Acoustical-Sensor Assemblies for Use in Flows

Ames Research Center, Moffett Field, California

Improved sensor assemblies have been invented for measuring acoustic signals in gases or liquids flowing in or around any of a variety of moving or stationary objects. Examples of such objects include land vehicles, aircraft, submarines, structures exposed to winds or natural water streams, and industrial equipment containing or surrounded by process streams. In comparison with older acoustical-sensor assemblies designed for use in flows, these assemblies offer reduced response to noise generated by flows at and near sensor-assembly surfaces and increased protection for fragile acoustical transducers. These assemblies can be made to discriminate against sounds coming from outside preferred ranges of directions. They can also be mounted flush with surfaces of objects to minimize flow-induced noise and drag.

The figure illustrates two assemblies within the scope of the invention. In each case, the sensor is a microphone or other dynamic-pressure transducer. Each sensor is mounted in a cavity, wherein it is recessed from a streamlined surface. The cavity is lined with a sound-absorbing foam filler. A hole in the filler tapers conically from the sensor at its narrow end to an opening on the streamlined surface at its wide end. The cone angle is chosen, along with other design parameters, to obtain the desired directional response.

Sensors Are Mounted Behind Screens at the apices of conical holes in sound-absorbing foam fillers.

The opening on the streamlined surface is covered with a screen or porous sheet, which prevents or reduces the propagation of surface-flow disturbances (turbulence) into the cavity while allowing the acoustic waves of interest to propagate to the sensor. The placement of the sensor in the recess behind the screen thus reduces the intensity of flow-induced noise arriving at the sensor. Moreover, spurious acoustic disturbances arriving from immediately upstream or downstream of the opening can be rejected by reflection from the shear layer in the flow adjacent to the screen. Because the fluid in the cavity is still or nearly still, relative to the fluid outside the cavity, the sensor is protected against damage by the flow. The screen also protects the sensor against impacts of particles entrained in the flow.

The sensor assembly shown in the upper part of the figure is mounted inside a streamlined body with a rounded fairing on its upstream end, for measuring sounds arriving at the surface of the body. The sensor assembly shown in the lower part of the figure is mounted on the wall of an object; this sensor assembly includes its own fairing, which protrudes into the flow beyond the wall boundary layer or free shear layer, for measuring sounds free of interference from turbulence in this layer.

This work was done by Fredric Schmitz, Sandy Liu, Stephen Jaeger, and W. Clifton Horne of Ames Research Center.

This invention has been patented by NASA (U.S. Patent No. 5,684,756). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to