Page 130 - Science Class 08

They are reciprocals of each other as shown in the suspended ball just touches one of its
the following formulae:
period = 1/frequency
or
T = 1/f
frequency = 1/period
or
f = 1/T
Have you ever thought that why the child’s
voice is shriller than that of an adult? It is
because frequency of vibra on plays an
important role in determining the shrillness or
the pitch of the sound. If the frequency of
vibra on is low, there is a sound of low pitch or
bass. Conversely, if the frequency of vibra on is
high, there is a sound of high pitch or shrill. As
the child’s voice has higher frequency than that
of an adult so as his voice is shriller than that of
an adult.
If a pendulum oscillates 50 mes in a second,
its frequency is 50 hertz (Hz). Clearly, a hertz
stands for one oscilla on per second.
HOW TO DISTINGUISH SOUND?
Two features of a sound help us to dis nguish
it from other sounds. These features are
loudness and pitch.
Let us read about them:
Loudness
We hear several sounds everyday. Some of
them are loud while others are low. What
makes a sound louder than others?
Let us perform an experiment to understand
this point clearly:
Suspend a light ball freely from a hook. Bring
a tuning fork and place it in such a way that
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prongs. Now, strike the fork gently. It will
produce a sound and the ball shall get away
to a certain distance.
Repeat the experiment telling the tuning
fork a bit harder this me. The sound
produced shall be louder and the displace-
ment will be larger.
It proves that the loudness of a sound
depends on the magnitude of displacement
of the vibra ng body. And, maximum
displacement of an oscilla ng body is called
its amplitude which depends on the force
with which the vibra ng body is hit. We can
check this fact by plucking at the strings of a
sitar.
Pitch
Pitch helps us dis nguish between low and high
sounds. Imagine that a singer sings the same
note twice, one an octave above the other. You
can hear a difference between these two
sounds. That is because their pitch is different.
Pitch depends on the frequency of a sound
wave. Frequency is the number of wavelengths
that fit into one unit of me. Remember that a
wavelength is equal to one compression and
one rarefac on. Even though the singer sang
the same note, because the sounds had
different frequencies, we heard them as
different. Frequencies are measured in hertz.
One hertz is equal to one cycle of compression
and rarefac on per second. High sounds have
high frequencies and low sounds have low
frequencies. Thunder has a frequency of only
50 hertz, while a whistle can have a frequency
of 1,000 hertz.
The human ear is able to hear frequencies of 20
to 20,000 hertz. Some animals can hear sounds
Science-8