Phase Speed and Group Speed
Propagation of beats => Superposition of two travelling waves of slightly different frequencies.
[after Display 14.4 from Ingard Fundamentals of Waves and Oscillations]
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Two waves with different phase speeds
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A non-sinusoidal wave changes shape as it propagates
Speed for each harmonic is proportional to harmonic number.
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Gaussian packet changes shape as it propagates
[From Elmore & Heald, Physics of Waves, section 14.7]
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