Wave interference occurs when two waves meet while traveling through the same medium. There are two types of interference - constructive and destructive. Constructive interference occurs when two waves are in phase, meaning crests meet crests and troughs meet troughs, increasing pressure. Destructive interference is when waves are out of phase, with crests meeting troughs, decreasing pressure. In sound waves, compressions and rarefactions can interfere constructively, with two compressions or rarefactions further compressing or decreasing pressure, while a compression and rarefaction will interfere destructively and cancel each other out. Controlling interference is important for concert hall design to reduce unwanted noise. Noise reduction systems also use destructive interference

2. Wave interference
• the phenomenon that occurs when two waves meet while
traveling along the same medium.
• The interference of waves causes the medium to take on a
shape that results from the net effect of the two individual
waves upon the particles of the medium.
• Constructive interference: when the two waves that meet
are in the same phase
• Destructive interference: when the two waves that meet are
in opposite phases

3. Interference in sound waves?
• One may then question how can sound waves
that do not possess upward and downward
displacements interfere constructively and
destructively?
• Sound is a pressure wave that consists of
compressions and rarefactions
• As a compression passes through a section of a
medium, it tends to pull particles together, creating
a high-pressure region
• As a rarefaction passes through a section of a
medium, it tends to push particles apart, thus
creating a low-pressure region.

4. Constructive interference
of sound waves
• Areas in which constructive interference
continuously occur are know as antinodes
• In sound waves antinodes are found either
when:
a compression of one wave meets up with a
compression of a second wave at the same location
in the medium, then the net effect is that that
particular location will experience an even greater
pressure
if two rarefactions (two low-pressure disturbances)
from two different sound waves meet up at the same
location, then the net effect is that that particular
location will experience an even lower pressure.

5. • Areas in which destructive interference
continuously occur are know as an antinode
• if two sound waves interfere at a given location in
such a way that the compression of one wave
meets up with the rarefaction of a second wave,
destructive interference results.
• The tendency of the compression to push particles
together is canceled by the tendency of the
rarefactions to pull particles apart
• the particles would remain at their rest position as
though there wasn't even a disturbance passing
through them

6. • Two speakers are set 1m apart and
produce identical tones
• The two sound waves traveled through
the air in front of the speakers, spreading
out through the room in spherical
fashion.
• In the diagram, the compressions of a
wavefront are represented by a thick line
and the rarefactions are represented by
thin lines.

7. Question #1
At which points
would constructive
interference occur
and what would
these locations be
called?

8. Answer to Question #1
• Constructive interference would occur at
points A and B as this is where two
crests or two compression wave fronts
are meeting
• These two points are antinodes as anti
nodes are where constructive
interference occurs

9. Importance of sound
wave interference
• Destructive interference of
sound waves becomes an
important issue in the
design of concert halls and
auditoriums. The rooms
must be designed in such
as way as to reduce the
amount of destructive
interference
• This can be done by
designing the walls,
ceilings, and so that they
absorb sound rather than
reflect it.
 The panels on the walls of this
high school band room will
absorb sound

10. Using destructive
interference to our
advantage
• Ex. Noise reduction systems
• Earphones have been produced
that can be used by factory and
construction workers to reduce the
noise levels on their jobs.
• the earphones capture sound from
the environment and use computer
technology to produce a second
sound wave that is one half cycle
out of phase
• The combination of these two
sound waves within the headset
will result in destructive
interference and thus reduce a
worker's exposure to loud noise.