12: Interference Lab - 1
12. PRELAB FOR INTERFERENCE
LAB
1.
INTRODUCTION
As you have seen in your studies of standing waves, a wave and its reflection can
add together “
constructively
” (peak meets peak, giving large amplitude) or
“
destructively
” (peak meets valley, canceling each other).
This type of
adding
is called
INTERFERENCE
, and it depends on the relative
phase
of the interfering waves.
Since
the direct and reflected waves are coming from the same source, they clearly start out
in
phase
.
Whether or not they come back together in phase depends on path length.
If the
difference in the two path lengths is 1,2,3,etc.
wavelengths
, then the waves will be back
in phase.
If there is an extra ½ wavelength, however, they will be out of phase.
2.
AN EXAMPLE OF MICROPHONE PLACEMENT
The next page contains an excerpt from an article on the importance of a
microphone placement.
Read the excerpt and answer the following questions:
1.
When the pressure wave is reflected from the table, is there a 180
o
phase shift?
(Think back to the standing waves in the open and closed tubes).
2.
Look at Fig. 4.10 from the excerpt.
Suppose that the direct sound travels 2
meters, and the reflected sound covers a greater distance of 2 ½ meters.
Therefore, the
difference in the two path lengths is
d
= 0.5m.
Constructive interference will occur when
d
=
λ
,
2
λ
,
3
λ
,
4
λ
, … , where
λ
is the wavelength.
Use the relation v =
λ
f
, (v – speed of
sound,
f
– frequency) to calculate the first four frequencies at which constructive
interference will occur.
Assume
v
= 344 m/s.
Then, calculate the first four frequencies at
which destructive interference (
d
= ½
λ
,
(1+
½)
λ
,
(2+
½)
λ
,
…)
will occur.