# Sound| NCERT Solutions| Class 9

NCERT Solutions of  Chapter: Sound. NCERT Solutions along with worksheets and notes for Class 9.

Page 162

(1) How does the sound produced by a vibrating object in a medium reach your ear?

Ans- Sound moves from point of generation to our ear through a medium.  When an object vibrates, it makes nearest particle of the medium to vibrate also.

First particle of the object vibrates. Then it displaced (break) equilibrium of the particle of the medium and exerts a force on the particle of medium. As a result of exerting force, the particle of medium displaced from its position of rest. Now the particle of medium starts vibrating and set another adjacent new particle of medium to vibrate till sound reaches to our ear. After displacing the other adjacent particle, first particle comes back to its original position. This series go on till we hear sound.

Page 163

(1) Explain how sound is produced by your school bell.

Ans- When hammer strike on the bell, the bell starts vibrating. The particle of medium (air) near bell gets set to vibrate by particle of bell. Then other particles of the air start vibrating till we hear sound.

(2) Why are sound waves called mechanical waves?

Ans- Mechanical waves travels through medium and particles of medium do not leave their places. Sound waves also travels through medium and articles of medium do not leave their places but transfer energy to adjacent particle.

(3) Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Ans- No, I can not hear any sound produced by my friend on the moon because there is no medium (air)on the moon and sound can not travel without medium.

Page 166 A

(1) Which wave property determines (a) loudness (b) pitch?

Ans-

(a) Loudness of the sound is determined by the amplitude of the vibration of sound.

(b) Pitch (Shrillness) is determined by the frequency of sound.

(2) Guess which sound has a higher pitch: guitar or car horn?

Ans – Guitar has higher pitch than car horn because strings of guitar produced high frequency sound. High frequency determined high pitch of the sound.

Page 166 B

(1) What are wavelength, frequency, time period and amplitude of a sound wave?

Ans- (a) The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called wavelength.

It is represented by Greek letter λ.

Its SI unit is metre(m).

(b) The number of oscillations per second is known as frequency of oscillations. Therefore, the number of compressions or rarefactions that crosses our ear per unit time is known as frequency of sound waves.

Its unit is hertz(Hz). It is usually represented by ν.

(d) The maximum extent of a vibration from the point of equilibrium is known as amplitude of vibration. In other words, the maximum displacement of an object or a vibration is known as amplitude.

(2) How are the wavelength and frequency of a sound wave related to its speed?

Ans- Wavelength of sound is inversely proportional to frequency of a sound wave.

Page 166 (C)

(1) Distinguish between loudness and intensity of sound.

Ans- Differences between loudness and intensity of the sound

 SN Loudness Intensity 1. It is a measure of the response of the ear to the sound. It is the amount of sound energy that passing each second through unit area. 2. It is vary from person to person. It is not vary person to person. 3. It depends on the sensitivity of the ear. It does not depend on the sensitivity of the ear.

Page 167

(1) In which of three media, air, water or iron, does sound travel the fastest at a particular temperature?

Ans-  The speed of the sound depends on density of the medium. The pressure exerted by the particles of the medium is related to the number of the particles of a medium in a given volume. Therefore, more density of the particles in the medium gives more pressure and vice versa. Therefore, propagation of sound can be considered as propagation of density variations or pressure variations in the medium.

Increasing order of density of given medium is given below –

Iron, water and air.

Therefore, sound travels the fastest at a particular temperature in iron.

Page 168

Page 169

(1) Why are the ceilings of concert halls curved?

Ans- To reduce reverberation, the roof and walls of the auditorium is covered with sound – absorbent materials like compressed fiberboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties.

Page 170

(1) What is the audible range of the average human ear?

Ans- The range of audible frequencies of human ear is approximately from 20 to 20,000 Hz.

(2) What is the range of frequencies associated with

(a) Infrasound?

(b) Ultrasound?

Ans- (a) The range of frequencies associated with infrasound is less than 20 Hz.

(b) The range of frequencies associated with ultrasound is more than 20 kHz.

Page 172

Exercises

(1) What is sound and how is it produced?

Ans- Sound is a form of energy which produces a sensation of hearing through vibrating a transmission medium such as gas, liquid and solid.

Sound can be produced by striking, scratching, plucking, rubbing, blowing or shaking different objects.

Eg:- (i) When we rub our hand, sound has produced.

(ii) When we strike a hammer on iron sheet, sound has produced.

(iii) Take a tuning fork and strike it on  a rubber pad. A sound is produced by vibrating the tuning fork.

(2) Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.

Ans-

When the sound object vibrates forward, it pushes and compresses the particles of medium in front of vibrating objects creating a region of high pressure. This region is called as compression (C).

