Laser-Assisted Sound Source Localization Inside A Solid Enclosure

Sound source localization is typically conducted in open space so that the line of sight is not blocked among individual sensors. This research presents a new technology that enables one to see acoustically through a solid Plexiglass enclosure to locate a sound source stationed inside at some arbitrarily selected locations. By using SODAR technology, non-invasive laser-assisted microphones, analog, and digital signal processing (band-pass filtering, fast Fourier transform algorithm, cross-correlations, and triangulations) satisfactory sound source localization results were obtained. In this project, the enclosure walls were flat; only the azimuth angle of a sound source could be determined. To obtain the Cartesian coordinates of a sound source inside a solid enclosure, it is necessary to measure the normal components of surface vibrations on a curved surface. Alternatively, one may measure vibration signals on two walls surfaces of an enclosure.At present, this see-through technology is accomplished using an array of laser vibrometers that can measure a minute amount of surface vibration signals on the surface of a solid enclosure. These vibrational signals are decoded and converted to acoustic signals, which are taken as the input data to another innovative technology known as Sonic Detection and Ranging (SODAR) to locate sound sources. Experimental validations of this new technology were conducted inside a solid enclosure made of plexiglasses of dimensions 1 X 1 x 1 m^3. A speaker was placed inside the enclosure at arbitrarily selected locations and emitted various acoustic signals at very low amplitudes. An array of six laser microphones were designed and built, which enabled one to measure the normal components of minute surface vibrations of enclosure walls, and subsequently, convert them to acoustic signals through band-pass filters and fast Fourier transform algorithm together with cross-correlations and triangulations to locate sound sources. It is worth mentioning that all experiments were conducted in a reverberant space. Satisfactory sound source localization results were obtained. It is noted, however, that in the present project, the enclosure walls were flat. Consequently, only the azimuth angle of a sound source could be determined. To obtain the Cartesian coordinates of a sound source inside a solid enclosure, it will be necessary to measure the normal components of surface vibrations on a curved, 3D surface. Alternatively, one may measure vibration signals on two walls surfaces of an enclosure. It is emphasized that SODAR algorithms were developed to determine multiple sound sources in 3D space simultaneously. Therefore, an extension of the present technology to determine the azimuth angle as well as the depth information of a sound source inside a solid enclosure is not an issue