Question - 21 : - A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 ms^-1.

Answer - 21 : -

Question - 22 : - Two children are a± opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.

Answer - 22 : -

Question - 23 : - The frequency of a sources/ sound is 100 Hz. How many times does it vibrate in a minute?

Answer - 23 : -

Question - 24 : - Does sound follow the same laws of reflection as light does? Explain.

Answer - 24 : -

Yes. Sound follows the same laws of reflection as that of light because,

(i) Angle of incidence of sound is always equal to that of angle of reflection of sound waves.

(ii) The direction in which sound is incident, the direction in which it is reflected and normal all lie in the same plane.

Question - 25 : -
Yes. Sound follows the same laws of reflection as that of light because,
(i) Angle of incidence of sound is always equal to that of angle of reflection of sound waves.
(ii) The direction in which sound is incident, the direction in which it is reflected and normal all lie in the same plane.

Answer - 25 : -

Time is inversely proportional to the speed. As the temperature increases, the speed increases. Thus on a hot day due to high temperature the speed of sound increases. Hence the time will decrease and we can hear the echo sooner.

Question - 26 : - Give two practical applications of reflection of sound waves.

Answer - 26 : - Reflection of sound is used in megaphones, horns and musical instruments such as trumpets and shehna. It is used in stethoscope for hearing patient’s heartbeat. Ceilings of the concert halls are curved, so that sound after reflection reaches all comers of the hall. (Any two practical applications can be written).

Question - 27 : - A stone dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Giving, g = 10 ms^-2 and speed of sound = 340 m s^-1.

Answer - 27 : -

Question - 28 : - A sound wave travels at a speed of 339 ms-1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?

Answer - 28 : -

Question - 29 : - What is reverberation? How can it be reduced?

Answer - 29 : -

The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation.

To reduce the undesirable effects due to reverberation, roofs and walls of the auditorium are generally covered with sound absorbent materials like compressed fiberboard, rough plaster or draperies. The seat materials are also selected having sound absorption properties.

Question - 30 : - What is loudness of sound? What factors does it depend on?

Answer - 30 : - The loudness of sound is determined by its amplitude. The amplitude of the sound wave depends upon the force with which an object is made to vibrate. Loud sound can travel a larger distance as it is associated with higher energy. A sound waves spreads out from its source. As it moves away from the source its amplitude as well as its loudness decreases.

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