CBSE Class 9 Science Sound Assignment

1. Sound : Sound is a form of energy which produces a sensation of hearing in our ears. 

2. Source of sound and its propagation : A source of vibration motion of an object is normally a source of sound.

 3. Characteristics of the medium required for the propagation of sound: 

(i) Medium must be elastic so that the medium particles have the tendency to return back to their original positions after the displacement. 

(ii) Medium must have the inertia so that its particles have the capacity to store the energy. The frictional resistance of the medium should be negligible to minimise the loss of energy in propagation. 

4. Types of waves 

(i) Mechanical waves : A mechanical wave is a periodic disturbance which requires a material medium for its propagation. On the basis of motion of particles the mechanical waves are classified into two parts.
(a) Transverse wave (b) Longitudinal wave
(a) Transverse wave : When the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave, the wave is known as the transverse wave. For example, waves produced in a stretched string.
(b) Longitudinal wave : When the particles of the medium vibrate along the direction of propagation of the wave then the wave is known as the longitudinal wave. For example sound wave in air.

(ii) Electromagnetic waves : The waves which do not require medium for propagation are called electromagnetic waves these waves can travel through vacuum also. For example, light waves, X-rays. 

5. Characteristics of a sound wave 

(i) Frequency : The number of vibrations per second is called frequency. 
The unit of frequency is hertz 

(ii) Amplitude: The maximum displacement of each particle from its mean position is called amplitude.
The S.I. unit of amplitude is metre (m). 

(iii) Time period: The time taken to complete one vibration is called time period.
Frequency= 1/(Time period) or v = 1/T 

(iv) Wavelength: The distance between two nearest (adjacent) crests or troughs of a wave is called its wavelength. 

(v) Velocity of wave: The distance travelled by a wave in one second is called velocity of the wave (or speed of the wave). The S.I. unit for the velocity of a wave is metres per second (m/s or ms^-1). 

(vi) Pitch : Pitch is the sensation (brain interpretation) of the frequency of an emitted sound and is the characteristic which distinguishes a shrill (or sharp) sound from a grave (or flat) sound. 

(vii) Loudness : It is a measure of the sound energy reaching the ear per second. 

6. Reflection of sound : When sound waves strike a surface, they return back into the same medium. This phenomenon is called reflection. 

7. Laws of reflection : Angle of incidence is equal the angle of reflection. The incident wave, the reflected wave and the normal all lie in the same plane. 

8. Echo : Phenomenon of hearing back our own sound is called an echo. It is due to successive reflection from the surfaces obstacles of large size. 

9. Relation between speed of sound, time of hearing echo and distance of reflection body : If t is the time at which an echo is heard, d is the distance between the source of sound and the reflecting body and v is the speed of sound. The total distance travelled by the sound is 2d.
speed of sound, v = 2d/t or d = vt/2 

10. Conditions for the formation of Echoes
(i) The minimum distance between the source of sound and the reflecting body should be 17.2 metres.
(ii) The wavelength of sound should be less than the height of the reflecting body.
(iii) The intensity of sound should be sufficient so that it can be heard after reflection. 

11. Reverberation : Persistence of sound after its production is stopped, is called reverberation. A short reverberation is desirable in a concert hall (where music is being played) because it gives ‘life’ to sound. Too much reverberation confuses the programmers and must be reduced to reduce reverberation. 

12. Range of Hearing : The audible range of sound for human beings extends from about 20 Hz to 20,000 Hz (one Hz = one cycle/s). Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound. Frequencies higher than 20 kHz are called ultrasonic sound or ultra sound. Ultrasound is produced by dolphins. 

13. Applications of ultrasound : The ultrasound is commonly used for medical diagnosis and therapy, and also as a surgical tool. It is also used in a wide variety of industrial applications and processes. Some creatures use ultrasound for information exchange and for the detection and location of objects. Also some bats and porpoises are found to use ultrasound for navi gation and to locate food in darkness or at a place where there is inadequate light for vision (method of search is called echolocation). 

14. Sonar : SONAR means Sound Navigation Rang-ing. In this sound waves (ultrasonic) are used [microwaves are absorbed by water)]. Sound waves are emitted by a source. These waves travel in water with velocity v. The waves re-flected by targets (like submarine bottom sea) are detected. 

Uses 

(i) The SONAR system is used for detecting the presence of unseen underwater objects, such as a submerged submarine, a sunken ship, sea rock or a hidden iceberg, and locating them accurately. 

(ii) The principle of SONAR is also used in industry of detection of flaws in metal blocks or sheets without damaging them. 

