Physics · Textbook solutions

Sound

Every solved example, exercise, and miscellaneous question — in the order the textbook teaches them. · 42 questions

8. Sound — worked examples

7 q

Solved Examples

Worked · 7
  1. Solved Ex.8.1
    The speed of sound in air is 330 m/s and that in glass is 4500 m/s. What is the ratio of the wavelength of sound of a given frequency in the two media?
  2. Solved Ex.8.2
    Suppose you are listening to an out-door live concert sitting at a distance of 150 m from the speakers. Your friend is listening to the live broadcast of the concert in another country and the radio signal has to travel 3000 km to reach him. Who will hear the music first and what will be the time difference between the two? Velocity of light =3×108= 3 \times 10^{8} m/s and that of sound is 330 m/s.
  3. Solved Ex.8.3
    Consider a closed box of rigid walls so that the density of the air inside it is constant. On heating, the pressure of this enclosed air is increased from P0P_0 to PP. It is now observed that sound travels 1.5 times faster than at pressure P0P_0, calculate P/P0P/P_0.
  4. Solved Ex.8.4
    A man shouts loudly close to a high wall. He hears an echo. If the man is at 40 m from the wall, how long after the shout will the echo be heard? (speed of sound in air = 330 m/s)
  5. Solved Ex.8.5
    When heard independently, two sound waves produce sensations of 60 db and 55 db respectively. How much will the sensation be if those are sounded together, perfectly in phase?
  6. Solved Ex.8.6
    A rocket is moving at a speed of 220 m/s towards a stationary target. It emits a wave of frequency 1200 Hz. Some of the sound reaching the target gets reflected back to the rocket as an echo. Calculate (1) The frequency of sound detected by the target and (2) The frequency of echo detected by rocket (velocity of sound = 330 m/s.)
  7. Solved Ex.8.7
    A bat, flying at velocity VB=12.5V_B = 12.5 m/s, is followed by a car running at velocity VC=50V_C = 50 m/s. Actual directions of the velocities of the car and the bat are as shown in the figure below, both being in the same horizontal plane (the plane of the figure): the line joining the car and the bat is horizontal, the car's velocity VC=50V_C = 50 m/s points forward and upward at 53∘53^\circ to that line, and the bat's velocity VB=12.5V_B = 12.5 m/s points forward and upward at 37∘37^\circ to the same line, the bat being ahead of the car. To detect the car, the bat radiates ultrasonic waves of frequency 36 kHz. Speed of sound at surrounding temperature is 350 m/s. There is an ultrasonic frequency detector fitted in the car. Calculate the frequency recorded by this detector. The ultrasonic waves radiated by the bat are reflected by the car. The bat detects these waves and from the detected frequency, it knows about the speed of the car. Calculate the frequency of the reflected waves as detected by the bat. (sin⁡37∘=cos⁡53∘≈0.6, sin⁡53∘=cos⁡37∘≈0.8)(\sin 37^\circ = \cos 53^\circ \approx 0.6,\ \sin 53^\circ = \cos 37^\circ \approx 0.8)

Exercises

35 q

Choose the correct alternatives

Practice · 5
  1. Choose the correct alternatives
    Ex Q.1 (i)
    A sound carried by air from a sitar to a listener is a wave of following type.
    1. A.
      Longitudinal stationary
    2. B.
      Transverse progressive
    3. C.
      Transverse stationary
    4. D.
      Longitudinal progressive
  2. Ex Q.1 (ii)
    When sound waves travel from air to water, which of these remains constant ?
    1. A.
      Velocity
    2. B.
      Frequency
    3. C.
      Wavelength
    4. D.
      All of above
  3. Ex Q.1 (iii)
    The Laplace's correction in the expression for velocity of sound given by Newton is needed because sound waves
    1. A.
      are longitudinal
    2. B.
      propagate isothermally
    3. C.
      propagate adiabatically
    4. D.
      are of long wavelength
  4. Ex Q.1 (iv)
    Speed of sound is maximum in
    1. A.
      air
    2. B.
      water
    3. C.
      vacuum
    4. D.
      solid
  5. Ex Q.1 (v)
    The walls of the hall built for music concerns should
    1. A.
      amplify sound
    2. B.
      reflect sound
    3. C.
      transmit sound
    4. D.
      absorb sound

