Page 129 - Science Class 08
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It is clear that the air crea ng sensa ons in the makes one oscilla on. It can also be taken as O
ears is not s ll air, but vibra ng air. to A, back to O, then to B and again back to O. In
So, a complete defini on of sound is as follows: both the cases, it covers double the distance
between A and B.
Sound is a sensa on produce inside the ears by
the vibra ng air that goes into the ear from Amplitude
outside. The amplitude of a wave is a measure of the
In science, we o en use a device, called tuning displacement of the wave from its rest posi on.
fork, to study sound. It is like a fork and is made The amplitude is shown on the graph below.
of thin metal.
Take a tuning fork and strike one of its prongs
with a hammer gently. Both the pongs will
start vibra ng. Bring this vibra ng fork near a
bead hanging freely from a hook. The bead
will bounce away from it again and again
Amplitude of a Wave
along with its vibra ons.
Now, bring your ear near the vibra ng fork. Amplitude is generally calculated by looking on a
You will hear a sound. If you bring the fork to graph of a wave and measuring the height of the
a stop by touching it, its sound will disappear. wave from the res ng posi on. The amplitude is
So, it is clear that sound is produced if there a measure of the strength or intensity of the
is a vibra ng or oscilla ng movement. wave.
For example, when looking at a sound wave,
TERMS RELATED TO SOUND the amplitude will measure the loudness of the
We have proved that sound is produced as a sound. The energy of the wave also varies in
result of an oscillatory mo on. This mo on is, direct propor on to the amplitude of the wave.
in fact, to-and-fro mo on. It can be clearly seen
Frequency and Period
in the pendulum of a wall clock. The picture
given below shows a pendulum oscilla ng to- The frequency of a wave is the number of mes
per second that the wave cycles. Frequency is
and-fro.
measured in Hertz or cycles per second. The
Three important terms are related to this
frequency is o en represented by the lower
movement are as follows:
case “f.”
Oscilla on
The period of the wave is the me between
wave crests. The period is measured in me
units such as seconds. The period is usually
represented by the upper case “T.”
A B
O The period and frequency are closely related to
Oscilla on of a Pendulum each other. The period equals 1 over the
Look at the diagram given above. The movement frequency and the frequency is equal to one
of the pendulum from A to B and then back to A over the period.
Science-8 129
