Transcript 9a – Beat Frequency

Beat Frequency
Tuning Fork Demos
•Two identical tuning forks
•Two non-identical tuning forks
The Interference Pattern
One Beat
Beat
• A Beat is created by interference
between two nearly identical sound
waves
• Beat – a periodic change in sound
intensity
• one beat is from loud to soft and back
to loud (constructive to destructive
and back to constructive interference)
How a Beat is produced
• when a rarefaction from one fork
coincides with a compression from
the other fork you get destructive
interference and soft sound
• when a rarefaction meets a
rarefactions (or 2 compressions) you
get constructive interference and
louder sound
• Beat Frequency – the number of beats heard
per second in Hertz
Beat frequency = number of beats
Total time
• Beat frequency is also the difference between
the frequencies of the two sources
Beat Frequency = Higher freq. – lower freq.
=  f2 - f1 
Sample Problem
A piano key with a frequency of
266 Hz is hit at the same time as a
tuning fork. Seven beats are
heard in 2.5 s. What are the
possible frequencies of the tuning
fork?
Questions
1.
2.
3.
4.
Explain how beats are created. Refer to the principle of superposition.
You are tuning a guitar by comparing the sound produced by the string
with the sound produced by a standard tuning fork. You notice a beat
frequency of 5 Hz when both sounds are present. As you tighten the
guitar string, the beat frequency rises steadily to 8 Hz. In order to tune
the string exactly to the tuning fork, should you continue to tighten the
string or loosen the string? Explain your answer.
An airplane mechanic notices that the sound from an aircraft with two
engines varies rapidly in loudness when both engines are running. What
could be causing this variation from loud to soft?
The two waveforms in Figure 5 have the same amplitude but different
frequencies. They are interfering with each other. Copy the two
waveforms into your notebook, and draw the resulting beat pattern.
Mark the points of constructive interference and destructive interference.