Oscillations, Waves, and Interactions - GWDG

80 D. Ronneberger et al.
Figure 6. Amplitude and phase of the pressure oscillation close to the wall of the unmodified
resonator section as functions of the axial coordinate; the pressure field was excited
by an incident sound wave. The flow velocity U was varied and the respective frequency f
was adjusted to the maximum of the transmission coefficient for each flow velocity. From
Ref. [29].
flow velocities, and the sound was introduced into the first cavity by means of three
loudspeakers, in this experiment. The greatest effect is obtained slightly above the
frequency of maximum sound amplification. In fact, the pressure drop and the sound
transmission exhibit a very similar dependency on the frequency and the flow velocity.
These phenomena have recently been observed also by Aurégan et al. [32] with cavities
which are much narrower than in our experiment; the ceramic skeleton of a catalytic
converter has been used by these authors for that purpose.
The resonator section can be used as a valve which is contolled by the amplitude
of superimposed sound and the response time of which (a few milliseconds) is
given by the width of the frequency interval of sound amplification. Lange and Ronneberger
[27] have demonstrated this possibility by active suppression of the sound
transmission through the resonator section at frequencies up to ca. 200 Hz. For this
purpose the pressure drop was adjusted to exactly follow the sound pressure in front