Beat frequency occurs when two sounds waves of slightly different frequencies are combined, resulting in a fluctuating volume caused by the interference between the waves. The difference between the frequencies is called the beat frequency, which can be used to tune instruments by listening for beats. The beat frequency formula is fbeat= |f1 - f2|, where f1 and f2 are the frequencies of the two sources. An example problem demonstrates how to use the beat frequency to calculate the pitch of a piano note based on the number of beats heard when played with a tuning fork.

1. BEAT FREQUENCY
Kathy Macarayan

2. WHAT IS A BEAT FREQUENCY
IN SOUND?
“The sound of a beat frequency or beat wave is a fluctuating volume caused
when you add two sound waves of slightly different frequencies
together. If the frequencies of the sound waves are close enough together, you
can hear a relatively slow variation in the volume of the sound. A good
example of this can be heard using two tuning forks that are a few frequencies
apart.”
Ron Kurtus, October 2013 (http://www.school-for-champions.com/science/
sound_beat_frequencies.htm#.VQZWZUJupFI)

3. APPLICATIONS OF BEAT
FREQUENCY
• The phenomenon of beats is most renowned for
its influence in tuning instruments.When two
frequencies are played simultaneously, instruments
are tuned to match the correct frequency of the
note; in other words, you are listening for beats.
When beats can no longer be detected, the
instrument is in tune.

4. BEAT FREQUENCY FORMULA
• Beat frequency can be solved by using the
equation
• fbeat= | f1 -f2 |
• where f1 and f2 are the frequencies of the two
sources

5. THE BEAT ENVELOPE
“If you add two waves of slightly different frequencies, the resulting
amplitude will vary or oscillate at a rate that is the difference
between the frequencies. That beat frequency will create a beat envelope
around the original sine wave.”
Ron Kurtus, October 2013 (http://www.school-for-champions.com/science/
sound_beat_frequencies.htm#.VQZWZUJupFI)
F1 (slightly slower
frequency)
F2 (slightly faster
frequency)
Because the frequencies are so close, you
would hear a frequency that is the average of
the two as well as the modulation of the
amplitude as the beat frequency defined
by the previous formula.

6. EXAMPLE
A tuning fork with a frequency of 560 Hz is sounded
together with a note played on a piano.Twelve beats are
heard in 4 seconds.What is the frequency or pitch of the
piano note?

7. SOLUTION
• First, you must figure out how many beats are
heard per second
• Beats per second = 12 beats/ 4 seconds = 3 Hz

8. SOLUTION
• Second, use the beat frequency formula to solve
• fbeat= | f1 -f2 |
• 3 Hz = |560 Hz- f2|
• There are two possible answers,
f2= 557 Hz, 563 Hz

9. MORE HELP
• If you are still confused about beat frequency,
check out this helpful youtube video by
Brightstorm
• https://www.youtube.com/watch?
v=ylBne5guWOc&spfreload=10

10. THANKS FOR
WATCHING

11. • Images:
http://www.taylorguitars.com/sites/default/files/hero-acoustic-guitars-browse-by-wood-taylor-guitars.jpg
http://avstrings.com/sites/default/files/imagecache/fullscreen/Metal%20Tuning%20Fork%20with%20Wood%20Block.jpg
http://www.uaudio.com/media/blog/2012/05/tune_hero.jpg