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Using smartphones as hydrophones: two experiments in underwater acoustics

During the last years, it has become increasingly clear that smartphones are valuable tools to be used almost everywhere. Until recently, a place that still resisted smartphone onslaught was the aquatic media. However, nowadays, many modern smartphones are waterproof and the performance of their microphones results sufficiently adequate to employ them as hydrophones. This capability gives rise to several interesting applications. Here, we describe two experiments in underwater acoustics which require two smartphones -- at least one should be waterproof. The first experiment consists in a simple time-of-flight measurement of the sound speed in water and the comparison with the corresponding value in air. The second experiment deals with the acoustic location -- or ranging -- of a distant object by comparing the time it takes for the sound to reach the object travelling in two different media (air and water in this case) with known sound speed.