Class 9 Science Chapter 11 Question Answers - Sound

Ques 1: Is sound wave longitudinal or transverse?
Ans: Sound wave is longitudinal in nature.

Ques 2: What is the relation between frequency (v) and time period of a sound wave?
Ans: v = 1/T
Frequency is inversely proportional to time period.

Ques 3: In which of the three media air, water or steel does sound travel the fastest?
Ans: Sound travels fastest in steel.

Ques 4: Which has a higher pitch—the sound of a whistle or that of a drum?
Ans: The sound of whistle has higher pitch.

Ques 5: What is pitch?
Ans: The way our brain interprets the frequency of an emitted sound is called the pitch.

Ques 6: How can we distinguish one sound from another having the same pitch and loudness?
Ans: The quality or timber of sound helps us to distinguish one sound from another having the same pitch and loudness.

Ques 7: What is the audible range of frequency for human beings?
Ans: The audible range of frequencies for human beings is 20 Hz to 20,000 Hz.

Ques 8: What is one Hz?
Ans: Hz is the unit of frequency, called as Hertz. One Hertz is equal to one cycle per second.

Ques 9: Define speed of sound.
Ans: The speed of sound is defined as the distance travelled per unit time by compression or rarefaction.

Ques 10: What is ‘note’ of sound?
Ans: The sound produced due to a mixture of several frequencies is called a note, it is pleasant to listen to.

Ques 11: Find the frequency of a wave whose time period is 0.002 second.
Ans: Frequency = 1/ Time preiod
Frequency = 1/0.002 = 500 Hz

Ques 12: What is the time period-of sound wave?
Ans: The time taken by two consecutive compressions or rarefactions to cross a fixed points is called the time period of the wave.

Ques 13: What is the minimum distance required to hear distinct echo?
Ans: The minimum distance of the obstacle from the source of sound should be 17.2 m.

Ques 14: What is reverberation?
Ans: The repeated reflection that results in the persistence of sound is called reverberation.

Ques 15: What is SONAR?
Ans: SONAR is—Sound Navigation and Ranging. It is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects by getting the reflection of sound.

Ques 16: What is ‘ultrasonic’ and ‘infrasonic’ sound wave?
Ans: Sound waves with frequencies below the audible range (less than 20 Hz) are termed as “infrasonic” and those sound waves with frequencies above the audible range (more than 20000 Hz) are termed as “ultrasonic”.

Ques 17: What should be the time interval between the originated sound and the reflected sound to be heard distinctly?
Ans: To hear a distinct sound the time interval between the originated sound and the reflected sound must be at least 0.1 second.

SHORT ANSWER TYPE QUESTIONS
Ques 1: What is a medium? Give two examples.
Ans: The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Example, air, water, metals.

Ques 2: Define wave-motion.
Ans: A wave is a disturbance that moves through a medium when the particles of the medium set neighbouring particles into motion. The particles of the medium do not move forward but the disturbance is carried forward.

Ques 3: What is ‘sonic boom’?
Ans: When an object just attains a supersonic speed, it causes shock waves in air. As a result there is large change in air pressure. This results in sonic boom.

Ques 4: Why does sound become faint with distance?
Ans: Sound is a form of energy. As it moves away from the source its amplitude as well as its loudness decreases. The energy also get transformed in vibration of the particles of the medium.

Ques 5: Why do we say that sound waves are longitudinal?
Ans: Longitudinal waves need medium for propagation. The sound energy travel in the same line as the particles oscillate.
———— > Sound energy
<————>Particles oscillation
It forms compression and rarefaction for the longitudinal wave motion. Sound wave shows all the characteristics of longitudinal wave so it is called as longitudinal wave.

Ques 6: Differentiate between longitudinal wave and transverse wave.
Ans:

Ques  7: What is crest and trough?
Ans: When a wave is propagated as represented below. A peak is called the crest and a valley is called the trough of a wave.


Ques 8: The maximum oscillation disturbance of particles of air forms crest and trough. What is echo? Why don’t we get echo in small room?
Ans: The distinct sound heard after reflection of sound from the source is called echo. For echo, the distance of reflecting surface from the source should be more than 17.2 m.

Ques 9: What is velocity of sound? Why does sound travel faster in summer season than in winter?
Ans: Velocity of sound is- the speed of sound in a given medium at a given temperature. As the temperature increases the speed of sound also increases, hence in summer the sound travels faster than in winter.

