Is human underwater hearing mediated by bone conduction?

In-air and underwater audiograms and directional hearing abilities were measured in humans. The lowest underwater thresholds were 2.8 µW/m or 3.6 mPa at a frequency of 500 Hz. The underwater hearing thresholds were 4-26 dB and 40-62 dB higher than in-air hearing thresholds when measured in intensity and pressure units, respectively. This difference is considerably smaller than what has been reported earlier. At frequencies below 1 kHz, when measured in units of particle velocity, the underwater threshold was much lower than published bone conduction thresholds, suggesting that underwater hearing is not always mediated by bone conduction pathways to the inner ear, as previously thought. We suggest it is the resonance of air in the air-filled middle ear that produces the low underwater thresholds, at least at frequencies below 1 kHz. The ability to determine the direction of a 700 Hz underwater sound source while being blindfolded was extremely poor, with submerged test subjects showing only coarse directional hearing abilities at azimuths of less than 50˚. The physical cues to sound direction are different in air and water, and the poor directional hearing abilities indicate that, in spite of low hearing thresholds, humans have no special adaptations to process directional acoustic cues under water.