The document discusses sound wave interference using two sound sources that produce varying sound intensities based on their alignment. When a person stands between the sources, they experience constructive interference (loud sound) if the waves are in phase and destructive interference (no sound) if the waves are out of phase. It also includes a simulation that illustrates these concepts and poses questions regarding sound perception based on the positioning of the sound sources.

1. 2 Sources Sound Wave
Interference
Ai Xin Wang
LO 8
March 16,2015

2. Sound waves interference pattern,
please see the simulation
below.(sound system is required)

3.  As I move the person along the horizontal line that
passes midway between the two sound sources, the
sound intensity never changes and stays at the
maximum--- very loud all the time.
 As I move the person along the slanted line, the
sound intensity changes. The sound is sometimes
loud and sometimes light.
Any observation?

4. Why? Please consider the picture below
• The two sound sources can be considered as
two waves that have the same frequency and
wavelength travelling in different directions.
• The blue and red lines represent the crests (
maximum amplitude) of the waves.
• We can tell that the black dots indicate where
the two waves are in phase- constructive
interference- maximum sound intensity.
• And the opened circles indicate that the two
waves are out of phase- destructive
interference- 0 sound intensity
IF we move along the direction of the black arrow (horizontally), we can experience
constructive interference (hearing loud sound) all the way, since only black dots appear.
IF we move along the direction of the purple arrow (slanted), we can experience on and off
constructive and destructive interference (hearing sometimes loud and sometimes light
sound), since both opened circles and black dots appear.

5. Q: Two point sources are 30 cm apart, and
both sources have the same wavelength and
frequency. Imagine you are standing on the
horizontal line that passes midway between
the two sources. What do you hear if the
two sources line up side by side? What do
you hear if one of the source is moved back
a bit?
Let’s do a problem now!

6. If the two sources line up side by side…
I hear constant
endless piercing
sound!
The reason for the person to hear constant endless piercing sound is that
he is standing on the line that undergoes constructive interference. The
amplitude of the two waves add up to produce the maximum resultant
wave. The crest of one wave coincides with the crest of the other,
indicated as blue dots on the diagram.

7. If the two sources line up front and back…
It’s nice
and quiet. I
love it！The reason for the person to hear no sound is that he is
standing at the point where the two waves are out of phase.
Meaning the line undergoes destructive interference. The two
waves completely cancel each other because the crest of one
wave coincides with the trough of the other. Resulting 0 as the
resultant wave.

8. The End 
Thanks for Watching