Voice-Calling Detection Distance with a Parabolic Microphone in Land Search and Rescue

IntroductionThe ability of a 55.9 cm parabolic microphone to increase detection range in search-and-rescue (SAR) sound sweeps for the purpose of responsive lost-person searching was examined.MethodsFive SAR personnel listened for 3 random words shouted once by persons simulating a responsive lost person at a target loudness of 88 dB (at 1 m) at various distances in coniferous lodgepole pine and deciduous aspen parkland forests. Intelligibility distance (where 50% of the shouted words were understood) and audibility distance (where 50% of the shouted words were audible but not intelligible), along with visual detection range, were determined. The lost person's unaided ear audibility of 5 whistle models and 1 portable train horn, blown at each searcher's parabolic microphone audibility distance, was also determined.ResultsThe parabolic microphone significantly increased both intelligibility and audibility distance by an average factor of 1.44 compared with the unaided ear. Intelligibility distance d_i^pmic with the parabolic microphone was well predicted by the equation <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>d</mml:mi><mml:mi>i</mml:mi><mml:mrow><mml:mspace></mml:mspace><mml:mi>p</mml:mi><mml:mi>m</mml:mi><mml:mi>i</mml:mi><mml:mi>c</mml:mi></mml:mrow></mml:msubsup><mml:mo>=</mml:mo><mml:mn>8805</mml:mn><mml:mspace></mml:mspace><mml:msup><mml:mi>e</mml:mi><mml:mrow><mml:mo>-</mml:mo><mml:mn>0.0978</mml:mn><mml:mspace></mml:mspace><mml:mi>d</mml:mi><mml:msub><mml:mi>B</mml:mi><mml:mrow><mml:mi>a</mml:mi><mml:mi>m</mml:mi><mml:mi>b</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:msup></mml:math>, where dB_amb is ambient dB at the listening location. Only the portable train horn could be heard by all participants with the unaided ear at the audibility distance of the parabolic microphone.ConclusionsThe use of a parabolic microphone significantly increased auditory detection range. When combined with the tested portable train horn, our data suggests that SAR sound sweeps for a responsive subject with a parabolic microphone can expect area coverage rates 44% greater than with the unaided ear and approximately 20 times that of visual searching.