Signal Processing and Propagation for Aeroacoustic Sensor Networks

Passive sensing of acoustic sources is attractive in many respects, including the relatively low signal bandwidth of sound waves, the loudness of most sources of interest, and the inherent difficulty of disguising or concealing emitted acoustic signals. The availability of inexpensive, low-power sensing and signal-processing hardware enables application of sophisticated real-time signal processing. Among the many applications of aeroacoustic sensors, we focus in this chapter on detection and localization of ground and air (both jet and rotary) vehicles from ground-based sensor networks. Tracking and classification are briefly considered as well. Elaborate, aeroacoustic systems for passive vehicle detection were developed as early as World War I [1]. Despite this early start, interest in aeroacoustic sensing has generally lagged other technologies until the recent packaging of small microphones, digital signal processing, and wireless communications into compact, unattended systems. An overview of modern outdoor acoustic sensing is presented by Becker and Gudesen [2]. Experiments in the early 1990s, such as those described by Srour and Robertson [3], demonstrated the feasibility of network detection, array processing, localization, and multiple target tracking via Kalman filtering. Many of the fundmental issues and challenges described by Srour and Robertson [3] remain relevant today. Except at very close range, the typical operating frequency range we consider is roughly 30 to 250Hz. Below 30Hz (the infrasonic regime) the wavelengths are greater than 10m, so that rather large arrays may be required. Furthermore, wind noise (random pressure fluctuations induced by atmospheric turbulence) reduces the observed signal-to-noise ratio (SNR) [2]. At frequencies above several hundred hertz, molecular absorption of sound and interference between direct and ground-reflected waves attenuate received signals significantly [4]. In effect, the propagation environment acts as a low-pass filter; this is particularly evident at longer ranges.