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Sound, 1. Sound : Sound is a form of energy which produces a sensation of hearing in our ears., , 2. Source of sound and its propagation : A source of vibration motion of an object is, normally a source of sound., , 3. Characteristics of the medium required for the propagation of sound:, , (i) Medium must be elastic so that the medium particles have the tendency to return back to, their original positions after the displacement., , (ii) Medium must have the inertia so that its particles have the capacity to store the energy., , The frictional resistance of the medium should be negligible to minimise the loss of energy in, propagation., , 4. Types of waves, , (i) Mechanical waves : A mechanical wave is a periodic disturbance which requires a material, medium for its propagation. On the basis of motion of particles the mechanical waves are, classified into two parts., , (a) Transverse wave, (b) Longitudinal wave, , (a) Transverse wave : When the particles of the medium vibrate in a direction perpendicular, to the direction of propagation of the wave, the wave is known as the transverse wave. For, example, waves produced in a stretched string., , (b) Longitudinal wave : When the particles of the medium vibrate along the direction of, propagation of the wave then the wave is known as the longitudinal wave. For example sound, wave in air., , (ii) Electromagnetic waves : The waves which do not require medium for propagation are, called electromagnetic waves these waves can travel through vacuum also. For example, light, waves, X-rays., , 5. Characteristics of a sound wave, Frequency : The number of vibrations per second is called frequency., The unit of frequency is hertz, , (ii) Amplitude: The maximum displacement of each particle from its mean position is called, amplitude., , The S.1. unit of amplitude is metre (m)., (iii) Time period: The time taken to complete one vibration is called time period., Frequency= 1/(Time period) or v = 1/T, , (iv) Wavelength: The distance between two nearest (adjacent) crests or troughs of a wave is, called its wavelength.

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(v) Velocity of wave: The distance travelled by a wave in one second is called velocity of the, wave (or speed of the wave). The S.I. unit for the velocity of a wave is metres per second (m/s, or ms-1)., , (vi) Pitch : Pitch is the sensation (brain interpretation) of the frequency of an emitted sound, and is the characteristic which distinguishes a shrill (or sharp) sound from a grave (or flat), sound., , (vii) Loudness : It is a measure of the sound energy reaching the ear per second., , 6. Reflection of sound : When sound waves strike a surface, they return back into the same, medium. This phenomenon is called reflection., , 7. Laws of reflection : Angle of incidence is equal the angle of reflection., The incident wave, the reflected wave and the normal all lie in the same plane., , 8. Echo : Phenomenon of hearing back our own sound is called an echo. It is due to successive, reflection from the surfaces obstacles of large size., , 9. Relation between speed of sound, time of hearing echo and distance of reflection body, , : If t is the time at which an echo is heard, d is the distance between the source of sound and the, reflecting body and v is the speed of sound. The total distance travelled by the sound is 2d., speed of sound, v = 2d/t or d = v2, , 10. Conditions for the formation of Echoes, , (i) The minimum distance between the source of sound and the reflecting body should be 17.2, metres., , (ii) The wavelength of sound should be less than the height of the reflecting body., (iii) The intensity of sound should be sufficient so that it can be heard after reflection., , 11. Reverberation : Persistence of sound after its production is stopped, is called, reverberation. A short reverberation is desirable in a concert hall (where music is being played), , because it gives ‘life’ to sound. Too much reverberation confuses the programmers and must be, reduced to reduce reverberation.

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12. Range of Hearing : The audible range of sound for human beings extends from about 20, Hz to 20,000 Hz (one Hz = one cycle/s). Sounds of frequencies below 20 Hz are called, infrasonic sound or infrasound. Frequencies higher than 20 kHz are called ultrasonic sound or, ultra sound. Ultrasound is produced by dolphins., , 13. Applications of ultrasound : The ultrasound is commonly used for medical diagnosis, , and therapy, and also as a surgical tool. It is also used in a wide variety of industrial, applications and processes. Some creatures use ultrasound for information exchange and for the, detection and location of objects. Also some bats and, , porpoises are found to use ultrasound for navi gation and to locate food in darkness or at, , a place where there is inadequate light for vision (method of search is called echolocation)., , 14. Sonar : SONAR means Sound Navigation Rang-ing. In this sound waves (ultrasonic) are, used [microwaves are absorbed by water)]. Sound waves are emitted by a source. These waves, travel in water with velocity v. The waves re-flected by targets (like submarine bottom sea) are, detected., , Uses, , (i) The SONAR system is used for detecting the presence of unseen underwater objects, such as, a submerged submarine, a sunken ship, sea rock or a hidden iceberg, and locating them, accurately., , (ii) The principle of SONAR is also used in industry of detection of flaws in metal blocks or, sheets without damaging them.