Hearing is the second most important channel, next only to vision, through which we learn about and appreciate the world around us. Through hearing, we understand speech, our chief medium for imparting and acquiring knowledge. It is, therefore, important that we know the mechanism of sensing sounds, which is called auditory sensation.
When an object vibrates by an external pressure, the air molecules around the object vibrate. The vibration spreads to the neighboring molecules, which create a wavelike movement of the molecules. If you have marked a stone thrown into a pond, you must have seen how the waves created near the stone move along to all sides of the pond. Sound waves produced by the vibration of an object move in the same way as the ripples move on the surface of the water in a pond. However, sound waves move much faster than the waves of water. The speed of sound waves under normal condition of temperature and atmospheric pressure is about 1130 feet per second. These sound waves are the physical stimuli for every thing we hear.
Sound waves have two components, pitch and loudness. The pitch of a sound is determined by its frequency, which is the number of expansions and contractions in the wave that occur in a second. Hertz is the unit of measurement for pitch. The loudness of a sound is determined by amplitude or height of the sound waves. This is the amount of contraction or expansion that occurs in each wave. The unit of measurement of amplitude or loudness is called as decibel (dB). The threshold of hearing is zero dB. How loud is zero dB? It is about as loud as the ticking of a watch 20 feet away in a quiet room. A sound wave of about 80 dB is perceived to be uncomfortably loud.
The sound waves are the physical energy received by our ears. Auditory sensation takes place when our ears receive the sound. Sound or auditory stimulation travels through the air in waves. Sound is caused by changes in air pressure that results from vibrations. Vibration is created by any object that vibrates, such as the vibration of a guitar string, vibration of the college bell, movement of a bus on the street, vibration of the vocal cord of a singer, and so on. Sound waves move in different directions in air. Thus, our ears come in contact with the sound waves and receive sound information. Sound waves can occur many times in one second. Our ear is sensitive to sound waves that vary from frequencies of 16 to 20,000 cycles per second.
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When sounds of different frequencies are generated together, as in case of musical sounds, we perceive a third tone. When there is harmony or consonance in the difference of frequencies, the sound is perceived as pleasant. But when there is disharmony or dissonance between them, the sound is perceived as unpleasant.
Sometimes in addition to producing the specified tone, a musical instrument may produce a number of tones that are greater in frequency. These are called overtones. Overtones resulting from vibrations elsewhere in the instrument and contributing to the richness of the sound are called timbers of a sound. Noise, on the other hand is produced because of a combination of dissonant sounds. It is perceived as unpleasant.