The Sound of Science - "Interference and Beats"

Jun 22, 2018

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If you ever listen to music, chances are you can hear right away if an instrument isn’t quite in tune with the rest of the group.  But very few people could accurately tune an instrument just by ear.  There are people who possess what musicians call “perfect pitch,” but most players today rely on digital tuners.  But how did musicians tune their instruments before there were electronic tuners?

One method was to use concepts called interference, and beats.  Interference is an effect that occurs when two or more waves interact with each other and combine to form a new wave.  If we strike two tuning forks with exactly the same frequency, in this case 256hz, the resulting sound waves will reinforce each other, and become louder. 

Jeremy Benson and Sam Watt (R) of NIU STEAM.

We call this constructive interference.  Before concert halls were designed by acoustic engineers, you could end up in a seat in a dead zone where the waves reflected off the walls in such a way as to cancel each other out. That is an example of destructive interference.

If we now listen closely to the sound of two tuning forks that have slightly different frequencies, we might hear something interesting.

You might have noticed that now the resulting sound was getting louder and quieter.  We had alternating constructive and destructive interference.  This creates an effect called beats.  The closer two waves are to one another, the slower this resulting beat frequency will be.  When two notes are perfectly in tune, the beats stop, and the interference becomes purely constructive.

Musicians can use this beat frequency to tune their instruments compared to a reference pitch.  The slower the resulting beat frequency becomes, the closer they are to being in-tune. Classical musicians often carry a tuning fork, while many a travelling musician knows that the dial tone on a hotel phone just happens to be an A.

Beats aren’t only used for musical instruments, though. Ultrasonic waves directed at human arteries can use these same beat frequencies to determine the rate of blood flow, and can help medical professionals identify the location of clots and obstructions. 

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