Sound of Class 9

When sound waves strike a surface, hey return back into the same medium. This phenomenon is called reflection.

The reflection of sound waves is similar to that of light rays. The only difference is that sound waves being larger in length. require bigger surfaces for reflection


  • Angle of incidence is equal to the angle of reflection.
  • The incident wave, the reflected wave and the normal, all lie in the same plane.


Take a smooth polished large wooden board and mount it vertically on the table. At right angle to the board, fix a wooden screen. One each side of the screen, place a long, narrow and highly polished tube 9inside). Place a clock at the end of the tube A. Move the tube B slightly from left to right, till a distinct tick of clock is heard. Measure the    and between tubes and wooden screen. It is found. This experiment illustrates the law of reflection.



Mega phone or speaking tube:

When we have to call someone at a far off distance (say 100m), we cup our hands and call the persons with maximum sound we can produce. The hands percent the sound energy from spreading in all directions. In the same way, the people use horn shaped metal tubes, commonly called megaphones. The loud speakers have horn shaped openings. In all these devises, the sound energy is prevented from spreading out by successive reflections from the horn shaped tubes.


(ii) Stethoscope :It is a medical instrument used frequently by doctors for making a rough diagnosis of the diseases existing inside the body at places which are either inaccessible or accessible only through major operations.


Working: The metal case C containing the diaphragm is gently pressed against the part of the body to be examined. The vibrations of this part are communicated to the diaphragm Dwhich starts vibrating. As shown in figure, these vibrations suffer multiple reflections in the tubes R, T, T1 and T2 and ultimately reach the earphones. The original sound produced by the part of the human body is exactly reproduced in the earphones and a preliminary diagnosis of the ailment is made.

  • Ear trumpet:It is a sort of machine used by persons who are hard of hearing. The sound energy received by the wide end of the trumpet is concentrated into a much smaller area at the narrow end by multiple reflections. This makes the otherwise inaudible sound audible to the user.
  • Hearing aid: An ear trumpet is a mechanical device helpful only to a person with mild hearing loss. A hearing aid is an electronic device which is battery operated and is used by people with severe hearing loss. A hearing aid is fitted with a microphone which converts sound waves into electrical signals. An electronic amplifier amplifies these signals which are then fed to a speaker in the hearing aid. The speaker converts the amplified electrical signals into sound which is sent to the ear for clear hearing.
  • Sound board :The sound waves obey the laws of reflection on the place as well as curbed reflecting surfaces. In order to spread sound evenly in big halls or auditoriums, the speaker (S) is fixed at the principle focus of the concave reflector. This concave reflector is commonly calledsounding board. The sound waves striking the sound board get reflected parallel to the principal axis.
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