Beats are heard from two sound sources which are at similar, but different frequencies.
The sound waves will go in and out of phase, adding and canceling at different times and producing a sound which increases and decreases in loudness at the beat frequency given by the following formula:
NCEA & Science Curriculum
Students can monitor any of the frequencies of sounds if they download a suitable Ap on their smartphone. (Search for Sound Analyzer). A free Ap that worked well for us is AKLite.
Using 2 signal generators and 2 speakers: Instructions
Connect the speakers to different signal generators set to perhaps 880 and 885 Hz. Point the speakers forward. Students should hear beats. Note that standard student signal generators are often not very accurate and not very stable, so you may have to adjust the signal generator frequencies to get beats. The signal generators used in this demonstration (not shown in photo above but see instructor notes) are more accurate than standard student signal generators.
The amplitudes of the signals do not need to be the same, and in the real world rarely are the same (e.g., tuning a piano using a tuning fork). You get beats with incomplete cancellation as well, although the sound amplitude does not go to zero. The 880 and 881 Hz signal below is an example of nearly complete cancellation while the amplitudes of the 880 and 885 Hz signals in the fast beats example are more different leading to less complete cancellation.
This can also be done on the computer using the combining sounds simulator (see Related Resources below).
See Instructor Notes for more information.
Using 2 tuning forks: Instructions
Use two identical tuning forks. Change the frequency of one of them by adding a piece of blu-tak to ne of the prongs. The small difference in frequency may only be apparent when the two tuning forks are sounded together and a beat is heard.
Using Soundcard Scope: Instructions
If you have a Windows PC, download Soundcard Scope. This can be used to generate two frequencies from your PC speakers and your microphone can pic up the sound and display it. All in one easy program.
Videos and sound files
These sound and video files can be used to show slower and faster beats. Most teachers will want the mp4s (m4vs) with graphics. Note that these are adapted from the sound files used by the late Professor Paul Callaghan on the Radio New Zealand radio show Saturday Morning with Kim Hill on 24 March 2007. The title of the show was “What is Sound?”
Download (right -click and save as, 400 KB): 880 881 And Beats.m4v
Download (right -click and save as, 400 KB): 880 885 And Beats.m4v
Sound Files (right-click and save as):
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.
Combining Sounds || Sound Interference
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