Q
1. An airplane produces a sound wave with frequency of 5 KHz and wavelength 30 m. In how much time would the sound wave cover the distance of 4 Km?

Solution

Frequency, ν = 5 KHz = 5000 Hz Wavelength, λ = 30 cm = 0.3 m Speed, v = ν λ = 0 .3 m x 5000 Hz = 1500 m/s Time = distance/ speed = 4000/1500 = 2.6 seconds

Q  2. How is sound produced in the following things? The buzzing of mosquito The sound of drums  The sound of guitar  The sound of trumpet

Solution

Vibration of mosquito wings produces buzzing sound. The sound of drum is caused by the vibration of its membrane when struck by drumsticks. The vibration of strings produces various sounds in guitar. The vibration of air inside tube of the trumpet produces sound.

Q  3. The frequency of a sound source is 600 Hz. The speed of sound in a medium is

330 m/s. What will be the linear distance between an adjacent crest and trough.

Solution

v = ν λ 330 = 600 x λ Hence, λ = 330/600 = 11/20 m The distance between adjacent crest and trough = λ/2 = 11/40 m

Q  4. What is meant by echocardiography?

Solution

It is used to study the heart-valve action. An image of the heart is obtained by getting ultrasonic waves reflected from various parts of the heart.

Q  5. What is reverberation?

Solution

The repeated reflection that results in the persistence of sound in a large hall is called reverberation.

Q  6. State laws of reflection of sound. Explain three applications based on reflection of sound.

Solution

(1) Angle of incidence = angle of reflection

(2) Incident sound, reflected sound and normal lie in same plane Applications:

(a) Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. In stethoscopes, the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.

(b) Megaphones are designed to send sound in a particular direction without spreading it in all directions. In it, a tube followed by a conical opening reflects sound successively to guide most of the sound waves from the source in the forward direction towards the audience.

(c) Generally the ceilings of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall.

Q  7. Do the medium particles have permanent displacement in wave motion?

Solution

No, medium particles simply oscillate to and fro about their equilibrium positions.

Q  8. What is an echo?

Solution

The sound returning back towards the source after getting reflected from a distant obstacle is called an echo.

Q  9. Sound produced by a thunderstorm is heard 10 s after the lightning is seen.

Calculate the approximate distance of the thunder cloud. (Given speed of sound = 340 m s–1)

Solution

Given that: Speed of sound = 340 m s–1 and t = 10 s Distance = Speed of sound × Time                = 340 m s–1 × 10 s →Distance = 3400 m

Q  10. When a person uses deodorant spray, the other person standing at a distance would hear the sound of spraying first and the fragrance of spray would reach him later. Why so?

Solution

The sound of spraying deodorant travels through the vibrations of air layers so it reaches first. But, the fragrance of deodorant reaches the other person through actual movement of air particles, therefore takes more time. This example shows that in sound waves there is no actual movement of particles from sound producing source to the ear otherwise the person standing at a distance would have heard and smelt the deodorant at same time.

Q  11. (i) Why is the ceiling and wall behind the stage of good conference halls or concert halls made curved?

(ii) Which property of sound leads to the formation of echoes? Briefly explain?

(iii) What is reverberation? What will happen if the reverberation time in a big hall is too long? How can we reduce it?

Solution

(i) Ceiling and walls are made curved so that sound after reflection reaches all corners of the hall.

(ii) Reflection leads to echoes. An echo is the phenomenon of repetition of sound of a source by reflection from an obstacle. To hear a distinct echo of a sound, the time taken by this sound to reach the listener after reflection should be (1/10) of a second. If speed of sound in air is 344 m/s, then the minimum distance of obstacle from the source of sound must be 17.2 m.

(iii) Reverberation- It is repeated reflection of sound that results in persistence of audible sound after the source has stopped emitting it. If the reverberation time in a big hall is too long, sound gets blurred, distorted and confusing to hear. This can be reduced by using sound absorbent material on roof, walls and seats.

Q  12. How does a megaphone work?

Solution

Megaphones are designed to send sound in a particular direction without spreading it in all directions. In these, a tube followed by a conical opening reflects sound multiple times to guide the sound waves in the direction towards audience.

Q  13. Name few instruments which are based on multiple reflection of sound?

Solution

Megaphone, Horns, stethoscope, trumpet, shehanais.

Q  14. Would the speed of sound on a warm day be different from the speed of sound on a cold day? If yes what would be the difference?

Solution

Speed of sound depends on the temperature, pressure and characteristics of the medium through which it is travelling. Therefore, the speed of sound would be greater on a warm day as compared to speed of sound on a cold day because with increase in temperature speed of sound increases.

Q  15. Can we hear the ringing of a mobile phone placed in a vacuum chamber?

Solution

No, we could not hear the ringing of a mobile phone placed in a vacuum chamber as sound waves needs a medium to travel and in vacuum there are no air particles.

Q  16. Can two astronauts talk on moon a they does on the surface of the earth?

Solution

No, the astronauts cannot talk on the surface of the moon because there is no atmosphere/medium present on the moon and sound cannot travel through vacuum. On earth air is present and hence they can talk with each other.

Q  17. What is cause of sensation of sound due to rolling of thunder?

Solution

Repeated reflection and reverberation is the cause of sound due to rolling of thunder.