Ques 10: Draw a graphical representation of the wave shape for
(a) low pitched sound
(b) a high pitched sound.
Ans:


Ques 11: Give two applications of echo/reflection of sound.
Ans: (i) Ships use reflection of sound technique “SONAR” which helps in locating the depth, distance, direction and speed of underwater objects.
(ii) Ceilings of concert halls are curved so that sound after reflection reaches all comers of the hall.

Ques 12: Define amplitude time period and frequency of sound wave.
Ans: Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called amplitude of the wave. Its unit is meter.
Time Period: The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave.
Frequency: The number of oscillation, occurring per unit time is called the frequency of sound wave.

Ques 13: A sound wave causes the density of air at a place to oscillate 1200 times in 2 minutes. Find the time period and frequency of the wave.
Ans: Frequency = 1200/2 ×  60 = 10Hz
Time period = ? Frequency = 1/T
∴ T = 1/Frequency = 1/10 = 0.1 s.

Ques 14: Give 3 uses of ultrasound.
Ans: Use of ultrasound:
1. Ultrasound is used to detect cracks and flaws in metal blocks.
2. It is used in ‘echo-cardiography’, the ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart.
3. It is used in ‘ultrasonography’, to detect the image of organs or to detect the abnormalities in the organs. It is also used to examine the foetus during pregnancy to detect congenital defects.

Ques 15: What is the function of middle ear?
Ans: Middle ear consist of three small bones called hammer, anvil and stirrup. These three bones receive the sound vibrations and increase the strength of these vibrations to amplify the vibrations received by ear-drum. These amplified vibrations are furthgr passed to the inner ear.
A ship sends out ultrasound that return from the seabed and is detected after 3.42 s.

Ques 16: If the speed of ultrasound through seawater is 1531 m/s. What is the distance of the seabed from the ship?-
Ans: Time between transmission and detection t = 342 s.
Speed of ultrasound in seawater = 1531 m/s.
Distance travelled by the ultra sound = 2 × deoth of sea = 2d
2d = speed of sound ×  time
= 1531 ×  3.42 = 5236 m
∴ 2d = 5236 m
∴ d = 5236/2 = 2618 m.
The distance of the seabed from the ship is 2618 m.

Ques 17: Distinguish between tone, note and noise.
Ans: Tone: A sound of single frequency is called a tone.
Note: The sound which is produced due to a mixture of several frequencies is called a note.
Noise: The sound which is produced due to a mixture of several frequencies but is unpleasant to the ear is called noise.

Ques 18: Establish the relationship between speed, wavelength and frequency of sound.
Ans: Speed of sound —» The distance travelled by a wave or a point on a wave (compression or rarefaction) per unit time.
Speed v = Distance/Time
v = λ/T
Distance = wavelength of the sound wave, it is the distance travelled by the sound wave in one time period (T) of the wave.
∴  v = λ/T   (as v = 1/T)
∴  v =  λv, Frequency = 1/Time period
Speed = Wavelength * Frequency.

Ques 19: Which wave property determines
(a) loudness? 
(b) pitch?
Name the characteristic of the sound which help you to distinguish your friend’s voice while talking in a dark room.
Ans: (a) Loudness is determined by amplitude.
(b) Pitch is determined by frequency.
The quality or timber of sound helps us to distinguish our friend’s voice while talking in a dark room.

Ques 20: A sound produces 13 crests and 15 troughs in 3 seconds. When the second crest is produced the first is 2 cm away from the source? Calculate.
(a) the wavelength 
(b) the frequency 
(c) the wave speed.
Ans:

(a) Wavelength = distance between two consecutive crests or troughs is 2 cm.
(b) Frequency = Number of troughs/Time  = 15/3 =5 Hz
(c) Wave speed = Distance/Time
Distance travelled by weve  = 15 * 2 = 30cm
Time = 3 seconds
∴ Wave speed = 30/3 = 10 cm/s.

LONG ANSWER TYPE QUESTIONS
Ques 1: Sound cannot travel in vacuum. Describe an experiment to demonstrate this.
Ans: Sound is a mechanical wave and needs a material medium to propagate. It cannot travel in vacuum and can be shown by the following experiment.
• Take an electric bell and an airtight glass bell jar. The electric bell is suspended inside the airtight bell jar. Switch ‘ON’ the electric bell.
• Now, connect the bell jar to vacuum pump.
• Pump out the air from the jar, the sound becomes fainter, although the same current passes through the bell.
• Pump out some more air from the jar, a very feeble sound is heard.
• When the air is completely removed from the jar, no sound is heard.


