound depends not only on the intensity of sound but also on the physical

conditions of the ear. The human ear is more sensitive to some frequencies than the others.

The loudness (L) of a sound is directly proportional to the logarithm of intensity i.e.,

L R log I

Do you know? L = K log I .......... (11.1)

Frequency of tuning fork depends on the mass where K is a constant of proportionality.
of its prongs. The greater the mass, the lower

the frequency of vibration which means the

lower the pitch.

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