Lesson 8 Sound
9th Science Guide: Hello, Students! This article contains all the Book Back Answers to the questions found at the end of each chapter in your 9th standard science book Lesson Sound. The guide includes a variety of question types such as multiple-choice, fill-in-the-blanks, assertion, and reason, as well as brief definitions and detailed explanations.
Sound Book Back Answer
Here are the answers to the book back questions for Lesson 8: Sound
I. Choose the correct answer:
1. Which of the following vibrates when a musical note is produced by the cymbals in an orchestra?
- stretched strings
- stretched membranes
- air columns
- metal plates
Ans: metal plates
2. Sound travels in the air
- if there is no moisture in the atmosphere.
- if particles of medium travel from one place to another.
- if both particles as well as disturbance move from one place to another.
- if disturbance moves.
Ans: if both particles as well as disturbance move from one place to another.
3. A musical instrument is producing continuous note. This note cannot be heard by a person having a normal hearing range. This note must then be passing through
- empty vessel
4. The maximum speed of vibrations which produces audible sound will be in
- ground glass
- dry air
- Human blood
Ans: ground glass
5. The sound waves travel faster
- in liquids
- in gases
- in solids
- in vacuum
Ans: in solids
6. If the speed of a wave is 340 ms-1 and its frequency is 1700 Hz, then wavelength λ for this wave in cm will be
7. Which of the following statement best describes frequency?
- the number of complete vibrations per second.
- the distance travelled by a wave per second.
- the distance between one crest of wave and the next one.
- the maximum disturbance caused by a wave.
Ans: the number of complete vibrations per second.
8. In the sound wave produced by a vibrating tuning fork as shown in the diagram, half the wavelength is represented by: Transverse Wave Wavelength Crust Crust
9. When the pitch of a note by a harmonium is lowered, then the wavelength of the note
- first decreases and then increases
- remains the same
10. The speeds of sound in four different media are given below. Which of the following is the most likely speed in ms-1 with which the two under water whales in a sea can talk to each other when separated by a large distance?
11. Which of the following can produce longitudinal waves as well as transverse waves under different conditions?
- TV transmitter
- tuning fork
12. The velocities of sound waves in four media P, O, Q, R and S are 18,00 km/h, 900 km/h, 0 km/h, and 1200 km/h respectively. Which could be a liquid medium?
II. Fill in the blanks.
1. Sound is a ________ wave and needs a material medium to travel.
2. Number of vibrations produced in one second is ________
3. The velocity of sound in solid is ________ than the velocity of sound in air.
4. Vibration of object produces _______
5. Loudness is proportional to the square of the _________
6. _________ is a medical instrument used for listening to sounds produced in the body.
7. The repeated reflection that results in persistence of sound is called ____________
8. A sound wave has a frequency of 4 k hz and wavelength 2 m. Then the velocity of sound is _________
9. Ultrasounds can also be used to detect cracks and flows in _________
Ans: metal blocks
10. In the inner ear, the pressure variations are turned into electrical signals by the _______
III. Match the following.
|1. Tuning fork||The point where density of air is maximum|
|2. Sound||Maximum displacement from the equilibrium position|
|3. Compressions||The sound whose frequency is greater than 20,000 Hz|
|4. Amplitude||Longitudinal wave|
|5. Ultasonics||Production of sound|
|Ans : 1 – E, 2 – D, 3 – B, 4 – C, 5 – A|
IV. Matrix matching.
|Loudness||Number of vibrations produced in unit time||decibel|
|Time period||The amount of sound produced / received||Metre|
|Amplitude||Distance travelled by sound in unit time||Hertz|
|Velocity of sound||The time required to produce one complete wave||Metre per second|
|Frequency||The maximum displacement from the mean position||second|
|Ans : 1 – B – A, 2 – D – E, 3 – E – B, 4 – C – D, 5 – A – C|
V. Answer in Brief
1. Through which medium sound travels faster, iron or water? Give reason.
Sound travels faster in iron.
- The speed of sound depends on the density of the medium
- Iron density is greater than water density.
2. Name the device which is used to produce sound in laboratory experiments.
Tuning fork is used to produce sound in laboratory experiments.
3. What is meant by supersonic speed?
When the speed of any object exceeds the speed of sound in air (330m/s) it is said to be travelling at supersonic speed
4. How does the sound produced by a vibrating object in a medium reach your ears?
when a rarefaction reaches it. In this way, the eardrum vibrates. 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 received from the sound wave to the inner ear. In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve and the brain interrupts them as sound.
5. You and your friend are on the moon. Will you be able to hear any sound produced by your friend?
- Sound needs a medium to propagate, so we will not be able to hear any sound on the moon.
