KEYWORDS: Acoustics, Signal processing, Linear filtering, Phased arrays, Interference (communication), Signal to noise ratio, Analog electronics, Surveillance, Antennas, Optical filters
A very large acoustic microphone array is described for use in the audio frequency band from 300 to 3000 Hz. The array used constant aperture processing to provide 10 wavelengths of aperture over the entire band, resulting in a narrow but constant beamwidth for all frequencies. The array was electronically steerable in azimuth over a +/- 80-degree range with no grating lobes. Several different embodiments have been constructed, and one design featured steerability in both the elevation and azimuth planes. This array used vertical subarrays at each horizontal array station that were pre-steered in elevation using a novel analog delay-and-sum beamformer. Signal processing was accomplished using a desktop PC. The large number of microphones used to construct the arrays demonstrated exceptional wind noise averaging in outdoor conditions. Array details, processing methodology, and test results are presented.
This paper describes development and application of a novel method to accomplish real-time solid angle acoustic direction finding using two 8-element orthogonal microphone arrays. The developed prototype system was intended for localization and signature recognition of ground-based sounds from a small UAV. Recent advances in computer speeds have enabled the implementation of microphone arrays in many audio applications. Still, the real-time presentation of a two-dimensional sound field for the purpose of audio target localization is computationally challenging. In order to overcome this challenge, a crosspower spectrum phase1 (CSP) technique was applied to each 8-element arm of a 16-element cross array to provide audio target localization. In this paper, we describe the technique and compare it with two other commonly used techniques; Cross-Spectral Matrix2 and MUSIC3. The results show that the CSP technique applied to two 8-element orthogonal arrays provides a computationally efficient solution with reasonable accuracy and tolerable artifacts, sufficient for real-time applications. Additional topics include development of a synchronized 16-channel transmitter and receiver to relay the airborne data to the ground-based processor and presentation of test data demonstrating both ground-mounted operation and airborne localization of ground-based gunshots and loud engine sounds.
As commercial operating systems such as Microsoft Windows move from the desktop to the embedded world and low-power, low-cost embedded CPU performance increases the distinction between dedicated embedded systems and laboratory demonstration systems is blurring. Signalscape, Inc. has developed a highly flexible acoustic signal processing system with real-time performance on Pentium II class desktop computers running Microsoft Windows. In a recently awarded DARPA program, Rockwell Science Center, Signalscape, and their partners are moving the flexibility and capabilities of Signalscape's many tool kits onto low-cost, low-power systems for unattended sensor applications. In this paper we discuss the results of the first demonstrations of acoustic beamforming using a small network of sensors.
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