Phased microphone arrays have become a well-established tool for performing aeroacoustic measurements in wind tunnels (both open-jet and closed-section), flying aircraft, and engine test beds. This paper provides a review of the most well-known and state-of-the-art acoustic imaging methods and recommendations on when to use them. Several exemplary results showing the performance of most methods in aeroacoustic applications are included. This manuscript provides a general introduction to aeroacoustic measurements for non-experienced microphone-array users as well as a broad overview for general aeroacoustic experts.

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A review of acoustic imaging methods using phased microphone arrays

The noise sources of landing commercial aircraft were examined with planar arrays consisting of 96 or 111 microphones mounted on an 8 m by 8 m plate under the glide path on the ground. It is shown that important airframe noise sources can be identified in spite of the presence of engine noise, i.e., landing-gear noise, flap side-edge noise, flap-gap noise, jet-flap interaction noise, slat-horn noise, slat-track noise. A surprising finding is a noise source near the wing tips of some aircraft which is tentatively called wake-vortex wing interaction noise. It is shown to be the by far strongest noise source (6 dB(A) louder than the engines) on a regional jet aircraft.

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Flyover Noise Measurements on Landing Aircraft with a Microphone Array

A new algorithm for estimating noncompact, distributed sources by means of phased array microphone measurements is presented and experimentally implemented to determine the noise source distribution in a subscale jet flow. Conventional beamforming techniques, developed for spatially well-separated point sources, can lead to significant errors when applied to reconstruct continuous source distributions such as jet noise. A new beamforming approach is developed for estimating such continuous source distributions. The objective is to recover the average source strength over a small region around each focus position as opposed to seeking the exact source strength at each spatial location as in conventional approaches. This strategy overcomes the drawbacks of conventional methods and yields a beamformer with uniform spatial resolution and accuracy over a large frequency range. The measurement technique is applied to the localization of broadband noise sources in a high-subsonic, heated, turbulent jet flow and shows good comparisons with prior measurements using other techniques.

Beamforming Algorithm for Distributed Source Localization and Its Application to Jet Noise

The development of the beamforming method (also called microphone antenna, phased array of microphones, acoustic telescope, or acoustic camera) is reviewed in this paper. The microphone antenna was invented by Billingsley (1974) and has since seen dramatic improvements due to the availability of better data acquisition and computing hardware. Recent mathematical and software developments invert the beamforming process and allow a quantitative determination of the sources. Beamforming is indispensable for the localization of sound sources on moving objects, on flying aircraft, on high-speed trains, on motor cars in motion, on open rotors like helicopter and wind turbine rotors. In these applications, the ability to follow the motion of the sources is important. The second important applications are source localization tests in the test sections of open and closed wind tunnels. The background noise suppression capability of the beamforming method is required here. The various applications are discussed with a long list of references. 1 Berlin Beamforming Conference

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History of acoustic beamforming

In this paper, a beamforming correction for identifying dipole sources by means of phased microphone array measurements is presented and implemented numerically and experimentally. Conventional beamforming techniques, which are developed for monopole sources, can lead to significant errors when applied to reconstruct dipole sources. A previous correction technique to microphone signals is extended to account for both source location and source power for two-dimensional microphone arrays. The new dipole-beamforming algorithm is developed by modifying the basic source definition used for beamforming. This technique improves the previous signal correction method and yields a beamformer applicable to sources which are suspected to be dipole in nature. Numerical simulations are performed, which validate the capability of this beamformer to recover ideal dipole sources. The beamforming correction is applied to the identification of realistic aeolian-tone dipoles and shows an improvement of array performance on estimating dipole source powers.

B. Barsikow

Papers

Wheel/rail noise generated by a high-speed train investigated with a line array of microphones