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Oct 2003

Volume 114, Issue 4, pp. 1695-2468

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Beampattern control of a microphone array to minimize secondary source contamination

Peter Jordan, John A. Fitzpatrick, and Craig Meskell

J. Acoust. Soc. Am. Volume 114, Issue 4, pp. 1920-1925 (2003); (6 pages) | Cited 1 time

Online Publication Date: 08 Oct 2003

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A null-steering technique is adapted and applied to a linear delay-and-sum beamformer in order to measure the noise generated by one of the propellers of a ⅛ scale twin propeller aircraft model. The technique involves shading the linear array using a set of weights, which are calculated according to the locations onto which the nulls need to be steered (in this case onto the second propeller). The technique is based on an established microwave antenna theory, and uses a plane-wave, or far field formulation in order to represent the response of the array by an nth-order polynomial, where n is the number of array elements. The roots of this polynomial correspond to the minima of the array response, and so by an appropriate choice of roots, a polynomial can be generated, the coefficients of which are the weights needed to achieve the prespecified set of null positions. It is shown that, for the technique to work with actual data, the cross-spectral matrix must be conditioned before array shading is implemented. This ensures that the shading function is not distorted by the intrinsic element weighting which can occur as a result of the directional nature of aeroacoustic systems. A difference of 6 dB between measurements before and after null steering shows the technique to have been effective in eliminating the contribution from one of the propellers, thus providing a quantitative measure of the acoustic energy from the other. © 2003 Acoustical Society of America.
Show PACS
43.38.Hz Transducer arrays, acoustic interaction effects in arrays
43.28.Tc Sound-in-air measurements, methods and instrumentation for location, navigation, altimetry, and sound ranging
43.50.Lj Transportation noise sources: air, road, rail, and marine vehicles

High-sensitivity photoacoustic leak testing

Eric Huang, David R. Dowling, Timothy Whelan, and John L. Spiesberger

J. Acoust. Soc. Am. Volume 114, Issue 4, pp. 1926-1933 (2003); (8 pages) | Cited 1 time

Online Publication Date: 08 Oct 2003

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The photoacoustic effect may be exploited for the detection and localization of gas leaks from otherwise sealed components. The technique involves filling the test component with a tracer gas, and radiating the component to produce photoacoustic sound from any leak site where tracer gas is present. This paper describes demonstration experiments utilizing 10.6-μ radiation from a carbon-dioxide laser and sulfur hexafluoride as a tracer gas for photoacoustic leak testing at leak rates between 6×10−5 cm3/s (1 cm3 in 4.6 h) and 5×10−9 cm3/s (1 cm3 in 6.3 years). The technique may reach or exceed the capabilities of the most sensitive commercial leak test systems using helium mass-spectrometers. In addition, comparison of the measured results to a simple scaling law suggests that tracer gas cloud geometry influences the photoacoustic signal amplitude. © 2003 Acoustical Society of America.
Show PACS
43.38.Zp Acoustooptic and photoacoustic transducers
43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect
43.60.Gk Space-time signal processing, other than matched field processing
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