How can humans, in air, hear sound generated underwater (and can goldfish hear their owners talking)?

Abstract

The air/water interface at the top of a body of water is often treated from below as a pressure release boundary, which it closely matches. The small discrepancy in that match, however, is enough to enable humans in air to hear sounds generated underwater, which would not be possible across a pressure release boundary. A discussion of this phenomenon, designed for teaching purposes and using no more acoustics than would be contained in a first-year undergraduate syllabus in acoustics, leads to a discussion of whether goldfish can hear their owners speaking. The analysis is then used to illustrate the care needed when comparing sound levels in air and water, a process which continues to lead to erroneous statements in the media and some academic articles.