Matric Notes Physics 10th Chapter 11 Sound Exercise Questions

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Physics 10th Chapter 11 Sound Exercise Questions

11.1 What is the necessary condition for the production of sound?

Ans. All sounds are produced by the vibrations of objects, Sound is a form of energy that travels in the form of longitudinal wave from one place to another.

11.2 What is the effect of the medium on the speed of sound? In which medium sound travels faster: air, solid or liquid? Justify your answer.

Ans. Speed of sound depends upon density of medium. Greater the density of medium, greater will be the  speed of sound wave. The speed of sound in liquid is five times than in gases and the speed of sound in solid is about fifteen times than in gases. Sound travels faster in solids because their density is greater than liquids and gases.

11.3 How can you prove the mechanical nature of sound by a simple experiment?

Ans. Sound waves always require a medium for its propagation which is shown in following experiment.

An electric bell which is suspended in the glass jar with the help of wires connected to a power supply. When we turn on the power supply the bell with ring and we can hear its sound. But when we start pumping out air with the help of vacuum pump the sound of bell becomes feeble and eventually dies out, and we cannot hear the sound, although the bell is ringing, when we again put air into the jar. We can hear the sound again which shows that sound waves require medium for their propagation. This experiment proves the mechanical nature of sound waves.

11.4 What do you understand by the longitudinal wave? Describe the longitudinal nature of sound waves.

Ans. Longitudinal waves are type of mechanical waves which travel in the form of compressions and rarefaction. The nature of sound wave can be explained by the following experiment.

When we strike the tuning fork on a rubber pad its prongs will begin to move between positions AOB. When prongs move from O to B, they exert pressure on the adjacent layer of air and compression is produced. The compressed layer of air compresses the layer of air next to it and so on. When prongs move from B to A, the pressure on layer of air is decreased and rarefaction is produced. This rarefaction is transferred to the air layer next to it and so on. So when the tuning fork moves back and forth rapidly, a series of compression and rarefaction is produced due to which sound waves propagate through air. As the direction of sound wave is along the direction of oscillating air molecules, which shows that sound waves are longitudinal in nature.

 

11.5 Sound is a form of wave. List at least three reasons to support the idea that sound is a wave.

Ans. Sound is a form of energy which obeys all the laws of reflection, rarefaction and diffraction.

11.6 We know that waves manifest phenomenon of reflection, refraction and diffraction. Does sound also manifest these characteristics?

Ans. Yes, Sound also manifest these characteristics. When sound is incident on the surface of a medium it bounces back into the first medium. This phenomenon is called reflection of sound or echo. Sound waves can be transmitted from one medium to other containing particles that can vibrate. On transmission their speed changes. Sound can also bend around obstacles that also prove that sound also shows the characteristic of diffraction.

11.7 What is the difference between the loudness and intensity of sound? Derive the relationship between the two.

Ans. Loudness: Loudness is the characteristic of sound by which loud and faint sound can be distinguished. The magnitude of the sensation produced in the ear by a called is called its loudness. It depends upon the intensity and also depends upon the physical condition of ear.

Intensity of sound: Sound energy passing per second through a unit area held perpendicular to the direction of propagation of sound waves is called intensity of sound. It is a physical quantity. Its unit is watt per square meter (Wm-2). It does not depend on the physical condition of the ear.

Relationship between loudness and intensity of sound Or Sound Intensity Level

For human ear, loudness of sound is not directly proportional to its intensity, but it is directly proportional to the logarithm of intensity. Or loudness of sound is directly proportional to the logarithm of sound.

Mathematically:

L= log⁡I

L= K log⁡I   ------(1)

Where ‘K’ is constant of proportionality.

If Lo represent the loudness of the faintest audible sound of intensity Io and L is the loudness of an unknown sound of intensity I, then above equation can be written as.

Lo=K  log⁡Io  ------(2)

Subtracting eq 2 from eq 1

 L- Lo=K log⁡I- K  logIo    

 L- Lo=K  (log⁡I - logIo)   

 L- Lo=K  log(I/Io)

Sound level or intensity level:

The difference between the loudness of two sounds (L – Lo) is called intensity level or sound level.

