A Sonar Experiment to Study Sound Propagation through Flames
Popular version of paper 2pPA14 presented at the 164th ASA Meeting, 2012 in Kansas City, Missouri.
Early in this research, following the automotive parking assist example, we experimented with an ultrasonic device that operated at 40 kHz and generated a narrow beam of sound. Unfortunately, we found that fire strongly distorted the sound beam and prevented it from making it through even small-sized laboratory fires. Experimental evidence and finite-element numerical modeling indicated that it was not the temperature and sound speed contrast of the fire that caused the distortion, as one might expect. Instead it was scattering of sound from the turbulent structure of the fire that caused the distortion. The wavelength of sound at 40 kHz is about 8 mm, which is smaller than the characteristic size of the turbulent structure of the fire, and hence the turbulence can easily scatter sound at that frequency.
Subsequently, we have experimented with a more advanced acoustic source, called a parametric array , that can create a much lower frequency sound in a narrowly focused beam. The lower frequencies (on the order of a few kiloHertz) are not as readily scattered by the turbulent structure of fire, yet the parametric source can still maintain a narrow beam at low frequencies, which greatly increases the spatial resolution possible from a sonar system using the source. In the initial results reported here, we found that acoustic signals from the parametric array can effectively penetrate moderate sized flames, reflect off of wall surfaces on the opposite side of the flames, and return back through the flames to a co-located acoustic receiver (a microphone) positioned near the source. Tracking these echoes, one can determine the distance to the wall and also create an image of the location of wall by scanning the source spatially, as shown in Fig. 2, along with a corresponding TIC image of the same fire.
This work was supported by internal R&D funds at Applied Research Laboratories at The University of Texas at Austin [http://www.arlut.utexas.edu/], a University Affiliated Research Center with broad experience in sonar development and signal processing.
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