Page 142 - Understanding NCERT Science 09

density of the medium oscillates between a
maximum value and a minimum value. The
change in density from the maximum value
to the minimum value, then again to the
maximum value, makes one complete
oscillation. The number of such oscillations
per unit time is the frequency of the sound
wave. If we can count the number of the
compressions or rarefactions that cross us
per unit time, we will get the frequency of
the sound wave. It is usually represented by
ν (Greek letter, nu). Its SI unit is hertz
(symbol, Hz).
The time taken by two consecutive
compressions or rarefactions to cross a fixed
point is called the time period of the wave. In
other words, we can say that the time taken
for one complete oscillation is called the time
period of the sound wave. It is represented by Fig. 11.7: Low pitch sound has low frequency and
high pitch of sound has high frequency.
the symbol T. Its SI unit is second (s).
Frequency and time period are related as as shown in Fig. 11.6(c). For sound its unit
follows: will be that of density or pressure.
1 The loudness or softness of a sound is
v =
T determined basically by its amplitude. The
amplitude of the sound wave depends upon
A violin and a flute may both be played at the force with which an object is made to
the same time in an orchestra. Both sounds
travel through the same medium, that is, air vibrate. If we strike a table lightly, we hear a
and arrive at our ear at the same time. Both soft sound because we produce a sound wave
sounds travel at the same speed irrespective
of the source. But the sounds we receive are
different. This is due to the different
characteristics associated with the sound.
Pitch is one of the characteristics.
How the brain interprets the frequency
of an emitted sound is called its pitch. The
faster the vibration of the source, the
higher is the frequency and the higher is
the pitch, as shown in Fig. 11.7. Thus, a
high pitch sound corresponds to more
number of compressions and rarefactions
passing a fixed point per unit time.
Objects of different sizes and conditions
vibrate at different frequencies to produce
sounds of different pitch.
The magnitude of the maximum
disturbance in the medium on either side of
the mean value is called the amplitude of the
Fig. 11.8: Soft sound has small amplitude and
wave. It is usually represented by the letter A,
louder sound has large amplitude.

SOUND 131

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