Intensity level =  L- Lo= K  log⁡(I/Io)-----(3)

Value of K

The value of K depends upon the unit of I and Io and on unit of intensity level. If the intensity I of any unknown sound is 10 times greater than the intensity of Io of the faintest audible sound. i.e. I = 10Io, and 

Intensity level of this sound is taken as unit, called Bel. Then value of K is 1.

Put k=1 in eq 3

Intensity level = L- Lo=log⁡(I/Io)

Unites of Intensity level:

Bel is the unit of sound level. It is a very large unit so a smaller unit decibel is used. It is abbreviated as dB.

1 bel = 10 dB

Intensity level =  L- Lo=10log⁡(I/Io)

11.8 On what factors does the loudness of sound depend?

Ans. Loudness of sound depends upon a number of factors. Some of them are listed below.

1. Amplitude of the vibrating body
2. Area of the vibrating body
3. Distance from the vibrating body
4. Intensity of sound

11.9 What do you mean by the term intensity level of the sound? Name and define the unit of intensity level of sound.

Ans. Sound level or intensity level: The difference between the loudness of two sounds (L – Lo) is called intensity level or sound level.

Intensity level = L- Lo= log⁡(I/Io) (Bel)

Unites of Intensity level:

Bel is the unit of sound level. It is a very large unit so a smaller unit decibel is used. It is abbreviated as dB.

1 bel = 10 dB

Intensity level =  L- Lo=log⁡(I/Io)(Bel)

Intensity level =  L- Lo=10log⁡(I/Io) (decibel)

11.10 What are the units of loudness? Why do we use logarithmic scale to describe the range of the sound intensities we hear?

Ans. Units of sound level is Bel and smaller unit of sound level is (dB). 1 bel = 10dB

Since the range of intensities is so wide and is difficult to calculate their sound level hence we use the scale by factor of ten in this way sound level is easily calculated by logarithm scale.

11.11 What is the difference between frequency and pitch? Describe their relationship graphically.

Ans. Pitch: Pitch is characteristics of sound due to which we can distinguish between shrill and grave sound. While frequency is the number of waves passing through a point in one second.

 

11.12 Describe the effect of change in amplitude on loudness and the effect of change in frequency on pitch of sound.

Ans. The loudness of the sound varies directly with the amplitude of the vibrating body. The sound produced by a sitar will be loud if we pluck its wires more violently. Similarly when we beat a drum forcefully, the amplitude of its membrane increases and we hear a loud sound.

Pitch is the characteristic of sound by which we can distinguish between a shrill and a grave sound. It depends upon the frequency. A higher pitch means a higher frequency and vice versa.

11.13 If the pitch of sound is increased, what are the changes in the following?

a. the frequency   b- the wavelength   c. the wave velocity   d. the amplitude of the wave.

Ans. (a) If the pitch of sound is increased frequency will also increase.

(b) With the increase of pitch wavelength of the waves decreases according to the relation. λ = v/f.

(c) If the pitch increases then velocity of the wave also increases according to the relation. V = fλ.

(d) Pitch is independent of amplitude of the sound wave so amplitude of the wave does not change with the change of pitch.

11.14 If we clap or speak in front of a building while standing at a particular distance, we rehear our sound after sometime. Can you explain how this happens?

Ans. If we clap or speak in front of a building while standing at a particular distance, we rehear our sound after sometime. This is due to reflection of sound wave from the surface of building.

11.15 What is the audible frequency range for human ear? Does this range vary with the age of people? Explain.

Ans. The range of the frequencies which a human ear can hear is called audible frequency range. It lies between 20Hz and 20,000Hz. Different people have different range of audibility. It also decreases with age. Young children can hear 20,000Hz but old people cannot hear sounds even above 15,000Hz.