Ques 2: Given that sound travels in air at 340 m/sec, find the wavelength of the waves in air produced by 20 kHz sound source. If the same source is put in a water tank, what would be the wavelength of the sound waves in water? (Speed of sound in water = 1480 m/s.)
Ans:  Speed of sound in air = 340 m/s.
Frequency = 20 kHz = 20* 10³ Hz
Wavelength = ?
∴ Speed = Wavelength × Frequency
v = λv

Speed of sound in water = 1480 m/s
Frequency = 20 ×  10³ Hz
Wavelength = ?
∴ Speed = Wavelength ×  Frequency
Wavelength = Speed /Frequency
= 1480/20 10³ = 0.074 m.

Ques 3: A child watching Dussehra celebration from a distance sees the effigy of Ravana burst into flames and hears the explosion associated with it 2 sec after that. How far was he from the effigy if the speed of sound in air that night was 335 m/sec?
Ans: Speed of sound in air = 335 m/s.
time required to  reach the sound = 2 sec
distance of the source of sound = ?
∴ Speed = Distance/Time
∴ Distance = Speed × Time
= 335 ×  2 sec
= 670 m.

ACTIVITY-BASED QUESTIONS
Ques 1: • Take a tuning fork and set it vibrating by striking its prong on a rubber pad. Bring it near your ear.
• Do you hear any sound?
• Touch one of the prongs of the vibrating tuning fork with your finger and share your experience with your friends.
• Now, suspend a table tennis ball or a small plastic ball by a thread from a support. Touch the ball gently with the prong of a vibrating tuning fork.
• Observe what happens and discuss with your friends.

Ans: • Yes, we heard sound.
• If we touch the ball with tuning fork set into vibration, the ball gets displaced from its mean position and starts moving.

Ques 2: • Fill water in a beaker or a glass up to the brim. Gently touch the water surface with one of the prongs of the vibrating tuning fork.
• Next dip the prongs of the vibrating tuning fork in water. .
• Observe what happens in both the cases.
• Discuss with your friends why this happens.
• Arrange them on a table near a wall.
• Keep a clock near the open end of one of the pipes and try to hear the sound of the clock through the other pipe.
• Adjust the position of the pipes, so that you can best hear the sound of the clock.
• Now, measure the angles of incidence and reflection and see the relationship between the angles.

Ans: In both the cases, sound will be produced by the tuning fork which produces ripples. But in case
(1) ripples are produced which will move up and down and in case
(2) ripples are produced which will move in sideways.

Ques 3: • Take a slinky. Ask your friend to hold one end. You hold the other end. Now stretch the slinky and give it a sharp push towards your friend.
• What do you notice? If you move your hand pushing and pulling the slinky alternatively, what will you observe?
• If you mark a dot on the slinky, you’ll observe that the dot on the slinky will move back and forth parallel to the direction of propagation of the disturbance.

Ans: When we give a small jerk a hump is produced and this travels forward. When we give a sharp push continuous disturbance is produced. When we give a push or pull to the slinky, slinky starts moving in the forward and backward direction parallel to the direction of propagation of the disturbance.

Ques 4: • Take two identical pipes. The length of the pipes should be sufficiently long.

• Lift the pipes on the right .vertically to a small height and observe what happens.
Ans: (i) Reflection of sound is similar to reflection of light i.e. Angle of incidence = Angle of reflection.
(ii) If we lift the pipe vertically to a small height, well not be able to hear the sound through the other end of the pipe because Angle of incidence * Angle of reflection. Therefore the reflected ray will not travel through the pipe B.

VALUE-BASED QUESTIONS
Ques 1: Raj noticed that his pet dog was frightened and trying to hide in safe place in his house when some crackers were burst in the neighbourhood. He realized the problem and he decided not to burst crackers during diwali or for any other celebrations.
(a) What must be the range of crackers sound?
(b) Name two diseases that can be caused due to noise pollution.
(c) Name the values of Raj reflected in above act.
Answer: (a) The range of crackers sound must be between 20 Hz to 20 kHz.
(b) Two diseases that can occur due to noise pollution are heart attack and high blood pressure.
(c) Raj reflects the value of respecting sensitivity for animals and caring for animals.

Ques 2: It is not advisable to construct houses near airports, in spite of that many new residential apartments are constructed near airports. Sumit files RTI and also complains the municipal office about the same.
(a) Why one should not reside near airport?
(b) Name other two places where there is noise-pollution.
(c) What value of Sumit is reflected in this act?
Ans: (a) The landing and taking off of the air-planes causes lot of noise pollution which may lead to deafness, high blood pressure and other health problems.
(b) The other two places where there is noise-pollution is, residing near the heavy traffic routes and railway stations or lines.
(c) Sumit shows participating citizen and moral responsibility values.