15. Human ear : It is a highly sensitive part of the human body which enables us to hear a sound. It converts the pressure variations in air with audiable frequencies into electric signals which travel to the brain via the auditory nerve.

The human ear has three main parts. Their auditory functions are as follows: 

(i) Outer ear : The outer ear is called `pinna’. It collects the sound from the suri-ounding. The collected sound passes through the auditory canal. At the end of the auditory canal there is a thin membrane called the ear drum or tympanic membrane. When compression of the medium produced due to vibration of the object reaches the ear drum, the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward when a rarefaction reaches. In this way the ear drum vibrates. 

(ii) Middle ear: The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear which act as levers. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear. 

(iii) 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 interprets them as sound.

Mark (1)

Q 1 What are the uses of SONAR technique?

Q 2 What does ‗SONAR‘ stand for ?

Q 3 Which part of the ear turns pressure variations into electrical signals?

Q 4 What is intensity of sound?

Q 5 Why sound wave is called longitudinal wave?

Q 6 What is a sound?

Q 7 Why sound waves are called mechanical waves?

Q 8 What is a crest and a trough in a wave?

Q 9 Can sound wave travel through vacuum?

Q 10 What is amplitude of a wave?

Q 11 What is characterized by the amplitude of the wave?

Q 12 What is characterized by the frequency of the sound wave?

Q 13 What is reverberation?

Q 14 What is the audible range of the average human ear?

Q 15 What should be the minimum distance between a sound source and reflector for a distinct echo?

Q 16 What is a wave?

Q 17 Explain why the ceilings of concert halls and conference halls made curved?

Q 18 What is law of reflection in sound?

Marks (2)

Q 19 Priya clapped her hand near a cliff and heard the echo after 5 second. If the speed of sound be 330m/sec, then find the distance of the cliff from priya?

Q 20 What are ‗infrasonic‘ and ‗ultrasonic‘?

Q 21 Calculate the time in which a tuning fork of frequency 234 Hz completes 26 vibrations.

Q 22 We hear thunder sound a few seconds after the flash is seen even though they occur simultaneously. Why?

Q 23 A sound wave has a frequency of 1500Hz and wavelength 25cm. How long will it take to travel 3 km?

Q 24 Why and how is reverberation removed in an auditorium or a big hall?

Q 25 State Laws of Reflection of Sound

Q 26 Harry standing 440 m away from a wall fired a gun and heard its echo 2 s later. How is an echo formed? Calculate the velocity of sound in air.

Q 27 What is the frequency of an oscillating body?
The frequency of a source is 80Hz. Find the number of times it vibrates in a minute.

Marks (3)

Q 28 Explain how ultrasounds help to detect cracks and flaws in metal blocks.

Q 29 Define time period of sound wave. What is its SI unit? How is it related with frequency?

Q 30 Explain, how compressions and rarefactions are produced in air near a source of sound.
Q 31 How does sound reaches our ears?

Q 32 What will happen to the loudness of a sound wave if its amplitude is doubled? Name a unit of sound. Is loudness affected by change in frequency?

Q 33 How do sound waves propagate?

Q 34 Are the particles of the medium dragged along with the wave as sound wave propagates through the medium? In which medium—solid, liquid or gas, does the sound wave travel fastest?

Q 35 Find the distance travelled by a sound wave of frequency 20 KHz, wavelength 1.7 cm in 10 s.

Q 36 Define frequency of a sound wave. What is its SI unit and how is it related to pitch? What is the frequency of the wave shown below?

Q 37 What do you understand by frequency of a sound wave? What is the relationship between velocity of sound wave and its frequency?
The wavelength of sound emitted by a source is 1.25 × 10–2 m. If the velocity of the sound is 342 m/s, what is the frequency of the sound?

Q 38 Explain that sound is a mechanical wave. How do sound waves travel in different media? What is the approximate speed of sound in air?

Q 39 What is the relevance of amplitude and of frequency of a vibrating body to sound produced by it?

Q 40 Define echo. A sonar echo takes 2.2s to return from a whale. Find the distance of the whale below the surface of the water.
[Given the speed of sound in sea water at 25^oC = 1533m/s]

Q 41 Define the terms.
1. Audible range of normal human ear
2. Ultrasonics
3. Infrasonics
When vibration is a necessary condition for production of sound, then why the vibrating pendulum does not produce sound?

Q 42 Define speed of sound. Name the factors on which the speed of sound depends. Why the intensity of sound of a speaker is larger immediately after rain?

Marks (5)

Q 43 Explain The structure of human ear.

Q 44 What is an echo? Calculate the minimum distance in air required from a surface reflecting sound to hear an echo at 20oC.

