Flow induced vibration and noise in a duct with a tensioned membrane

In a flow duct, turbulence is a source of excitation which gives rise to wall vibrations which subsequently radiate sound, and the sound thus radiated is called the break-out noise. In this study, however, the vibration and noise radiation by a segment of flexible wall is intended for noise reduction in a flow duct. The difference between the present configuration and the break-out noise radiation is that (a) the flexible wall in this case is supported by a solid-walled cavity, and (b) the flexible wall made of a thin metal membrane is highly stretched. Any incident wave would induce vibration on the membrane, and the upstream radiating wave represents sound reflection. With sound reflection, the transmitted wave is reduced as the total energy derives from the incident sound. The device is called a drum-like silencer, and its theory has been developed so far without considering the flow effect (J. Acoust. Soc. Am. 112:2014-2035). A question arises as to whether the turbulence induced vibration would give extra noise which adds to the sound energy flux downstream as its source does not come from the incident sound waves from the upstream. Also, flow induced instability has to be excluded before the device can be used in a flow duct. Experimental studies are conducted using both wind tunnel tests and insertion loss (IL) tests with natural ventilation flows with high turbulence intensity. It is found that the flow turbulence does induce a certain amount of vibration, but such vibration is clearly not a manifestation of flow induced flexural instability. The measurement of IL also proves that the performance in a turbulent flow duct reproduces the IL found with the same rig without flow, implying that there is no significant noise radiation caused by the turbulence induced vibration.