As people grow older their ears become less sensitive to higher frequencies. Children under the age of five and some animals, such as dogs can hear up to 25 kHz (1 kHz = 1000 Hz). The audible range of sound for human beings extends from about 20 Hz to 20000 Hz (one Hz = one cycle/s). Generally the ceilings of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall, Sometimes a curved soundboard may be placed behind the stage so that the sound, after reflecting from the sound board, spreads evenly across the width of the hall In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound, Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. Megaphones or loudhailers, horns, musical instruments such as trumpets and shehanais, are all designed to send sound in a particular direction without spreading it in all directions, In these instruments, a tube followed by a conical opening reflects sound successively to guide most of the sound waves from the source in the forward direction towards the audience. What is the distance of the reflecting surface from the source, given that the speed of sound is Hence, the distance between the cliff and the personġ. In 5 s sound has to travel twice the distance between the cliff and the person. What is the distance of the cliff from the person if the speed of the sound, v is taken as 346 m s –1? ExampleĪ person clapped his hands near a cliff and heard the echo after 5 s. The seat materials are also selected on the basis of their sound absorbing properties. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster or draperies. In an auditorium or big hall excessive reverberation is highly undesirable.
The repeated reflection that results in this persistence of sound is called reverberation. The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces, such as the clouds and the land.Ī sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. Echoes may be heard more than once due to successive or multiple reflections. This distance will change with the temperature of air. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17.2 m. Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least (344 m/s) × 0.1 s = 34.4 m. If we take the speed of sound to be 344 m/s at a given temperature, say at 22 ✬ in air, the sound must go to the obstacle and reach back the ear of the listener on reflection after 0.1s. To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s. The sensation of sound persists in our brain for about 0.1 s. This sound which we hear is called an echo. If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later.
You can make the pipes using chart paper. Take two identical pipes, as shown in Fig.An obstacle of large size which may be polished or rough is needed for the reflection of sound waves. The directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three are in the same plane. Like light, sound gets reflected at the surface of a solid or liquid and follows the same laws of reflection as you have studied in earlier classes. Sound bounces off a solid or a liquid like a rubber ball bounces off a wall.
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