Determination of transmission loss of materials from array measurements using nearfield acoustical holography/inverse boundary element methods in a transmission loss facility

Abstract

A new approach for the measurement of transmission loss based on NAH is discussed in this paper. Two array configurations were tested to determine the transmission loss of sound isolation materials applied to an aircraft fuselage panel in Boeing’s interior noise test facility (INTF). The first configuration consisted of a 120 element planar array positioned conformal to the fuselage panel surface in the anechoic side of the TL facility. A second configuration used 50 microphones in a spherical array of diameter 0.4 meters placed in front of the panel. The normal intensity on the surface of the fuselage panel isolation layer was reconstructed using NAH/IBEM (nearfield acoustical holography/inverse boundary element methods) using partial fields generated from a set of reference accelerometers. The panel excitation was random plane wave fields generated in the reverberation room of the TL facility. This new approach measures the TL in narrow bands or 1/3 octave bands from 50 to 2 kHz, and is shown to be very accurate when compared with conventional TL measurements. This approach also provides the normal panel vibration for each partial field so that panel modes can be uncovered and studied. Furthermore, the spherical array provides a reconstruction of the vector acoustic intensity in the volume in front of the panel so that the magnitude and direction of the transmitted energy fields can be mapped.

Comparison of different measurement technologies for the in-flight assessment of radiated acoustic intensity

“…These coherent partial fields were then propagated to the treatment boundary using patch IBEM methods 3 , and the radiated intensity distribution and integrated sound power was computed for the different treatment configurations (Table 1). The insertion loss of the treatment was found as described in Sec.…”

“…The insertion loss of the treatments was measured with the different measurement techniques 3,4 to determine the ability of the microphone arrays to measure changes in the sound power radiated from the sidewall under various treatment conditions and excitation mechanisms. In all, these treatments were studied while applied to four different fuselage test articles, two were in laboratory environments and two were sidewall sections of a flight test aircraft.…”