Answer briefly

Practice · 20
  1. Answer briefly.
    Ex Q.2 (i)
    Wave motion is doubly periodic. Explain.
  2. Ex Q.2 (ii)
    What is Doppler effect?
  3. Ex Q.2 (iii)
    Describe a transverse wave.
  4. Ex Q.2 (iv)
    Define a longitudinal wave.
  5. Ex Q.2 (v)
    State Newton's formula for velocity of sound.
  6. Ex Q.2 (vi)
    What is the effect of pressure on velocity of sound?
  7. Ex Q.2 (vii)
    What is the effect of humidity of air on velocity of sound?
  8. Ex Q.2 (viii)
    What do you mean by an echo?
  9. Ex Q.2 (ix)
    State any two applications of acoustics.
  10. Ex Q.2 (x)
    Define amplitude and wavelength of a wave.
  11. Ex Q.2 (xi)
    Draw a wave and indicate points which are (i) in phase (ii) out of phase (iii) have a phase difference of π/2\pi/2.
  12. Ex Q.2 (xii)
    Define the relation between velocity, wavelength and frequency of wave.
  13. Ex Q.2 (xiii)
    State and explain principle of superposition of waves.
  14. Ex Q.2 (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
  15. Ex Q.2 (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
  16. Ex Q.2 (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
  17. Ex Q.2 (xvii)
    Explain what is meant by phase of a wave.
  18. Ex Q.2 (xviii)
    Define progressive wave. State any four properties.
  19. Ex Q.2 (xix)
    Distinguish between traverse waves and longitudinal waves.
  20. Ex Q.2 (xx)
    Explain Newtons formula for velocity of sound. What is its limitation?

Solve the following problems

Practice · 10
  1. Solve the following problems.
    Ex Q.3 (i)
    A certain sound wave in air has a speed 340 m/s and wavelength 1.7 m for this wave, calculate a) the frequency b) the period.
  2. Ex Q.3 (ii)
    A tuning fork of frequency 170 Hz produces sound waves of wavelength 2 m. Calculate speed of sound.
  3. Ex Q.3 (iii)
    An echo-sounder in a fishing boat receives an echo from a shoal of fish 0.45 s after it was sent. If the speed of sound in water is 1500 m/s, how deep is the shoal?
  4. Ex Q.3 (iv)
    A girl stands 170 m away from a high wall and claps her hands at a steady rate so that each clap coincides with the echo of the one before. a) If she makes 60 claps in 1 minute, what value should be the speed of sound in air? b) Now, she moves to another location and finds that she should now make 45 claps in 1 minute to coincide with successive echoes. Calculate her distance for the new position from the wall.
  5. Ex Q.3 (v)
    Sound wave A has period 0.015 s, sound wave B has period 0.025. Which sound has greater frequency?
  6. Ex Q.3 (vii)
    At what temperature will the speed of sound in air be 1.75 times its speed at N.T.P?
  7. Ex Q.3 (viii)
    A man standing between 2 parallel eliffs fires a gun. He hearns two echos one after 3 seconds and other after 5 seconds. The separation between the two cliffs is 1360 m, what is the speed of sound?
  8. Ex Q.3 (ix)
    If the velocity of sound in air at a given place on two different days of a given week are in the ratio of 1:1.1. Assuming the temperatures on the two days to be same what quantitative conclusion can your draw about the condition on the two days?
  9. Ex Q.3 (x)
    A police car travels towards a stationary observer at a speed of 15 m/s. The siren on the car emits a sound of frequency 250 Hz. Calculate the recorded frequency. The speed of sound is 340 m/s.
  10. Ex Q.3 (xi)
    The sound emitted from the siren of an ambulance has frequency of 1500 Hz. The speed of sound is 340 m/s. Calculate the difference in frequencies heard by a stationary observer if the ambulance initially travels towards and then away from the observer at a speed of 30 m/s.