POINTS TO REMEMBER
•Sound is
produced due to vibration of different objects.
• Sound
travels as a longitudinal wave through a material medium.
• Sound
travels as successive compressions and rarefactions in the medium.
• In sound propagation, it is the energy of
the sound that travels and not the particles of the medium.
• Sound cannot travel in vacuum.
• The change
in density from one maximum value to the minimum value and again to the maximum
value makes one complete oscillation.
• The
distance between two consecutive compressions or two consecutive rarefactions
is called the wavelength, λ.
• The time
taken by the wave for one complete oscillation of the density or pressure of
the medium is called the time period, T.
• The number
of complete oscillations per unit time is called the frequency (n), n = 1/ T .
• The speed v, frequency n, and wavelength λ,
of sound are related by the equation, v = nλ.
• The speed
of sound depends primarily on the nature and the temperature of the
transmitting medium.
• The law of
reflection of sound states that the directions in which the sound is incident
and reflected make equal angles with the normal to the reflecting surface and
the three lie in the same plane.
• For
hearing a distinct sound, the time interval between the original sound and the
reflected one must be at least 0.1 s.
• The persistence of sound in an auditorium is
the result of repeated reflections of sound and is called reverberation.
• Sound properties such as pitch, loudness and
quality are determined by the corresponding wave properties. • Loudness is a
physiological response of the ear to the intensity of sound.
• The amount
of sound energy passing each second through unit area is called the intensity
of sound.
• The
audible range of hearing for average human beings is in the frequency range of
20 Hz – 20 kHz.
• Sound
waves with frequencies below the audible range are termed “infrasonic” and
those above the audible range are termed “ultrasonic”. • Ultrasound has many
medical and industrial applications.
• The SONAR
technique is used to determine the depth of the sea and to locate under water
hills, valleys, submarines, icebergs, sunken ships etc.
1. Explain how sound is produced by
your school bell.
Answer: When the bell
continues to move forward and backward, it creates a series
of compressions and rarefactions. In this way a
bell produce the sound .
2. Why sound waves are called
mechanical waves?
Answer: Sound waves need the
material medium to vibrate and propagate. Hence, these waves are known as
mechanical waves. Sound waves propagate through a medium because of the
interaction of the particles present in that medium.
3.Suppose you and your friend are on
the moon. Will you be able to hear any sound produced by your friend?
Answer: Sound needs a medium
to propagate. Since there is no material medium on the moon due to absence of
atmosphere, you cannot hear any sound on the moon.
4.Which wave property determines (a)
loudness, (b) pitch?
Answer: (a) Amplitude of
sound wave determines the loudness of a sound. The amplitude of a sound
directly proportional to amplitude of wave. If the amplitude of a sound is
large, then the sound produced will also be loud.
(b) Frequency of sound wave determines
the pitch of a sound . The pitch of a sound is proportional to its frequency.
High pitched sounds have high frequency
5.Guess which sound has a higher pitch:
guitar or car horn?
Answer: The frequency of the
vibration of a sound produced by a guitar is greater than that produced by
a car horn. As we know that the pitch of a sound is directly proportional
to its frequency, the guitar has a higher pitch than a car horn.
6. What are wavelength, frequency, time
period and amplitude of a sound wave?
Answer: Wavelength: The
distance between two consecutive compressions or two consecutive
rarefactions is known as the wavelength. Its SI unit is metre (m).
Frequency: The number of oscillations
produced by vibrating body in one second is known as the frequency of a sound
wave. It is measured in hertz (Hz).
Amplitude: The maximum height reached
by the crest or trough of a sound wave is called its amplitude.
7.How are the wavelength and frequency
of a sound wave related to its speed?
Answer: Speed of sound wave
is product of wavelength and frequency of a sound wave
Speed ( v) = Wavelength × Frequency = ν
λ
Calculate the wavelength of a sound
wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.
Frequency of the sound wave, v = 220 Hz
Speed of the sound wave, v = 440 m/ s
Speed = Wavelength × Frequency
Wavelength of a sound wave = Speed of
the sound wave/ Frequency of the sound wave
= 440 m/s ÷ 220 Hz = 2m
Hence, the wavelength of the sound wave
is 2 m.
8.A person is listening to a tone of 500 Hz sitting at a distance of 450 m from
the source of the sound. What is the time interval between
successive compressions from the source?
Answer: You know that the time taken to
complete two successive compressions or two consecutive rarefactions
is called the time period of the wave.
Time period of the wave = 1/Frequency Hence, time period is reciprocal of the
frequency of the wave
Now, Time period of the wave=1/500=0.002 second
9. Distinguish between loudness
and intensity of sound.
Answer: a) Intensity of sound waves is
defined as the average energy transported per second per unit area
perpendicular to the direction of propagation.
b) It is measured in Js-1m-2 or Wm-2.
c) The intensity of sound in air depends on the square of the frequency and the
square of the amplitude.
a) The loudness of sound is defined as the degree of sensation of sound
produced in the ear
b) The loudness of a sound depends on its amplitude.
c) The amplitude of a sound decides its intensity, which in turn is perceived
by the ear as loudness.
10. In which of the three media, air, water or iron, does sound travel the
fastest at a particular temperature?
Answer: The speed of sound depends on the
nature of the medium. Sound travels the fastest in solids. Its speed decreases
in liquids and it is the slowest in gases. Therefore, for a given temperature,
sound travels fastest in iron.
11. An echo returned in 3 s. What is the distance of the reflecting surface
from the source, given that the speed of sound is 342 m/s ?
Answer: Speed of sound, v = 342 m/ s Echo
returns in time, t = 3 s
Distance travelled by sound = v × t = 342 × 3 = 1026 m
In the given time interval, sound
has to travel a distance that is twice the distance of the reflecting surface
and the source.
Hence, The actual distance of the
reflecting surface from the source =1026/2=513m
12. Why are the ceilings of concert halls curved?
Answer: Since, concert halls
are big, so audience at the back rows of the hall may not hear clear sound of
speaker. To overcome this problem, the ceiling of the concert halls is made
concave. Concave ceiling helps the sound wave to reflect and send to farther
distance which makes the concert hall enable to send clear sound to the
audience even sitting in back rows of hall.
13. What is the audible range of the average
human ear?
Answer: 20 Hz to 20000 Hz
14. What is the range of frequencies
associated with
(a) Infrasound
(b) Ultrasound
Answer:
a) Infrasound: Less than 20 Hz (b) Ultrasound: More than 20000 Hz
15. A submarine emits a SONAR pulse,
which returns from an underwater cliff in 1.02 s. If the speed of sound in salt
water is 1531 m/s, how far away is the cliff ?
Answer:To return the SONAR pulse back, its
wave has to travel two way.
Here, given,
Velocity (v) of sound wave = 1531m/s
Time (T) = 1.02 s
Thus, Distance = speed X time
Distance=1531 ms^(-1)×1.02 s=1561.62 m
So,the distance between the source and reflecting
surface=1561.62÷2=780.81m
