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Impedance Matching
Friday 5
We have been saying that if the impedance is matched between two media, then there
will be no reflection of waves as energy passes from one medium to another.
If a wave
pulse passes from medium number 1 with impedance
Z
1
and strikes a medium number 2
that has impedance
Z
2
, the ratio of the amplitude of a reflected wave pulse
A
r
to the
incident wave pulse
A
i
is given by:
21
1/
r
i
AZ
Z
Z
−
=
+
Plot of the ratio
A
r
/
A
i
given the ratio
Z
2
/Z
1
Fig. 1
Notice that if a wave reflects from a low impedance (
Z
2
less than
Z
1
) then the reflected
pulse has the same sign and nearly the same amplitude as the incident pulse.
If it reflects
from a high impedance (
Z
2
greater than
Z
1
) then the sign is inverted.
This is what we saw
with reflections on the wave machine in lecture.
If the impedance ratio is 1, this means that the impedances are the same and then the
amplitude of the reflected wave is zero.
This is called "impedance matching".
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 Spring '08
 SKIFF

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