Peoples Physics Book Version-3

11.1. The Big Idea http://www.ck12.org

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Common Frequencies







fbeat=|f 1 −f 2 | Beat frequency

fn= 2 nvL|integer n Standing waves restricted or unrestricted at both ends

fn= 4 nvL|odd integer n Standing waves restricted at one end

Doppler Shifts

fo=f

v+vo

v−vs

fo(observed frequency) is shifted up when source and observer moving closer

fo=f

v−vo

v+vs

foobserved frequency) is shifted down when source and observer moving apart, where

vis the speed of sound,vsis the speed of the source, andvois the speed of the observer

Key Applications

• Constructive interferenceoccurs when two waves combine to create a larger wave. This occurs when the
  peaks of two waves line up.


• Destructive interferenceoccurs when two waves combine and cancel each other out. This occurs when a peak
  in one wave lines up with a trough in the other wave.


• When waves of two different frequencies interfere, phenomena known asbeatsoccur. The frequency of a beat
  is the difference of the two frequencies.

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