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Module 1: Sound - Standing waves

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Physics - Standing waves

Standing waves

The expression 'standing wave [1]' appears to be a contradiction as, by
definition, a wave must be moving. This expression refers to a situation
where a number of waves are interfering and the result creates the
impression of a wave that is standing still. Examples of a standing wave
include the vibration of a guitar string after it has been plucked.

All physical objects have natural frequencies at which they can most
readily vibrate. For strings, this depends on the length, tension and mass
of the string. The frequency at which an air column will vibrate is
determined by its length (and less so by its diameter).

View the glass shattering video.

Whenever a musical instrument is played, it is forced to vibrate by the
musician. If this forced frequency is equal to the natural frequency of the
instrument, the amplitude of the vibrations add together and resonance
occurs. The resultant sound has a large amplitude, that is, it is quite
loud. Resonance [2] is defined as the outcome of a situation when the
forcing frequency is equal to the natural frequency of an object. If an
inexperienced person bows a violin, a loud pure note does not occur because
the forcing frequency will not match the natural frequency at which the
string will vibrate. When resonance is occurring there is the maximum
possible transfer of energy from the forcing item to the instrument.

Wave motion video.

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[1] http://alison.com/#
[2] http://alison.com/#