When the sound object vibrates backward, it creates a region of low pressure. This region is called as rarefaction (R).

As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the medium. In this way, compressions and rarefactions are produced in air near a source of sound.

(3) Cite an experiment to show that sound needs a material medium for its propagation.

Ans- Experiment

Take a airtight jar, an electric bell and a vacuum pump. Suspend the bell inside the jar and fit vacuum pump with jar. Now, press the bell switch, we can hear sound of bell. Now start vacuum pump, when air is pump out from the jar gradually, the sound of bell become fainter. When all air is pump out from the jar, we can not hear the sound of the bell.

Therefore, it is clear that sound needs a material medium for its propagation.

(4) Why is sound wave called a longitudinal wave?

Ans- Longitudinal waves are waves in which particles of medium do not leave their places but oscillate back and forth their position of rest.

Sound waves also propagate in the same way. Therefore Sound waves are longitudinal waves.

(5) Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?

Ans- Loudness and pitch of the sound will help us to identify your friend by his voice while sitting with others in a dark room.

(6) Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?

Ans- Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen because speed of the sound is slower than the speed of the light.

(10) Does sound follow the same laws of reflection as light does? Explain.

Ans– Yes , sound does follows the same laws of reflection as light.

The sound gets reflected at the surface of a solid and liquid.

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 at the point of incidence and the three lie in the same plane.”

(11) When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?

Ans- Echo is reflection of the sound which reaches to us after 0.1 second. The speed of the sound increases with the temperature. Therefore, we can hear echo on hotter day but time interval should be greater than 0.1 second.

(12) Give two practical applications of reflections of sound waves.

Ans- Echo and reverberation are two examples of the reflection of the sound waves.

(15) What is the reverberation? How can it be reduced?

Ans- The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation.

To reduce reverberation, the roof and walls of the auditorium is covered with sound – absorbent materials like compressed fiberboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties.

(16) What is loudness of sound? What factors does it depend on?

Ans- Loudness is a physiological response of the ear to the intensity of the sound.

Loudness of the sound is depends on the amplitude of the vibration of sound.

Eg:-  (a) If the amplitude of vibration is large, the voice of sound produced will be loud.

(b) If the amplitude of vibration is small, the voice of sound produced will be weak or soft.

(17) Explain how bats use ultrasound to catch a prey.

Ans- Bats emits and detects reflections of ultrasonic waves. The high – pitched ultra-sonic waves reached to the bat’s ear after reflecting from the obstacles or prey. Then, the bat comes to know place of the prey or obstacle. Therefore, the bat can catch its prey.

(18) How is ultrasound used for cleaning?

Ans- It is used for cleaning purpose of spiral rod, electronic components, odd shaped parts etc. The object is placed in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the particles of dust, grease get detached and drop out. Therefore, the object gets cleaned.

(19) Explain the working and application of a sonar.

Ans- Sonar is a device which is used to measure the distance, direction and speed of underwater objects by using ultrasonic waves.

It consists of a transmitter and a detector and is installed in a boat or a ship.

The transmitter produces ultrasonic waves and transmitted them. These waves travel through underwater and after striking the object on the seabed, they get reflected back. The detectors sensed them and converts ultrasonic waves into electrical signals which is interpreted. The distance of the object inside the sea can be calculated by multiplying speed of the sound and time interval between transmission and reception of the ultrasound.

Let time interval between transmission and reception of the ultrasound = t

Let speed of the sound through seawater = v

The total distance 2d = vt

This method is called echo – ranging. The sonar technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs, sunken ships etc.

(21) Explain how defects in a metal block can be detected using ultrasound.

Ans- Ultrasounds can be used to detect cracks and flaws in metal blocks.

Ultrasonic waves are allowed to pass through the metal blocks and detectors are used to detect the transmitted waves. If there is very small flaw or defect, the ultrasound gets reflected back warning the presence of defect.

(22) Explain how the human ear works.

Ans- The outer ear is called pinna which collected sounds from the surroundings and passes through the auditory canal. At the end of the auditory canal there is a thin membrane called eardrums. The eardrum is like a stretched rubber sheet. When our ear receives compressions of medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrums inward. Similarly, eardrums move outward when a rarefaction of the medium reaches to the eardrum. In this way eardrum begins vibrate and sends vibrations to the inner ear. The vibrations are amplified several times by three bones (The hammer, anvil and stirrup) in the middle ear. The middle ear transmits the amplified pressure variations to the inner ear. Here, the pressure variations are turned in to electrical signals are sent by the cochlea. These electrical signals are sent to the brain via the auditory nerve and the brain interprets them as sound.

Helping Topics

Sound

Worksheet Class 9