Peoples Physics Concepts

http://www.ck12.org Chapter 11. Wave Motion and Sound

11.3 Beat Frequencies

• Explain the concept of destructive and constructive interference in the context of beat frequencies.

Students will learn the concept of destructive and constructive interference in the context of beat frequencies.

Key Equations

fbeat=|f 1 −f 2 |; two interfering waves create a beat wave with frequency equal to the difference in their

frequencies

Guidance

• 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, a phenomenon known asbeatingoccurs. The frequency
  of a beat is the difference of the two frequencies.

Example 1

You want to find out the frequency of a tuning fork. When you strike the unknown fork and a fork which is known

to create a sound at 100 Hz at the same time, you hear a beat frequency of 3 Hz. You strike the unknown fork a

second time, but this time with a tuning fork rated for 105 Hz and you hear a beat frequency of 2 Hz. What is the

frequency of the unknown fork?

Solution

Each test gives us two possibilities for a frequency. Based on the first test, the unknown fork could either be rated

for 97 Hz or 103 Hz. Based on the second test, the frequency of the unknown fork could either be 103 Hz or 107

Hz. The two tests agree on 103 Hz, so that must be the frequency of the unknown tuning fork.

Watch this Explanation

MEDIA

Click image to the left for use the URL below.

URL: http://www.ck12.org/flx/render/embeddedobject/305