NCERT Class-9 Science Solutions: Chapter 12-Sound Part 3

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Question 13:

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

Answer:

It is because the sound produced by the pendulum is of much lower frequency than that of bee. The sound produced by bee's wing is continuous where as the sound producer by the pendulum is just once or twice. Thus the sound from the pendulum by the time we hear, vanishes in the air

Humming bees produce sound by vibrating their wings which is in the audible range. In case of pendulum the frequency is below which does not come in the audible range.

Question 14:

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

Answer:

If any explosion takes place at the bottom of a lake. Infrasound type of shock waves in water will take place

When an explosion takes place in the bottom of the lake then shock waves are produced in the form of a semi-eclipse.

These waves have larger pressure and travel faster than sound waves.

These shock waves do not have long duration and they last for only milliseconds.

Question 15:

Sound produced by a thunderstorm is heard after the lightning is seen. Calculate the approximate distance of the thunder cloud. (Given speed of sound .)

Answer:

Given,

Time and speed

We know that,

Question 16:

For hearing the loudest ticking sound heard by the ear, find the angle in the Figure

Figure shown the find the angle

Figure Shown the Find the Angle

Figure shown the find the angle

Answer:

We know that, in laws of reflection, the angle of incidence (x) is always equal to the angle of reflection (x),

Since, AOB is a straight line

Therefore,

Question 17:

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

Answer:

The ceiling of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall uniformly. That sound after reflection reaches the target audience.