I don’t think we need to call on a surfer/physicist to solve this little mystery. In fact, there’s another important factor here that shapes the sounds that reach our ears: the properties of the air itself in which the sound travels. Variations in air temperature, humidity, and wind speed/direction all affect how sound travels, causing all kinds of interesting effects.

Consider the temperature first. Sound travels faster in warm air, and slower in cooler air (other factors being equal). Earth scientists have long known that, in the lower atmosphere, air temperature tends to decrease with altitude (no wonder we like to escape to the cool of the mountains on hot summer days). Near the surface, the temperature profile can change dramatically, as the sun heats the surface by day, the surface cools by night, and weather systems come and go. If the temperature profile were perfectly flat (that is, if it were the same temp at the surface as at, say, 10 meters up), then the surf sounds would travel straight toward your ears (plus lots of reflections from the ground). But that’s a very rare case, since the atmosphere is always in motion. On mornings when temperatures decrease with altitude, the surf sounds will tend to refract upward, where the speed of sound is slower. Much of the surf sound thus gets deflected up and away from your ears, making it appear softer. On mornings, however, when the temperature increases with altitude (what meteorologists call a “temperature inversion”), then the surf sounds will tend to be deflected downward: you’ll hear sounds that would previously have soared high over your head, plus all the reflections from the ground and the sea surface. (It’s just as if the sound were trapped in a tunnel between the waves and your ears — very little sound gets lost up, up and away.) The result is that the surf would sound louder.

The wind gradient (how the wind speed changes with altitude) also affects the loudness of the sound, as does the humidity gradient. But I think this is enough for now.

These atmospheric-acoustic affects can have profound consequences on our lives. Some research suggests that they may even have contributed to the outcomes of wars — see “Acoustic Shadows in the Civil War”.

For a little more detail about how temperature gradients affect sound propagation in the air, see Dr. Dan Russell’s “Refraction of sound waves”. It includes some nice animations.

Liked your recording and comments on source for the sound components.

Bob Hays, PhD, Biology (ret).