Frequency and Amplitude

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Category: Frequency and Amplitude

Published on Friday, 15 January 2010 17:33

Written by Bruce Gale

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Most people, even those with little more than an elementary education, can usually define sound fairly accurately by describing it as the result of some kind of vibration that can be detected by the ear.  In most cases, these vibrations are carried to us through the air, but they can just as easily be communicated through some other sort of homogeneous medium such as water, wood, or even one’s own skull.   Sound travels away from its source in a series of waves as adjacent particles swing into and away from each other. 

We become aware of all this activity when vibrating air molecules set the eardrum in motion.  To observe this effect in the music room, touch the skin of a timpani or snare drum lightly with one finger when the band is playing a forte passage.  It is very likely that you will actually be able to feel the vibrations as the membrane responds to the sound waves in the air.

Such effects, of course, are well known.  But what distinguishes mere noise from what we generally regard as musical tones?  A simple working definition is that noise consists of a jumble of irregular frequencies, whereas a musical note has a regular or constant vibration.

We identify high and low notes by the speed of the air vibrations associated with them.  In other words, a note that has a very fast frequency (produced, for example, on a rapidly vibrating guitar string), is described as being high, whereas notes with low frequencies (as in the case of a more slowly vibrating guitar string) are described as being low. 

Physicists measure frequencies in terms of cycles per second or Hertz (Hz).  The lowest frequency audible to humans is about 20 Hz and the highest is about 25,000 Hertz.  There is, however, a great deal of variation between individuals.  Generally speaking, young people can hear sounds at the extreme ends of these ranges much more easily than adults or retirees.  Animals also have quite different hearing ranges.  Dogs, for example, can hear very high frequencies inaudible to humans, while whales can hear very low ones.

Volume, on the other hand, refers to the amplitude rather than the frequency of the vibration.  The greater the initial force used to start a sound source vibrating, the greater the amplitude and thus the louder it sounds.

Other things being equal, the pitch of a vibrating guitar or piano string remains the same regardless of how forcefully it is struck or how soft it becomes as the resistance of the surrounding air gradually brings the string to a standstill.  The distinction is as vital for musicians as it is for physicists.  A common problem in just about all secondary school wind bands, for example, is the tendency for the ensemble to go out of tune when playing crescendos or diminuendos.  Careful practice and constant listening is often needed before young musicians are able to maintain a constant pitch while varying the volume (amplitude) of the sound on their respective instruments