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.

 Sound

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 1 Explain with an example how sound is produced?

Q 2 Describe with the help of a diagram how sound propagates through air?

Q 3 What do you understand by the terms compression and rarefaction?

Q 4 Explain with example to show that sound cannot travel through vacuum?

Q 5 What are longitudinal waves?

Q 6 Sound waves are longitudinal. Prove?

Q 7 What are the main characteristics of sound waves?

Q 8 Explain crest and trough with the help of the graphical representation of sound wave?

Q 9 Explain the terms Wavelength and Amplitude?

Q 10 Explain the difference between Frequency and Pitch of a sound wave?

Q 11 Explain time period with respect to a sound wave?

Q 12 Frequency of a wave motion is 250 Hz. What is its time period?

Q 13 What do you mean by an Oscillation of a sound wave?

Q 14 Derive a relation between speed, frequency and wavelength of a sound wave?

Q 15 A sound wave has a frequency of 2 kHz and wavelength 40 cm. Calculate its speed?

Q 16 Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium?

Exam Questions NCERT Class 9 Science Chapter 12 Sound

Question. A sonar echo takes 2.2 s to return from a whale. How far away is the whale?
Ans : Total time taken by the signal = 2.2 s
So, time taken the signal to reach the whale = 1.1 s
Distance of the whale = d (assume)
Speed of sound in sea water at 25°C = 1533 ms–1
So, distance of the whale,
                             d = Speed of the signal × Time taken
or                          d = 1533 m s–1 × 1.1 s = 1686.3 m

Question. What do you mean by a wave?
Ans : A wave is a vibratory disturbance in a medium which carries energy from one point to another without being a direct contact between the two points.

Question. What does wave transfer-matter or energy?
Ans : Energy.

Question. Where is density of air higher at compression or at rarefaction?
Ans : At compression.

Question. A gun is fired in the air at a distance of 660 m, from a person. He hears the sound of the gun after 2 s. What is the speed of sound?
Ans : Distance travelled by sound = 660 m,
            Time taken by the sound = 2 s,
                 Speed of sound in air = ?
                   So, Speed of sound = Distance/Time
                                                =660/2s=330m/s
Thus, the speed of sound in the air is 330 m/s.

Question. Guess which sound has a higher pitch-guitar or car horn?
Ans : Guitar has a higher pitch because it has higher frequency.

Question. What is intensity of sound?
Ans : The amount of sound energy passing through unit area each second is called the intensity of sound.

Question. Why are longitudinal waves called pressure waves?
Ans : Sound waves travels in the form of compression and rarefactions, which involve change in pressure, and volume of the air. Thus, they are called pressure waves.

Question. Sound travels faster on a rainy day than on a dry day. Why?
Ans :Sound travels faster on rainy day because the velocity of sound increases with increase in humidity. On rainy day, humidity is more thus, velocity of sound is also more.

Question. How moths of certain families are able to escape capture?
Ans : Moths of certain families can hear high frequency sounds (squeaks) of bat as they have sensitive hearing equipment. Thus, they get to know when a bat is near by and hence, able to escape its capture.

Question. What is SONAR?
Ans : SONAR (Sound Navigation And Ranging) is a technique for determining water depth and locating underwater objects, such as reefs, submarines and schools of fish.

Question. Define one hertz.
Ans : One hertz is one vibration per second.

Question. Define wavelength.
Ans : It is the distance between two nearest points in a wave which are in the same phase of vibration.

Question. What is the audible range of the average human ear?
Ans : An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.

Question. What is sound and how is it produced?
Ans : Sound is mechanical energy which produces a sensation of hearing. When an object is set into vibrations, sound is produced.

Question. Why is sound wave called as longitudinal wave?
Ans : Sound wave is called longitudinal wave because the particles of the medium vibrate in the direction of the propagation of wave.

Question. Flash and thunder are produced simultaneously. But, thunder is heard a few seconds after the flash is seen, why?
Ans : The speed of light is 3 × 108 ms–1 whereas that of sound is 344 ms–1 in air. Thus, flash of lightning is seen at once, but sound takes few seconds to reach our ears.

Question. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?
Ans : No. of vibrations produced in 1 s = 100
 No. of vibrations produced in 60 (sec) = 1 min =
                                          100 × 60 = 6000.

Question. Name the two types of mechanical waves.
Ans : The two types of mechanical waves are :
(i) Transverse wave and
(ii) Longitudinal wave.

Question. Name the term associated with the travelling disturbance in a medium.
Ans : Wave.

Question. Do waves transport energy?
Ans : Yes.

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