Antonov An-12BK. Image courtesy Richard Vandervord via Wikimedia Commons. CC BY-SA 4.0 International. |
What are beat frequencies?
When
two sound waves of different frequencies interact, the waves subtract
and the difference in frequency creates a tremolo effect, resulting in a
periodic rise and fall in volume. (Basically the peaks or troughs
combine and reinforce or cancel each other out. ) The phenomenon is
sometimes noticed when two musical instruments with different pitches
are played together. Beat frequencies also occur with radio waves,
electrical signals and coherent light waves.
The
animated GIF below show a red and green sound wave, initially at the
same frequency and in phase. The two waves add together to form a blue
sine wave that's greater in amplitude. As the frequency of the green
wave slowly changes, the resultant blue waveform changes in amplitude,
modulated by a beat frequency. See link also in the comments to a
YouTube demo, showing the phenomenon with two tuning forks.
Anyone
who's doing their Junior Cert will probably be able to do the maths and
find the resultant of two sine waves of different frequencies added
together, requiring the trigonometric identities at the end of the maths
tables to do the calculations (Are these still available from the
Government Publications Sales Office?)
More information on beat frequencies here:
https://en.wikipedia.org/wiki/Beat_(acoustics)
https://en.wikipedia.org/wiki/Beat_(acoustics)
Beat frequency. The blue waveform is the sum of the red and green waves. Animated GIF courtesy Adjwilley via Wikimedia Commons. CC BY-SA 3.0 Unported. |