Effect of directional array size on the measurement of airframe noise components

Abstract

A study was conducted to examine the effects of overall size of directional (or phased) arrays on the measurement of aeroacoustic components. An airframe model was mounted in the potential core of an open-jet windtunnel, with the directional arrays located outside the flow in an anechoic environment. Two array systems were used; one with a solid measurement angle that encompasses 31.6° of source directivity and a smaller one that encompasses 7.2°. The arrays, and sub-arrays of various sizes, measured noise from a calibrator source and flap edge model setups. In these cases, noise was emitted from relatively small, but finite size source regions, with intense levels compared to other sources. Although the larger arrays revealed much more source region detail, the measured source levels were substantially reduced due to finer resolution compared to that of the smaller arrays. To better understand the measurements quantitatively, an analytical model was used to define the basic relationships between array to source region sizes and measured output level. Also, the effect of noise scattering by shear layer turbulence was examined using the present data and those of previous studies. Taken together, the two effects were sufficient to explain spectral level differences between arrays of different sizes. An important result of this study is that total (integrated) noise source levels are retrievable and the levels are independent of the array size as long as certain experimental and processing criteria are met. The criteria for both open and closed tunnels are discussed. The success of special

A Microphone Array Method Benchmarking Exercise using Synthesized Input Data

“…This is remarkable in view of the low complexity of the method. 11 The good performance of SPI can, however, be explained by considering the TUD results. TUD did not directly apply SPI, but solved a Least Squares minimization problem to find the best match between the CSM without diagonal and a line source of unknown strength.…”

Effect of surface treatment on array microphone self-noise

“…Microphone array measurements have been used to quantify jet and airframe noise sources in closed [10][11][12] and open 13,14 wind tunnel test sections with much success. Although in-flow wind tunnel array measurements are not burdened by the need to apply shear layer corrections to American Institute of Aeronautics and Astronautics 2 multiple source paths, the background noise level at the microphones is high because of the turbulent boundary layer flowing over the exposed microphone diaphragms.…”

“…Another method, often used during single microphone acquisitions, is to subtract out the tunnel-empty background noise 13 . While straightforward in theory, the task of evaluating the tunnel empty-noise in a consistent manner often requires multiple test runs to achieve an accurate statistical set 17 .…”

“…While straightforward in theory, the task of evaluating the tunnel empty-noise in a consistent manner often requires multiple test runs to achieve an accurate statistical set 17 . Processing techniques, such as replacing the covariance matrix diagonal with zeroes or an averaged spectra, may also reduce the influence of flow noise, but this method is limited somewhat by the necessity for completely incoherent self noise at each microphone 13,16 .…”

Effects of Angle of Attack and Velocity on Trailing Edge Noise

“…TE noise). The method proposed, after taking into account spatial source coherence, is to use an Array Calibration Function (equivalent to peak-output-level plots for line sources from Brooks and Humphreys [13]) to relate peak levels to total line (TE noise) source strength. This was in preference to applying the noise integration method of Ref.…”