Human presence at sea is normally on the surface, and the sounds that we produce within the water are
rarely given much consideration. The air-sea interface creates a substantial sound barrier. Sounds waves in
the water are reduced in intensity by more than a factor of a thousand when crossing the air-sea boundary.
This means that we are effectively insulated from the noise produced by rotating propellers that drive our
ships or by high-intensity sonars used to measure the depth or probe the interior of the sea. The conflict
between human and marine mammal use of the sea is fundamentally a consequence of the fact that we do
not inhabit the same sound environment. Marine mammals live with their ears in the water, and we live,
even at sea, with our ears in the air.