Page 143 - Understanding NCERT Science 09
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of less energy (amplitude). If we hit the table Example 11.1 A sound wave has a
hard we hear a louder sound. Can you tell frequency of 2 kHz and wave length
why? A sound wave spreads out from its 35 cm. How long will it take to travel
source. As it moves away from the source its 1.5 km?
amplitude as well as its loudness decreases.
Louder sound can travel a larger distance as Solution:
it is associated with higher energy. Fig. 11.8 Given,
shows the wave shapes of a loud and a soft Frequency, ν = 2 kHz = 2000 Hz
sound of the same frequency.
Wavelength, λ = 35 cm = 0.35 m
The quality or timber of sound is that
We know that speed, v of the wave
characteristic which enables us to distinguish
= wavelength × frequency
one sound from another having the same pitch v = λ ν
and loudness. The sound which is more
= 0.35 m 2000 Hz = 700 m/s
pleasant is said to be of a rich quality. A sound The time taken by the wave to travel a
of single frequency is called a tone. The sound
distance, d of 1.5 km is
which is produced due to a mixture of several
frequencies is called a note and is pleasant to
listen to. Noise is unpleasant to the ear! Music
is pleasant to hear and is of rich quality. Thus sound will take 2.1 s to travel a
distance of 1.5 km.
Q 1. Which wave property determines Q uestions
uestions
(a) loudness, (b) pitch?
1. What are wavelength, frequency,
2. Guess which sound has a higher
time period and amplitude of a
pitch: guitar or car horn?
sound wave?
frequency of a sound wave
The speed of sound is defined as the 2. How are the wavelength and
distance which a point on a wave, such as a related to its speed?
compression or a rarefaction, travels per unit 3. Calculate the wavelength of a
sound wave whose frequency is
time. 220 Hz and speed is 440 m/s in
We know, a given medium.
speed, v = distance / time
4. A person is listening to a tone of
l 500 Hz sitting at a distance of
=
T 450 m from the source of the
sound. What is the time interval
Here λ is the wavelength of the sound wave. It between successive compressions
is the distance travelled by the sound wave in from the source?
one time period (T) of the wave. Thus,
The amount of sound energy passing each
second through unit area is called the intensity
v = λ ν
of sound. We sometimes use the terms
“loudness” and “intensity” interchangeably,
or v = λ ν but they are not the same. Loudness is a
That is, speed = wavelength × frequency. measure of the response of the ear to the sound.
The speed of sound remains almost the Even when two sounds are of equal intensity,
same for all frequencies in a given medium
we may hear one as louder than the other
under the same physical conditions. simply because our ear detects it better.
132 SCIENCE
Rationalised 2023-24
