Page 131 - Science Class 08

at even higher frequencies. The reason we Here are some more examples of pitch:
cannot hear dog whistles, while they can, is v When a drum is beaten, it vibrates but not
because the frequency of the whistle is too with very high frequency. So, its sound is
high to be processed by our ears. Sounds that not very sharp, i.e., its pitch is low.
are too high for us to hear are called ultrasonic.
v Blow a whistle and observe the sharpness
Ultrasonic waves have many uses. In nature, of its sound. It is far sharper than the
bats emit ultrasonic waves and listen to the sound of a drum as the small stone inside it
echoes to help them know where walls are or moves with a very large frequency.
to find prey. Captains of submarines and other
v The voice of a child is much more shriller
boats use special machines that send out and
than that of a grown-up man. The reason is
receive ultrasonic waves. These waves help
that frequency of the vocal cords of a child
them guide their boats through the water and
is far higher than that of an adult’s.
warn them when another boat is near.

Take a thin long metal strip and place it on the edge of a table such that a larger part of it
projects out of the edge. Pressing hard its part on the table, just jerk its free end downwards.
The metal strip will vibrate. Try to listen the sound produced by it and note its sharpness in your
mind.
Now, shorten the part of the strip projec ng out of the table-edge and repeat the process. The
strip will vibrate, but not with the same frequency. So, the sound will be far less sharp too.

SOUND AND AUDIBILITY
As men oned earlier, human sound is also
produced due to the vibra on of the vocal cords
that are there in the throat. These cords can
vibrate with 60 to 13000 vibra ons a second.
We have also proved that loudness of sound
depends on its amplitude while its pitch
depends on the frequency of the vibra ng body.

So, it is clear that audibility of a sound depends
on these two factors. The louder and sharper a Human ear
sound, the more audible it will be. Pitch plays a
v Human ears cannot hear sounds with a
major role in making a sound audible than
loudness. In other words, frequency (number frequency of less than 20 Hz per second
of vibra ons) in a unit me count a lot. because these frequencies are infrasonic.
Human Ears v Also, human ears cannot hear sounds with
a frequency of over 20,000 Hz per second
Nature has gi ed audibility to different
creatures in various degrees. because these frequencies are ultrasonic.

Humans have their limita ons of hearing as Remember that dogs have a far be er hearing
given: power than human beings. That is why they are

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