Beats occur when two sounds waves of slightly different frequencies interfere with each other, causing the resulting amplitude to increase and decrease over time. The rate at which the amplitude oscillates between high and low volumes is called the beat frequency. The beat frequency is equal to the difference between the frequencies of the two interfering waves. For example, if a 100 Hz tone and 105 Hz tone are played together, their 5 Hz difference in frequency would cause the amplitude to beat at 5 Hz.

1. Learning Objective
---Beats
Hedy Liang (19403147)
Lab Section: L2F

2. What is a beat?
When we listen to two waves that have
slightly different frequencies, we hear the
amplitude increasing and decreasing as a
function of time. This type of variation in
amplitude is called a beat.
Beats

3. Beats
Beats are caused by the interference of two vaces at the
same point in space.
The plot below shows the periodic increase and decrease
for two sine waves.

4.  The point C in the graph is Constructive
interference and Point D is destructive
interference.
Beats

5. Beat Pattern
Beat Pattern

6.  The beat frequency refers to the rate at
which the volume is heard to be
oscillating from high to low volume.
 For example, if two complete cycles of
high and low volumes are heard every
second, the beat frequency if 2Hz. The
beat frequency is always equal to the
difference in frequency of the two notes
that interfere to produce the beats.
Beat Frequency

7. For equal amplitude sine waves, the sum rule is
followed by the equation:
The beat frequency is given by
Sum and Difference Frequencies

8. Beat Frequency
A 100 Hz tone and a 105 Hz tone
are played simultaneously. What
is the beat frequency?
Check Your Understanding

9. (B) 5Hz
The amplitude will be large for
both the positive and negative
crests; therefore, the amplitude
modulations have the same
frequency as the difference
between the two frequencies.
Answer