Sound is a form of energy produced by a vibrating body. Sound requires a medium for its propagation. Sound does not propagate in vacuum. Sound is produced due to the vibration of an object.
The to and fro or back and forth motion of an object is called vibration. To produce pleasant sounds, a number of musical instruments have been developed. Some instruments produce sound due to the vibration of membranes, some due to the vibration of strings, and some others produce sound due to the vibration of an air column. The maximum displacement of a vibrating particle from its mean or equilibrium position is called its amplitude. The time taken by the vibrating particle for one full vibration or oscillation is called the time period of vibration. The number of vibrations per second is called the frequency. Frequency is measured hertz (Hz).
Sound produced by any means has the following characteristics, namely, loudness, pitch or shrillness, and quality or timbre.
The loudness of sound depends on its amplitude. The loudness of sound is proportional to the square of the amplitude. A roar of a lion is louder than a woman’s voice. The pitch of sound depends on its frequency. If frequency is more, then the pitch or shrillness is more. The pitch of a woman’s voice is more and it is shriller then a man’s voice. The loudness of sound is measured in Decibel (db). If loudness exceeds 80 db, then the sound becomes physically painful.
Not all sound produced by vibrating bodies is audible. The human ear can only recognise sounds of frequencies in the range of 20 Hz and 20,000 Hz. This range of frequency is called audible sound. Some animals like dogs and snakes can hear sounds of frequencies greater than 20,000 Hz. Sounds of frequencies less than 20 Hz are called infrasonic sounds, while sounds of frequencies greater than 20,000 Hz are called ultrasonic sounds.