Sound can travel through some medium like air. It can not travel in vacuum. Thus for the sound to be audible to the ears, the sound source and ear must be connected by an elastic medium like air. The characteristics of audible sound are discussed below.
1. Frequency of Sound
Frequency of pitch of sound is defined as the number of cycles or vibrations per second. The highest audible sound (e.g., whisle) has a frequency of 20000 cps (cycles or vibrations per second). The lowest audible sound has a frequency of 20 cps (e.g., whispering). The frequency is a measure of the quality of sound.
2. Intensity of Sound
The intensity of sound is defined as the flow of sound energy per second through unit area. The intensity of sound is the strength of the sensation received by the human ear. Intensity of sound is a purely physical quantity. But loudness of sound depends on the characteristics of ear.
3. Measurement of Sound
The range of intensity of sound is very large. The loudest sound is about 1013 times the sound which is just audible by the human ear. There is a wide range of sound levels, it is realised that a scale has to be adopted as a guidance. The intensity of sound is measured on a logarithmic scale due to wide range of variation of the intensity of sound. Bel is the measure of intensity of sound named after Graham Bell, the inverter of telephone.
As the unit of bel is comparatively large, hence a shorter practical unit decibel (db) equal to 1/10th of a bel unit. The range of audible sound to painful noise varies from 1 to 1013 which is covered on logarithmic scale between 1 and 130 db units. Thus one db unit is approximately the smallest change of sound intensity which the human ear can hear. Table 28.1 (Sharma, 1988) shows the rating to sound generally experienced.
Table 28.1 Intensity of sound and comfort
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