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.

Using three microphone array configurations at two aircraft body stations on a Boeing 777-300ER flight test, the acoustic radiation characteristics of the sidewall and outboard floor system are investigated by experimental measurement. Analysis of the experimental data is performed using sound intensity calculations for closely spaced microphones, PATCH Inverse Boundary Element Nearfield Acoustic Holography, and Spherical Nearfield Acoustic Holography. Each method is compared assessing strengths and weaknesses, evaluating source identification capability for both broadband and narrowband sources, evaluating sources during transient and steady-state conditions, and quantifying field reconstruction continuity using multiple array positions.

Near-field acoustical holography (NAH) microphone array measurement technologies are currently being investigated for use in aircraft for determining the sound intensity that is radiated through a trimmed aircraft fuselage. Two types of microphone array geometries are studied in this effort, spherical and conformal to the sidewall. Due to the complexity of the interior sound field and in-flight excitation, questions have arisen concerning the capability of array techniques to measure these desired acoustic quantities. The main questions relate to the agreement between array measurements and traditional two-microphone intensity probe measurements and the sensitivity of array measurements to backside sources present in the aircraft cabin. Thus, an investigation was performed in which the NAH array results were compared to intensity probe results measured in the NASA Langley Structural Acoustic Loads and Transmission facility, the Boeing Interior Noise Test Facility, and in flight on the NASA Aries 757 aircraft. An overview of this test sequence and a comparison of NAH, IBEM, and two-microphone intensity probe measurements are made. These comparisons show the relative behavior of the different measurement techniques in both idealized laboratory and realistic in-flight environments. Conclusions are drawn concerning the use of array measurements to assess the sound intensity transmitted through an aircraft sidewall in-flight.

Transmission loss is an important metric used to assist in the selection of materials for noise control and design. In order to assess and improve the performance of transmission loss measurements, two different techniques were compared: traditional microphone and/or matched pair microphone intensity anechoic chamber-side techniques, and nearfield acoustical holography techniques. Intensity and sound power measurements using traditional methods were made on a test panel installed in an anechoic-reverb facility transmission loss window, then repeated using a conformal nearfield microphone array and nearfield acoustical holography processing techniques. The full vectored intensity components were used to assess the radiation characteristics of the test panel. This assessment quantifies the acoustics as panel radiation, flanking path contamination, and boundary effects. Data are presented to qualify the best estimate for transmission loss by rejecting contamination. Also, the dynamic range of the transmission loss measurement is compared between traditional and nearfield acoustical holography techniques with high transmission loss panel samples.