Oscillations, Waves, and Interactions - GWDG

Oscillations, Waves and Interactions, pp. 73–106
edited by T. Kurz, U. Parlitz, and U. Kaatze
Universitätsverlag Göttingen (2007) ISBN 978–3–938616–96–3
urn:nbn:de:gbv:7-verlag-1-04-5
Sound absorption, sound amplification, and
flow control in ducts with compliant walls
D. Ronneberger and M. Jüschke
Drittes Physikalisches Institut, University of Göttingen
Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
Abstract. Efficient damping of narrow-band noise in ducts is commonly achieved with
a lining which has a resonance tuned to the frequency band of the noise. However, with
superimposed mean flow and with high quality factors of the resonators, large sound amplification
(up to 30 dB) is observed even if only a short section (less than two diameters
long) of the circular duct is provided with the lining. Jointly with the sound amplification, a
considerable increase of the static pressure drop (by more than 100 % at high sound pressure
amplitudes) along the lined duct section is observed. The most important experimental results
will be reviewed and the physical mechanisms behind these phenomena are thoroughly
discussed. The theoretical investigation of the wave propagation and of the stability of the
flow within the resonator section happens to be an unexpectedly high challenge. The various
common approaches based on mode decomposition and axial homogeneity of the flow
result in dispersion relations which largely diverge from the experimental results. While
a convective instability has been observed to cause the considered phenomena, the flow is
predicted to be subject to absolute instability even if the interaction between the coherent
and the turbulent instability waves are included by a first approximation. So we conjecture
that the spatial development of the mean flow which for its part depends on the spatial
development of the instability waves has to be taken into account.
1 Introduction
Sound propagation in channels plays an important role in many technical problems,
e. g. when noise from machines or from the outside is propagated through
air-conditioning duct systems or when the noise from turbofans is to be reduced.
Duct acoustics is also an obligatory subject in lectures on technical acoustics, and
the design of sound absorbing ducts is a problem, which is treated typically in engineering
sciences. So, why – the reader might ask – are we concerned with this subject
in a Physics Institute since many years? In fact, it is more than fifty years ago that
Fridolin Mechel, a student of Erwin Meyer at that time, was instructed to investigate
the performance of acoustically treated ducts that carry a mean flow. Only a few
years earlier, Lighthill [1] had published his famous paper on the sound production
in unsteady flows which has triggered extensive research work in aero-acoustics.