# Maharashtra Board Class 11 Physics Chapter 8 Sound Solution

## Maharashtra Board Class 11 Physics Solution Chapter 8 – Sound

Balbharati Maharashtra Board Class 11 Physics Solution Chapter 8: Sound. Marathi or English Medium Students of Class 11 get here Sound full Exercise Solution.

 Std Maharashtra Class 11 Subject Physics Chapter 8 Chapter Name Sound

1.) Choose the correct alternatives

i) A sound carried by air from a sitar to a listener is a wave of following type.

(A) Longitudinal stationary

(B)Transverse progressive

(C) Transverse stationary

(D) Longitudinal progressive

Sound wave is longitudinal progressive wave.

ii) When sound waves travel from air to water, which of these remains constant?

(A) Velocity(B) Frequency

(C) Wavelength (D) All of above

Frequency is property of source and not the medium. Hence frequency remains constant when sound travel from air to water.

iii) The Laplace’s correction in the expression for velocity of sound given by Newton is needed because sound waves

(A) Are longitudinal

(B) Propagate isothermally

(D) Are of long wavelength

Newton considered the sound wave propagation as a isothermal but Laplace corrected it by adiabatic propagation.

iv) Speed of sound is maximum in

(A) Air (B) water

(C) Vacuum (D) solid

We know that speed of the sound in medium in decreasing order as solid>liquid>gas.Therefor speed of sound is maximum in solid.

v) The walls of the hall built for music concerns should

(A) Amplify sound (B) reflect sound

(C) Transmit sound (D) absorb sound

To avoid echoes for music concerns the wall of the hall is built to absorb sound.

i) Wave motion is doubly periodic. Explain.

Answer- doubly periodic means periodic in both time as well as space. Wave repeats itself after equal interval of time hence called periodic in time while wave motion repeats itself after equal distance hence called periodic in space hence wave motion is  doubly periodic in nature.

ii) What is Doppler Effect?

Due to relative motion between source of sound and listener of sound there is change in frequency of sound heard by listener. This apparent change in frequency of sound due to this relative motion is called Doppler effect of in sound.

iii) Define a transverse wave.

Answer- A wave in which particles of the medium vibrate in a direction perpendicular to the direction of propagation of wave is called transverse wave.

iv) Define a longitudinal wave.

Answer-A wave in which particles of the medium vibrate in a direction parallel to the direction of propagation of wave is called longitudinal wave.

v) State Newton’s formula for velocity of sound.

Answer- according to newton’s formula the velocity of sound depends on elasticity and density of the medium. Newton formulated the relation as

V = √(E/ρ)

Where E is the proper modulus of elasticity of medium and ρ is the density of medium.

vi) What is the effect of pressure on velocity of sound?

Answer-According to Laplace’s formula velocity of sound in air is

According to Boyles law, at constant temperature PV = constant.

We can conclude that at constant temperature velocity of sound does not change with change in pressure.

vii) What is the effect of humidity of air on velocity of sound?

Let ρm and ρd be the densities of moist and dry air respectively. If vm and vd are the speeds of sound in moist air and dry air then

We know that dry air has more density than moist air therefore vm>vd .hence the speed of sound in moist air is greater than speed of sound in dry air. i.e speed increases with increase in the moisture of air.

viii) What do you mean by an echo?

When the original sound reflects from some rigid surface at a distance from the source of sound a repetition of original sound is produced called as echo.

ix) State any two applications of acoustics.

1.) At very high power level, ultrasound is selective destroyer of pathological tissues in treatment of arthritis and certain type of cancer.

2.) Reflection of ultrasonic waves from regions in the interior of body is used for ultrasonic imaging. It is used for prenatal (before the birth) examination, detection of anamolous conditions like tumor etc. and the study of heart valve action.

x) Define amplitude and wavelength of a wave.

Amplitude- Amplitude of a wave motion is the largest displacement of a particle of the medium from its rest position.

Wavelength- : Wavelength is the distance between two successive particles which are in the same state of vibration.

xi) Draw a wave and indicate points which are (i) in phase (ii) out of phase (iii) have a phase difference of π/2.

i) Points in phase- A and E, C and G, B and H.

ii) Points which are out of phase-Hand F, Band D.

III) Points which have a phase difference of π/2- A and B, B and C, C and D.

xii) Define the relation between velocity, wavelength and frequency of wave.

Relation between velocity, wavelength and frequency of wave is given as,

Velocity = distance/time

We know that wavelength (λ) is the distance covered by wave in time perid of (T). therefore

Wave Velocity (V) = (wavelength (“λ” ) )/(time period(T))

(V) = ( (“λ” ) )/((T))

V = nλ where n is frequency and given as n = 1/T.

xiii) State and explain principle of superposition of waves.

When two or more waves travelling through a medium arrive at a point of medium simultaneously, each wave produces its own displacement at that point independent of the others. Hence the resultant displacement at that point is equal to the vector sum of the displacements due to all the waves.

If y1 and y2 are displacements due to first wave and second wave respectively then according to superposition theorem the resultant displacement will be

xiv) State the expression for apparent frequency when source of sound and listener are

i) moving towards each other

ii) moving away from each other

Answer- According to Doppler effect of sound the apparent frequency is given by

Where,

n = apparent frequency of sound due to relative motion

n0= original frequency of sound

v = velocity of sound in air

Vs = velocity of source

VL = velocity of listener

Where the upper signs (in both numerator and denominator) should be chosen when source and listener comes close to each other while lower signs should be used when they comes away from each other.

i) When source of sound and listener are moving towards each other here we have to use upper signs hence apparent frequency is given by

ii) When source of sound and listener are moving away from each other here we have to use lower signs hence apparent frequency is given by

xv) State the expression for apparent frequency when source is stationary and listener is

1) Moving towards the source

2) Moving away from the source

Answer-according to Doppler Effect of sound the apparent frequency is given by

Where,

n = apparent frequency of sound due to relative motion

n0= original frequency of sound

v = velocity of sound in air

Vs = velocity of source

VL = velocity of listener

Where the upper signs (in both numerator and denominator) should be chosen when source and listener comes close to each other while lower signs should be used when they comes away from each other.

when source is stationary and listener is Moving towards the source here vs=0 and listener moving towards source so use upper signs hence apparent frequency is given by,

when source is stationary and listener is Moving away the source here vs=0  and listener moving away source so use lower signs hence apparent frequency is given by,

xvi) State the expression for apparent frequency when listener is stationary and source is.

i) moving towards the listener

ii) Moving away from the listener

Answer-according to Doppler Effect of sound the apparent frequency is given by

Where,

n = apparent frequency of sound due to relative motion

n0= original frequency of sound

v = velocity of sound in air

Vs = velocity of source

VL = velocity of listener

Where the upper signs (in both numerator and denominator) should be chosen when source and listener comes close to each other while lower signs should be used when they comes away from each other.

i) when listener is stationary and source is moving towards the listener e. vL=0 as source is moving towards listener so use upper signs hence

ii) when listener is stationary and source is moving away the listener e. vL=0 as source is moving away listener so use lower signs hence

xviii) Define progressive wave. State any four properties.

The wave in which disturbance is travelling in medium from one point to the other point is called progressive wave.

Properties of progressive wave,

• Progressive waves are doubly periodic in nature i.e. periodic in time as well as periodic in space.
• All particles of medium having same amplitude and frequency.
• All particles are in motion i.e. no particle is at state of rest.
• These waves are of two types longitudinal and transverse progressive waves.

xix) Distinguish between transverse waves and longitudinal waves.