A correction methodology has been developed to account for the signal interference. The first side lobe of the beam pattern was 13.4 dB down from the peak response at 8 kHz and at an angle of 23° from the normal vector: these characteristics made it possible to obtain good acoustic signals from the model when the model was located at a distance of 1.11 m from the array. Data were acquired at 12,321 scan points in a plane encompassing the model. From these data, aerodynamic noise from sources as small as 6 mm on the model surface could be identified easily.

The microphone signals were digitized at a rate of 153,600 samples per second on 104 channels simultaneously by use of analog-to-digital converter circuits and a computer. The resulting maximum acoustic frequency was 60 kHz with a bandwidth of 300 Hz. The data for frequencies <2 kHz were found to be of marginal utility because the microphone beam pattern at those frequencies was too wide. The data for frequencies >32 kHz were found to be of marginal utility because at those frequencies, the sources were too weak and the side lobes too strong. The frequency limits of 2 and 32 kHz correspond to limits of 140 and 2,240 Hz, respectively, on the full-scale aircraft.

A sound-convection correction was included in the processing of the data so that sources appeared to come from the model rather than being swept downstream. The acoustic sources were depicted, one frequency at a time, as color contours on the scan plane with the model outline superimposed, as shown in Figure 2. Various integration schemes have been developed to compute the combined effects on a listener and to generate narrow band and third octave acoustic spectra.

Ten airframe noise sources that might be important to approach and landing noise of the full-scale aircraft were identified in the study. The relative strengths of these sources and their dependences on the configuration of the aircraft were documented. Although the data were scaled to the frequencies for the fullscale aircraft, no extrapolation to full scale flyover was performed.

This work was done by Paul T. Soderman of Ames Research Center. For further information, contact the Ames Technology Partnerships Division at (650) 604-2954. ARC-14967