Chapter 12: Sound

The given graph (Fig.12.2) shows the displacement versus time relation for a disturbance travelling with a velocity of 1500 m s–1. Calculate the wavelength of the disturbance.

Soln: T= 2×10⁶s

Frequency v= 1/T

= 10⁵ hz

Wavelength λ v/V=5×10⁵m

11. Which of the above two graphs (a) and (b) (Fig.12.3) representing the human voice is likely to be the male voice? Give the reason for your answer.

Soln:

The pitch of a male voice is lighter than the pitch of a female; hence the graph represents the male voice.

12. A girl is sitting in the middle of a park of dimension 12 m × 12 m. On the left side of it, there is a building adjoining the park, and on the right side of the park, there is a road adjoining the park. A sound is produced on the road by a cracker. Is it possible for the girl to hear the echo of this sound? Explain your answer.

Soln:

Echo can be heard if the gap between the original sound and the reflected sound received by the listener is around 0.1 sec.

Sound Velocity × time interval

= 344×0.1

= 34.4 m

Here, sound reflects from the building and then reaches the girl, who is much smaller than the required distance. Hence echo cannot be heard.

13. Why do we hear the sound produced by the humming bees while the sound of vibrations of a pendulum is not heard?

Soln:

Humming bees produce the sound by beating their wings, and the frequency of the sound they produce will be in the range of 20 Hz to 20000 Hz, which is audible. On the other hand, the pendulum produces sound less than 20 Hz, which is below the audible range, and we don’t hear the sound of pendulum vibrations.

14. If any explosion takes place at the bottom of a lake, what type of shock waves in the water will take place?

Soln:

The answer is longitudinal waves

15. The sound produced by a thunderstorm is heard 10 s after the lightning is seen. Calculate the approximate distance of the thundercloud. (Given the speed of sound = 340 m s–1.)

Soln:

Distance = speed ×time

Here speed = 340ms^-1

Time = 10s

= 340× 10 = 3400m.

16. For hearing the loudest ticking sound heard by the ear, find the angle x in Fig.12.4.

Soln:

The angle of incidence is always equal to the Angle of reflection

Angle of Incidence =90⁰-50⁰=40⁰

Angle of reflection= angle of incidence=40⁰

Hence angle x is 40⁰

17. Why are the ceiling and wall behind the stage of good conference halls or concert halls made curved?

Soln:

The ceiling and wall behind the stage of good conference halls or concert halls made curved to ensure the reflected sound reaches equally to all the audience.