Quantification of Airframe Noise Using Microphone Arrays in Open and Closed wind Tunnels

Abstract

The reliability of the phased array technique for quantifying airframe noise was assessed in the DNW-LLF open and closed wind tunnel sections. Acoustic measurements were performed on wing noise from a 1:10.6 scaled Airbus A340 model, using a 4-m diameter out-of-flow microphone array in the open jet and a 1-m diameter wall array in the closed test section. Apart from the differences in array resolution, the source characteristics are generally similar for both test sections. The open jet results show that, although the absolute integrated array level can be too low due to e.g. coherence loss, the relative sound levels determined with the array (i.e. differences between configurations) are accurate within 0.5 dB. The difference between the absolute array levels in the closed test section and the farfield levels in the open jet is smaller than 3 dB for all frequencies. The relative array levels in the closed test section agree with the open jet within 1 dB, provided that the flow conditions-and therefore the noise source characteristicsare similar.

Three-Dimensional Acoustic Source Mapping with Different Beamforming Steering Vector Formulations

“…Acoustic source mapping techniques using beamforming methods have been widely applied for the study of acoustic sources (e.g., for trains [2], aeroacoustic testing [3,4], airframe noise [5,6], noise source characterization at a helicopter [7], and jet noise [8]). These methods use the signals from an array of acoustic sensors (mostly microphones) to filter out the signal from a source at an assumed location.…”

Three-dimensional beamforming of dipolar aeroacoustic sources

“…To improve the dynamic range of the method, the diagonal terms of the CSM can be removed (a step known as diagonal removal). The removal of the diagonal terms improves the ability of the array to identify sources by "subtracting" the high amplitude autospectral terms that can be dominated by phenomena that are correlated with their own signal, i.e turbulent boundary layer noise/flow noise, electrical self-noise, rather than an acoustic source in the far field [17]. Hence, this step is generally required when the microphones are exposed to flow as in a closed wind tunnel or an open jet wind tunnel with low level recirculation.…”

“…Coherence loss between microphones may also be caused scattering of sound by turbulence in the flow and generally increases with increasing frequency and distance between microphones [17]. Therefore, at higher frequencies, microphones in the outer part of the array may be completely incoherent with one another, reducing the effective aperture of the array [21].…”

A plasma-based non-intrusive point source for acoustic beamforming applications

“…Narrowband spectra are computed using a conventional beamforming normalization method which approximates integration [32]. Integration bounds are specified as 70.1 m in the x-direction and 70.35 m in the y-direction, selected to capture as much of the trailing edge of the airfoil as possible, while rejecting some sidewall contamination at higher frequencies.…”