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In this paper we evaluate the innate ability of mobile phone speakers to produce ultrasound and the possible uses of this ability for accurate indoor positioning. The frequencies in question are a range between 20 and 22 KHz, which is high enough to be inaudible but low enough to be generated by standard sound hardware. A range of tones is generated at different volume settings on several popular modern mobile phones with the aim of finding points of failure. Our results indicate that it is possible to generate the given range of frequencies without significant distortions, provided the signal volume is not excessively high. This is preceded by the discussion of why such ability on off-the-shelf mobile devices is important for Location Based Services (LBS) applications research. Specifically, this ability could be used for indoor sound trilateration positioning. Such an approach is uniquely characterized by the high accuracy inherent to sound trilateration, with little computational burden on the mobile device, and no specialized hardware or audible noise. Combined with a fast internet connection and the sensors present in modern smartphones, such as accelerometers and magnetometers, our approach makes mobile phones a suitable platform for indoor LBS positioning.
Filonenko, V., Cullen, C. & Carswell, J. (2010) Investigating Ultrasonic Positioning on Mobile Phones. International Conference on Indoor Positioning and Indoor Navigation (IPIN), Zürich, Switzerland, September.