11.16 Explain that noise is a nuisance.

Ans. Noise Pollutions:

Noise pollution has become a major issue of concern in big cities. Noise is an undesirable sound that is harmful for health of human and other species.

Sources of noise pollution:

Transportation equipment and heavy machinery are the main sources of noise pollution. For example, noise of machinery in industrial areas, loud vehicle horns, hooters and alarms.

Negative effects of sound:

Noise has negative effects on human health as it can cause conditions such as hearing loss, sleep disturbances, aggression, high stress levels. Noise can also cause accidents by interfering with communication and warning signals.

Safe Level of Noise:

A safe level of noise depends on two factors:

1. the level (volume) of the noise; and
2. the period of exposure to the noise.

The level of noise recommended in most countries is usually 85-90 dB over an eight-hour workday.

Ways to reduce noise

Pollution can be replaced to acceptable level by replacing the noisy machinery with environment friendly machinery and equipment, putting sound-reducing barriers, or using hearing protection devices.

11.17 Describe the importance of acoustic protection.

Ans. The technique or method used to absorb undesirable sounds by soft and porous surfaces is called acoustic protection.

Explanation:

Soft, porous materials, such as draperies and rugs absorb large amount of sound energy and thus quiet echoes and soften noises. Thus, by using such material in noisy places we can reduce the level of noise pollution. However,

1. If the surfaces of class rooms or public halls are too absorbent, the sound level may be low for the audience.
2. Sometimes, when sound reflects from the wall, ceiling, and floor of a room, the reflecting surfaces are too reflective and the sound becomes grabbled. This is due to multiple reflections called reverberations.
3. In the design of lecture halls, auditorium or theater halls, a balance must be achieved between reverberations and absorption. It is often advantageous to place reflective surfaces behind the stage to direct sound to the audience.
4. Generally the ceilings of lecture halls, conference halls and theatre halls are curved so that sound after reflection may reach all the corners of the hall. Sometimes curved soundboards are placed behind the stage so that sound after reflection is distributed evenly across the hall.

11.18 What are the uses of ultrasound in medicine?

Ans. 1- In medical field, ultrasonic waves are used to diagnose and treat different ailments. For diagnosis of different diseases, ultrasonic waves are made to enter human body through transmitters. These waves are reflected differently by different organs, tissues or tumors. The reflected waves are then amplified to form an image of the internal organs of the body on the screen. Such an image helps in detecting the defects in these organs.

2- Powerful ultrasound is now being used to remove blood clots formed in the arteries.

3- Ultrasound can also be used to get the pictures of thyroid gland for diagnosis purposes.

Extra Question:

11.19 Is any medium required fro propagation of sound?

Ans. Yes, material medium is required for propagation of sound, i.e. solid, liquid, and gas.

11.20 Write the name of characteristic of sound?

Ans. There are five characteristics of sound.

1. Loudness of sound
2. Intensity of sound
3. Pitch
4. Quality of sound
5. Music and noise

11.21 Distinguish between noise and musical sound?

Ans. The sound which has pleasant effect on ear is called musical sound. The sound which has jarring effect is called noise. The frequency and amplitude of musical sound change in a regular manner and in noise frequency and amplitude does not change in regular manners.

11.22 What is white whistle?

Ans. Some people use silent whistle to call dogs whose frequency lies between 20,000 Hz to 25,000 Hz. It is silent for human but not for dogs because their audible frequency range is much more than human audible frequency range.

11.23 Describe the compression and rarefactions produced in the sound wave?

Ans. Compression: Compression is places where air pressure is slightly higher than the surrounding air pressure due to high density of air particles.

Rarefactions: Rarefactions are the regions correspondence to low air pressure due to low density of air particles.

11.24 What is higher frequency range of the following? a. Bats b. Mice c. Dogs d. Cats

Ans. Bats: Bats can hear frequencies up to 120,000 Hz.

Mice: Mice can hear frequencies up to 100,000 Hz.

Dogs: Dog can hear frequencies up to 35,000 Hz.

Cats: Cat can hear frequencies up to 25,000 Hz.