Q  18. Wavelength of a siren in air is 1.2m. And if same siren is blown under water it has wavelength 6 m in water. Find the velocity of sound in water, if the speed of sound in air is 330 m/s.

Solution

Let v1, vbe the velocities of sound in air and water respectively. Wavelength of sound waves in air λ1 = 1.2 m Wavelength of sound waves in water λ2 = 6 m Since, source of sound is same, frequency will be same v= νλ1 330 m/s = ν x 1.2 ν = 330/1.2                                         …

(1)   Velocity of sound in water v= ν  λ2 ν = v2/6                                               …

(2) From equation 1 and 2 we get 330/ 1.2 = v/6 v= 330 x 6/1.2 = 1650 m/s is the velocity of sound wave in water.

Q  19. Sound of engine of a submarine would travel faster in water or in air?

Solution

Sound of engine of a submarine could be detected earlier (with help of instruments called hydrophones) in water as compared to in air. As sound waves travel faster in liquids as compared to in gases.

Q  20. How far does sound travel in air when a tuning fork of frequency 12 Hz makes 30 vibrations? Velocity of sound is 336 m/s.

Solution

Q  21. List three characteristics of sound waves. State the factors on which each of these characteristics depends.

Solution

1. Loudness- The sensation produced in the ear which enables us to distinguish between a loud and a faint sound is called loudness. Loudness is determined basically by the amplitude of the wave.

2. Pitch- It is that characteristic of sound which helps in differentiating between a shrill and a grave sound. It depends on the frequency of sound wave.

3. Quality or timbre- The characteristic of a sound which distinguishes it from another of the same pitch and loudness is called quality. It depends on the wave-form of the sound wave.

Q  22. List in tabular form two differences between longitudinal waves and transverse waves. Name a wave which does not require a material medium for its propagation.

Solution

Longitudinal wave Transverse waves

(i) The particles of the medium vibrate in a direction parallel to the direction of propagation of the wave.

(i) The particles of the medium vibrate in a direction perpendicular to the direction of propagation of wave.

(ii) Compressions and rarefactions are formed.

(ii) Crests and troughs are formed.

(iii) Eg.- sound waves

(iii) Eg.- Light waves Light waves, radio waves, etc. do not require any material medium for their propagation.

Q  23. Why ceilings of concert halls and conference halls are made curved?

Solution

The ceilings of concert halls and conference halls are generally curved. It is done to ensure that the sound after reflection from the ceilings reaches all corners of the hall.

Q  24. How is ultrasound used in various industries?

Solution

Ultrasound can be used for various commercial uses as follows:

1)Ultrasound is used in industry for detecting flaws (cracks, etc) in metal blocks. If there is a crack in metal block then ultrasounds get reflected and do not reach the ultrasound detector, thereby indicating a flaw.

2)Ultrasound is used in medical industry to investigate internal organs of the human body such as liver, gall bladder, uterus, kidneys, pancreas, heart, etc.

3)Ultrasound is also used to remove kidney stones by first breaking the kidney stones into fine granules which then could pass through urine.

4)Ultrasound is also now a days used extensively to see the well being of foetus (unborn child) inside the mother’s uterus.

5)Ultrasound is used in sonar apparatus to measure the depth of sea and to locate objects under sea like shipwrecks, submarines, sea rocks, hidden ice-bergs etc.

Q  25. Why do animals like dogs etc get uneasy before earthquakes?

Solution

Earthquakes produce low frequency infrasound before the main shock waves begin, therefore the animals like dog which could hear sounds below 20 hz, i.e infrasound get a warning and become disturbed.

Q  26. How does a hearing aid work?

Solution

A hearing aid is a battery operated electronic device. It receives sound through a microphone which converts the sound waves to electrical signals. These electrical signals are amplified by an amplifier and sent to speakers of hearing aid. The speaker converts the amplified electrical signal to sound and sends to the ear for clear hearing.

Q  27. In which medium the speed of sound will be greater -air or water? Give reason for your answer.

Solution

Sound travels faster in water than air because particles of water are closer to each other, that is water is denser than air, and hence vibrations could travel faster.

Q  28. (a) What is the role of hammer bone, anvil bone, stirrup bone and cochlea of human ear in hearing a sound?

(b) Write a difference between pitch of sound and loudness.

Solution

(a) The vibrations due to the ear drum are amplified several times by three bones hammer, anvil and stirrup in the middle ear. The pressure variations in the inner ear are turned into electrical signals by the cochlea.

(b) S.No Pitch Loudness 1 It is the way in which the brain interprets the frequency of a given sound. It is the physiological response of the ear to the intensity of sound.

2 It depends on frequency of the wave. It depends on amplitude (or intensity) of the wave.

Q  29. (i) Why is sound called a longitudinal wave?

(ii)Sound is produced due to vibrating motion, then why a vibrating pendulum does not produce sound?

Solution

(i) In a sound wave, particles of medium vibrate along the direction of motion of the wave. Thus, the sound wave is a longitudinal.

(ii) The frequency of the vibrating pendulum does not lie within the audible range (20Hz to 20,000Hz), and hence it does not produce audible sound.

Q  30. Does the sound of an exploded cracker in air travel faster than the sound produced by a humming bee? Why?

Solution

In air, both the sound, that is sound of an exploded cracker and the sound produced by a humming bee travel with same speed in air. It is because, the speed of sound in a medium is independent of the frequency of the sound.

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