Sound and Changes in Hearing
Like the eye, the human ear is adapted for hearing in the air environment, and can pick up sounds between the frequencies of 20 and 20 000 cycles.
In the air medium, the speed of sound propagation is 340m/sec. at 8ºC, and low frequency sounds (bass sounds) reach longer distances than high frequency sounds (high-frequency sounds).
Thanks to the distance of the two ears (about 20cm), the temporal gap with which the sound is detected by each of them is transmitted to the brain, which allows us to know immediately the direction of its origin.
In water , at the same temperature of 8ºC, the speed of sound propagation is 1435m/sec, more than four times the propagation speed in the air. In waters with higher temperature, this speed is even higher, reaching 1500m/sec. In these conditions, the time lag with which sound is detected by each ear is too short and the brain has great difficulty in assessing the direction of the sound origin.
That’s why the sound seems to reach the diver from all directions, particularly if it’s low frequency, such as the bass sounds produced by the propeller of a slightly rotating motor, which begin to be heard at great distance. In the case of bass sounds, at best, the notion of approximation or distance from the sound source can be found, due to the increase or decrease in sound intensity, depending on the situation.
If the sound has a very high frequency (sounds produced by the propeller of a very rotating motor) it is already possible to reference the direction of the sound origin, but in this case the sound begins to be heard when it is already much closer, with the inconvenience of being produced by a fast boat.
In view of this situation the diver must take special care when he hears a sound during the ascent, as he may risk being rammed by an approaching vessel, especially if it is a fast vessel.