Q 45 What is SONAR stands for? For what purpose it is used?
Suppose a sonar apparatus is attached to the ship and sends ultrasonic waves in the sea. When the waves reach the bottom of the sea, they are reflected back. If the ultrasonic waves take 6 seconds to travel from the ship to the bottom of the sea and back to the ship, then what will be the depth of the sea? [speed of sound in water = 1500ms^-1].

Q 46 Explain how defects in a metal block can be detected using ultrasound.

Q 47 Discuss SONAR.

Q 48 Explain the working and applications of SONAR.

Q 49 Rajan and Mohan are standing at opposite ends of an aluminium rod. Rajan strikes the end of the rod with a stone. Calculate the ratio of times taken by the sound waves in air and in aluminium to reach Mohan. [Velocity in aluminium = 6420m/s]

Q 50 Explain the statement ‗Wave transfers energy and not the matter‘ with the help of an activity.

Most Important Questions

Q 1 Fill in the blanks:

1. Sound is produced due to ________of different objects.

2. Sound travels as a _________ wave through a material medium.

3. Sound travels as successive _________ and ___________ in the medium.

Q 2 Fill in the blanks:

1. The distance between two consecutive compressions or two consecutive rarefactions is called the ___________.

2. The change in the density or pressure from a maximum value to minimum and then back to the maximum is called an ___________.

3. The number of periodic oscillations per unit time is called the ____________ of the sound wave.

Q 3 Fill in the blanks:

1. The loudness of sound is proportional to the _________ of the amplitude of the vibration.

2. The speed of sound __________as it moves from solid to gaseous state.

3. In any medium the speed of sound __________ with increase in temperature.

Q 4 Explain with an example how sound is produced?

Q 5 Describe with the help of a diagram how sound propagates through air?

Q 6 What do you understand by the terms compression and rarefaction?

Q 7 Explain with example to show that sound cannot travel through vacuum?

Q 8 What are longitudinal waves?

Q 9 Sound waves are longitudinal. Prove?

Q 10 What are the main characteristics of sound waves?

Q 11 Explain crest and trough with the help of the graphical representation of sound wave?

Q 12 Explain the terms Wavelength and Amplitude?

Q 13 Explain the difference between Frequency and Pitch of a sound wave?

Q 14 Explain time period with respect to a sound wave?

Q 15 Frequency of a wave motion is 250 Hz. What is its time period?

Q 16 What do you mean by an Oscillation of a sound wave?

Q 17 Derive a relation between speed, frequency and wavelength of a sound wave?

Q 18 A sound wave has a frequency of 2 kHz and wavelength 40 cm. Calculate its speed?

Q 19 Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium?

Q 20 Calculate a) the wavelength b) the time period of a tuning fork of frequency 512 Hz that is set to vibrate. Velocity of sound in air is 320 m/s.

Q 21 What is the wavelength of sound waves produced in air by a vibrating tuning fork whose frequency is 256 Hz when the velocity of sound in air is 330 m s^-1?

Q 22 Why the flash of lightning due to collision of clouds is seen much before the thunder, although both occur simultaneously?

Q 23 What do you understand by speed of sound? How does it depend upon the medium and temperature?

Q 24 What is reflection of sound? Explain the laws of reflection with respect to sound?

Q 25 What is Echo. Explain the conditions that have to be satisfied to hear an echo?

Q 26 Why do echoes produced in an empty auditorium usually decrease when it is full of audience?

Q 27 What is reverberation?

Q 28 A girl clapped his hands near a cliff and heard the echo after 5 s. What is the distance of the cliff from the person if the speed of the sound, velocity of sound in air is taken as 346 m/s?

Q 29 A boy fires a gun and hears the echo 2 seconds later. If he is 480 m away from a wall, calculate the velocity of sound in air?

Q 30 Explain the term audible range, Infrasonic frequencies, Ultrasonic frequencies?

Q 31 Explain how is the principle of echo used by
(a) The bat during its flight at night.
(b) The dolphin to locate small fish as its prey.

Q 32 Figure shows a waveform of frequency 50 Hz in a string. The numbers in the string represent distance in centimeter. For this wave motion, find:
(a) wavelength
(b) amplitude, and
(c) velocity

Q 33 What is ultrasound? Give some important applications of ultrasound?

Q 34 How Ultrasound is used to detect defects in metals?

Q 35 What is SONAR?

Q 36 Explain the working of SONAR.

Q 37 Suppose a ship sends a pulse of ultrasound and receives an echo 0.2 seconds later. If the speed of sound in water is 1000 m/s calculate its depth?

Q 38 With the help of a diagram, explain the structure of a human ear?