Description:
Each time one is confronted with unwanted narrow-band noise which propagates along a duct, it suggests itself to make use of resonators in the suppression of this noise. However, resonators may be highly sensitive to superimposed mean flow: Detuning and drastic decay of the quality factor is encountered in many cases, nevertheless, also the contrary may happen, namely the increase of the acoustic energy in the resonator. This may result in self-excited oscillations or in the amplification of the incident sound. The paper gives a review of these phenomena, and it will focus on the physical effects behind these phenomena. Simple models that are based on the governing physical effects have been developed and are compared to experimental results with different configurations of resonators in flow ducts. In the case of sound amplification the amplitude of the organized motion of the fluid may become very high so that the nonlinearity of the interaction with the mean flow becomes significant. The state of knowledge and the open questions in this respect will be addressed as well.