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2.1. The science of sound

Sound is the vibration of air molecules or variation in air pressure that can be sensed by the ear. The pattern and rate of audible vibrations give sound its unique quality. The range of auditory perception is approximately 20 cycles (or fluctuations) of air pressure per second to 20,000 cycles per second. Air pressure fluctuations outside of this range are not audible to the human ear and are called subsonic (less than 20 cycles per second) and ultrasonic (more than 20,000 cycles per second) vibrations.

Sound is generated by a friction-producing force such as a drum stick hitting a cymbal, a bow moving across a cello string, or a vibrating speaker cone that sets the surrounding air molecules into motion. From the point of impact or disturbance, sound waves or patterns of vibrating air molecules radiate outwards through the atmosphere to the ear like the ripples of water on a pond. Figure 2-1 shows a sound produced by a cello as it emanates from a speaker cone.


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