Resource Lesson Beats: An Example of Interference
Rapid changes in loudness, known as beats, occur when two tones very close in frequency are heard at the same time. The number of beats is equal to the difference in two nearly similar frequencies.

This "line drawing" shows how two patterns of slightly different spacing show the phenomena of beat interference. The pattern is best seen when you look from a distance of about a meter.

When two tuning forks of similar frequencies are played simultaneously, the frequency of the beats, beat frequency, is the different between the two frequencies.

beat frequency = |f2 - f1|

While the pitch at which those beats are heard is the average of the two frequencies.

beat pitch = ½(f2+f1)

Refer to the following information for the next two questions.

Suppose you sound two tuning forks simultaneously: one fork has a frequency of 256 hz and the other has a frequency of 260 hz.
 How many beats would be heard each second?

 What is the pitch of these beats?

 Directions: With this EXCEL worksheet you can experiment with changing the frequencies of two in-phase point sources. Just enter any new frequencies you want to compare in these respective cells: green frequency in G1 and the red frequency in G2. The blue wave represents the resulting areas of constructive and destructive interference between the red and green frequencies you specified. If you count the "number" of high-amplitude regions of constructive interference, you can "see" how many beats occur in a given "second" in the simulation.   Beats Demonstration When finished, close your file without saving changes, thank-you.

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