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Periodic Phenomena
Kundt's tube with analog outputs
Ref : 22204
6. Theoretical study of a sound tube
A sound wave is produced at the left end of the pipe by the loudspeaker powered by a LFG
(not supplied). It propagates in the air's elastic medium and is reflected from the end of the
tube.
The superposition of the two waves – incident wave and reflected wave – gives rise to a
vibratory state: the air in the tube is subjected to a series of compressions and rarefactions
of sections of air constituting the space in the tube.
For specific values of the frequency of sound waves, length of the tube and speed of sound
in air, the vibration inside the pipe can present itself in the form of standing waves.
6.1
In the vibratory states observed, immobile points called displacement nodes and points
with maximum vibration called displacement antinodes can be distinguished.
The displacement antinodes correspond to constant pressure (or to zero variations in
pressure): these are pressure nodes. Conversely, the displacement nodes coincide with
the area of maximum pressure variations or pressure antinodes.
The electret-condenser microphone, the scanning sensor of the vibratory state in the tube
is sensitive to air pressure: it thus detects the pressure nodes N and antinodes V (as
well as the intermediary states).
At the loudspeaker level (sound source): the membrane of the loudspeaker vibrates and
constitutes a maximum displacement region (displacement antinode). It is thus a
pressure node N.
The position of the other end depends on its state:
6.2
For specific conditions, the pressure nodes (N) have almost zero values and the pressure
antinodes (V) have maximum amplitude. This phenomenon corresponds to a state of
resonance of air in the pipe with the vibrating membrane of the loudspeaker.
ENGLISH
Description of the vibratory state
•
If the end is open: the pressure there is constant (atmospheric pressure) and we
have a pressure node N
•
If the end is closed (plug installed), the displacement of air is zero; as a result, we
have there a pressure antinode V.
Conditions for the existence of standing waves
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