- Moon doesn’t have any atmosphere
6. What should an object do to produce sound?
- An object should vibrate to produce sounds.
- When the object vibrates, nearby air and things also vibrate then produce the sound.
7. Can sound travel through a vacuum?
No, Sound travel through a vacuum. Sound needs a material medium to travel.
8. Name the physical quantity whose SI unit is ‘hertz’. Define.
Frequency: The number of vibrations produced in one second is called the frequency of the wave. * It is denoted as “n”
VI. Answer in detail.
1. Describe with a diagram, how compressions and rarefactions are produced.
The wave that propagates with compressions and rarefactions are called longitudinal waves. In longitudinal waves the particles of the medium move to and fro along the direction of propagation of the wave. Sound is also a longitudinal wave.
Sound can travel only when there are particles which can be compressed and rarefied. Compressions are the regions where particles are crowded together. Rarefactions are the regions of low pressure where particles are spread apart. A sound wave is an example of a longitudinal mechanical wave.
2. Verify experimentally the laws’ reflection of sound.
The laws of reflection are:
- The angle in which the sound is incident is equal to the angle in which sound is reflected.
- Direction of incident sound, direction of the reflected sound and the normal are in the same plane.
Take two identical pipes as shown in below. You can make the pipes using chart paper. The length of the pipes should be suf ciently long as shown in f gure. Arrange them on a table near 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 angle of incidence and ref ection and see the relationship between the angles. Lif the pipes on the right vertically to a small height, and observe what happens.
3. List the applications of sound
Megaphones, loud speakers, horns, musical instruments such as nathaswaram, shehnai and trumpets are all designed to send sound in a particular direction without spreading it in all directions. In these instruments, a tube followed by a conical opening ref ects sound successively to guide most of the sound waves from the source in the forward direction towards the audience.
Stethoscope is a medical instrument used for listening to sounds produced in the body. In stethoscopes these sounds reach doctor’s ears by multiple ref ections that happen in the connecting tube.
Applications of ultrasonic waves:
- Ultra sound can be used in cleaning technology. Minute foreign particles can be removed from objects placed in a liquid bath through which ultrasound is passed.
- Ultrasounds can also be used to detect cracks and faws in metal blocks.
- Ultrasonic waves are made to refect from various parts of the heart and form the image of the heart. Tis technique is called ‘echo cardiography’.
- Ultrasound may be employed to break small ‘stones’ formed in the kidney into fne grains. Tese grains later get fushed out with urine.
SONAR stands for Sound Navigation And Ranging. The Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.
The electrocardiogram (ECG) is one of the simplest and oldest cardiac investigations available. It can provide a wealth of useful information and remains an essential part of the assessment of cardiac patients. In ECG the sound variation produced by heart is converted into electric signals.
4. Explain how does SONAR work?
SONAR stands for Sound Navigation And Ranging. Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects. The Sonar consists of a transmitter and a detector and is installed at the bottom of boats and ships.The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the seabed, get reflected back and are sensed by the detector.
The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted. The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between transmission and reception of the ultra sound
Let the time interval between transmission and reception of ultrasound signal be ‘t’ and the speed of sound through sea water be 2d = v × t. This method is called echo-ranging. Sonar technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs etc.
5. Explain the working of the human ear with a diagram.
The outer ear is called the ‘pinna’. It collects the sound from the surroundings. The collected sound passes through the auditory canal. At the end of the ear is the eardrum or tympanic membrane. When a compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward when a rarefaction reaches it. In this way, the eardrum vibrates. 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 received from the sound wave to the inner ear. In the inner ear, the pressure variations are turned into electrical signals by the cochlea.
These electrical signals are sent to the brain via the auditory nerve and the brain interrupts them as sound.
VI. Numerical problems.
1. The frequency of a source of sound is 600 Hz. Calculate the number of times it vibrates in a minute.
Frequency of sound is = 600Hz
1 sec = 600Hz
1 min = 60 sec
60 sec = 60 x 600
Source of vibrate 36000 times.
2. A stone is dropped from the top of a tower 750 m high into a pond of water at the base of the tower. Calculate the number of seconds for the splash to be heard?
(Given g = 10 m s–2 and speed of sound = 340 m s–1)
|height (s) =||750m|
|Speed of sound (U) =||0|
|g =||10 ms-2|
|Speed of sound; =||340 ms-2|
|ஃ S||Ut1 + 1/2at12|
|750 =||0 (t1) + 1/2 x 10 x t12|
|t12 =||750/50 = 150|
|Time (t2) =||distance / speed|
|t2 =||2.205 s|
|The time for splash sound heard at the top is t =||t1 + t2|
|=||12.25 + 2.205|
|t =||14.455 s|
Last Updated: 1st July 2023