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f1 = Hz f2Hz 
A microphone is placed between two loudspeakers. The speakers are connected to two sources of sound that have variable frequencies, f1 and f2.  The graph shows the sound waves arriving at the microphone, as a functions of time, from each speaker, and also shows the sum of the two waves. Change the frequency of either sound source (45 Hz < f1, f2 <  50 Hz), and watch the changing interference between the two sound waves. Study the phenomenon of beats and verify that the beat frequency is correct.


  1. What happens as the two frequencies get closer together?
  2. What happens as the two frequencies get farther apart?
  3. Does it matter which speaker has the higher frequency?
  4. What happens if the two frequencies are identical?
  5. Remember that it is the difference between the two sound frequencies that determines the beat frequency. 


See Serway and Beichner, Physics For Scientists and Engineers 5th Edition, Section 18.7

Illustration written by Steve Mellema