Nearly all sound energy is reflected at the air-water interface

Evidence

The impedance of water is about 3400 times larger than the impedance of air. You may have noticed that if you are underwater, it is very difficult to hear someone above water, even if he is shouting. Using formula 1.3 for the amount of energy reflected, we find that about 99.9% of the sound arriving from the air is reflected from the water surface. Sound launched within water travels quite well; if it reaches the surface, it reflects back down. Notice from formula 1.3 that the percent of energy reflected is the same, no matter which side of the interface the energy is approaching from. — Eric J. Heller, Why You Hear What You Hear- An…, 98

Categorization

• CANDY: A fun elevator fact explaining why you can’t hear people above water while underwater — and the reverse is equally true.
• XREF: Connects to standard room acoustics / loudspeaker design impedance-matching problems.