StAB - Flow Impedance Test Facility

Flow Impedance Test Facility

Photograph of the Flow Impedance Test Facility

The Flow Impedance Test Facility (FITF) is used to characterize the acoustic performance of full-scale acoustic liner materials for use in commercial aircraft engine nacelles. The FITF contains four major measurement apparatuses. The first is a raylometer, which is used to measure the DC flow resistance of acoustic absorbers. This fully automated device is currently capable of measurements over a velocity range of 0.001 to 5 meters/sec, which is well beyond the range of raylometers commonly used in industry. The second apparatus is a pulse impedance tube, in which a time-domain approach is used to determine the acoustic characteristics of test samples at high sound pressure levels. This is achieved via a high intensity sound source, capable of producing pulses up to 170 dB.

The third apparatus is a 2"x2" normal incidence impedance tube. Sound waves are impinged onto the surface of acoustic absorbers mounted onto the end of this apparatus. This sound is reflected from the sample, setting up standing wave patterns that are used to determine the absorptive qualities of the absorbers. The last, and most significant, apparatus is a grazing flow impedance tube. This 2"x2" cross-section wind tunnel is used to measure the acoustic properties of sound absorption materials in the presence of mean flow up to Mach 0.5. The normal incidence and grazing incidence tubes are typically operated over a frequency range of 500 to 3000 Hz, with incident sound pressure levels ranging from 100 to 160 dB (maximum level is test specimen dependent).

Recent studies include testing a set of acoustic liners fabricated by AlliedSignal Engines in the normal incidence and grazing incidence tubes. Results of these tests, conducted in 1998, were used to design two acoustic liners currently mounted in business jets. During 1999 and 2000 a series of tests was conducted with 28 test liners fabricated by GE and BF Goodrich. These liners were tested in each of the four major impedance test facilities (NASA LaRC, GE, BF Goodrich and Boeing) to determine the cause of variability between previous results acquired at the industry labs. The data acquired in the Flow Impedance Test Facility was used to assess the validity of the data acquired by the industry labs. Improved techniques for the measurement and prediction of noise reduction in aircraft engines using modern acoustic liners have been developed.