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DECEMBER 04, 2012

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Sound is a form of energy which produces a sensation of hearing. Sound waves are longitudinal in character and propagate through solids, liquids and gases. Sound requires a medium to travel in i.e. it cannot travel in vacuum. 





Human ear is sensitive to frequencies from about 20 Hertz (waves less than this threshold are infrasonic) to 20,000 Hertz. Sound waves having a frequency of more than 2000 Hz. are called ultrasonic waves. We are not sensitive to both infrasonic and ultrasonic waves. Sound waves originate in the vibration of bodies like strings, air columns and plates. In human beings, the vocal chords present in the throat vibrate to produce sound.




In air, the speed of sound is affected by pressure, temperature, density, humidity and wind speed. In dry air at 0oC, its speed is about 332 m/s. It travels faster in damp air and also the speed increases with temperature.





The intensity of sound is expressed in decibels (dB). Beyond 85 Db, sound pollution is said to result e.g. a busy traffic point at peak hours. The intensity of the loudest painless sound equals 120 dB, with normal conversation having an intensity of about 65 dB. The intensity of sound from a nearby airplane is about 120 dB. The intensity varies inversely as a square of the distance from the source.





Sound waves show the phenomena of reflection, refraction, interference, and diffraction just like the light waves. Sound waves which are approximately periodic create pleasing sensations (if there are no sudden changes in loudness) e.g. musical sounds, humming of a bee etc. Sound waves with no regularity or a superposition of periodic waves having very large number of components is perceived as noise.




The three characteristics of a sound wave are


(i)  loudness

(ii)  pitch and

(iii)  quality




The loudness of sound is determined by its intensity or the degree of sensation produced in the ear drum. Pitch is directly proportional to the frequency of sound. The higher the pitch, the shriller is the sound. Women, in general, have a voice with a higher pitch. Quality is that characteristic of sound which can help us distinguish  between two sounds of the same pitch and loudness. It is due to tonal quality that different musical instruments produce different sounds. The quality of sound also enables us to recognize a person’s voice while talking on a telephone.



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