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Beat Frequencies

When waves of two frequencies interfere a third wave is perceived by the observer.

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What instances might cause beat frequency to occur, and to what degree of effectiveness? 


Credit: Marco Lazzaroni
Source: http://www.flickr.com/photos/motumboe/3293600150/sizes/o/in/photolist-623yzu-6g2LHa-aaEHa1-9Jaw1g-8BDfGP-8BDfEM-8BGme1-84T8tX-84WgBh-84TcWi-84TdnR-84Tdw2-84TkcD-84Wm5N-84Wd7d-84Thbz-84WqJY-84TawT-84WoHq-84Wgto-84T88F-84Wjxm-84Tcn4-84Tj8n-84Tb8M-84Wpyj-84WmUh-84T98n-84WehC-84Wmco-84TiiT-84Wczy-84WpVb-84T5we-84TaNc-84TdMr-84Wc8q-84WcWd-84WqzL-84Wkam-84Tjon-84Wq2o-84T8T4-84TcHX-84Thpp-84T9VM-84TaUB-84Tjzc-84WddW-84WoWN-84Wp9E/
License: CC BY-NC 3.0

You are at a rock concert and a band is getting ready to perform on stage. Two guitarists are warming up and are trying to play the same exact notes simultaneously. However, they are not synchronized in one instance. This causes you to hear a humming, pulsating sound from the echo of the two guitars, aside from the notes being played. This indicates beat frequency, which is the result of two sound waves of different frequencies interfering with each other.

Different notes in music have different frequencies (this chart shows the frequencies and wavelengths of many different musical notes). However, the notes each guitarist played may still have been the same note. This would mean that either one or both of the guitars was not tuned properly.

Creative Applications

1. Do you think beat frequency can be used to find the exact note played by the second guitar, given the note played by the first? Explain.

2. What kind of interference do the two sound waves experience to produce beats?

3. Would a difference in notes or a difference in tuning produce a larger beat frequency?

4. Our ears can only detect beats with a frequency of 7 Hz or less. Use this to explain why we cannot hear beat frequency when any two different notes are played on a piano.


The following video gives a better idea of beat frequency physically through graphs, with sound to match.

Courtesy of DrDaveBilliards.

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