Enhanced Modulation Spectrum Using Space-Time Averaging For In-Building Acoustic Signature Identification

For most buildings, virtually all subsystems such as fans, generators, and motors generate acoustic energy. This acoustic energy can weakly penetrate walls and pass through hallways and conduits. The propagation paths are complex and, given the typically low energy of received acoustic signals, present challenges for the detection and classification of the subsystems. While conventional approaches would not be expected to work under these difficult conditions, a key observation can be made: these types of subsystems produce line spectra which consist of harmonics of a fundamental frequency. Acoustic propagation effects then strongly affect the relative energy of these harmonics. Modulation spectra, which make use of the frequency spacing instead of the relative energy of the harmonics, are especially insensitive to these frequency-dependent acoustic attenuation affects. When combined with temporal averaging and 3-dimensional spatial (over an array of acoustic sensors) processing, enhanced modulation spectra offer a new approach to the detection and classification of building subsystems which produce sound