Background: Inspired by the development of a swimmer approaching detection system which emitted underwater alert sound (Ohgi, et al., BMS2023), we began conducting research on the physiology of swimmers` underwater hearing. Aim: This study aimed to investigate the auditory characteristics of swimmers underwater in different conditions. Methods: Two healthy collegiate competitive swimmers with normal hearing wore snorkels and performed the following tasks in the 25m indoor pool: a stationary seated position with their heads submerged, tethered and non-tethered front crawl strokes. The experiment was conducted with the swimmers not wearing swimming caps over their ears. We conducted an octave band test (128Hz to 8 kHz) underwater to determine the sound levels (around 70 to 180 dB (0dB re. 1*Pa/*Hz) at which each frequency could be heard. The underwater speaker was settled up positioned at 1m depth. As the volume was gradually increased and decreased, the swimmers reacted their perception of each pure tone by pressing a waterproof contact switch. As for the tethered and non-tethered swimming, the swimmers swam moderate tempo and speed with crawl stroke. Results: The study showed that the frequencies most easily heard by swimmers while sitting still shift toward higher frequencies when they switch to tethered front crawl stroke. Furthermore, it was found that during non-tethered front crawl stroke, sounds emitted directly from behind the swimmer could not be heard even at a distance of about 5 meters, whereas sounds emitted from the front can be heard even at a distance of 25 meters. Our non-tethered front crawl stroke was still preliminary experiment, so that we could not validate statistical differences between subjects. However, our results demonstrated that underwater hearing is strongly influenced by the relationship between the sound source location and the swimmer`s direction of travel. Conclusions: We conducted underwater octave band tests to assess swimmers` hearing underwater while both stationery, tethered and non-tethered front crawl swimming. The results showed a tendency for the lower limit of audibility to shift toward higher frequencies when transitioning to tethered swimming. We also confirmed that underwater hearing exhibits anisotropy during front crawl swimming.
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