Surf on Singing Beach 
One cold and overcast November morning, I took my microphones out to Singing Beach, in Manchester, Massachusetts, hoping to record its famous singing sands. Unfortunately, it had rained a few hours before I arrived, leaving the sand too damp to sing.
But, as luck would have it, there was a heavy ocean swell that day: 4- and 5-foot breakers were rolling onto the beach, filling the air with a marvelous and surprisingly steady roar of sound. I stood on shore in dry sand, pointing my shotgun mic towards the area some 25 yards offshore where the waves were breaking.
{the red cable snaking out into the surf was for hydrophones used in a different recording}
Listen to the magnificent slowly rising pitch of the “noise” that follows each breaker’s crash, as the tumbling water rolls ashore, breaking into ever-smaller wavelets, ripples, and popping bubbles of foam. It’s a subtle sound, easily missed during a brisk walk along the beach. But stand still, close your eyes, and face out towards the surf and let your ears alight on the enveloping field of sound that washes over you, and you will hear it, too.
It’s a gorgeously rich natural sound, whose physical origin is surprisingly complex (see the Tech Note, below). For us, standing alone on the keen edge of the shore, it’s enough to turn our eyes and ears oceanward, as each wave announces the end of its journey that began days before, unseen, somewhere in the mid-Atlantic. See the falling, curling wall; feel the thump underfoot and the spitting spray; hear the crash of sound and the rising surge of pitch as foam cascades up the shore: In a thousand voices the wave shouts out its name one final time: I was here! Hear me! Hear me!
Singing Beach lived up to its name today.
Technical note
Here’s how I begin to make sense of the physics of this sound. Upon breaking, a wave disintegrates into a froth of millions of air bubbles trapped in seawater. Within a fraction of second each bubble ruptures, sending out a broad-spectrum pulse of acoustic energy into the air. But upon breaking in this turbulent foam, each bubble in turn spawns a host of smaller ones, which likewise burst into ever-smaller bubbles. So the average size of bubbles tends to decrease as the wave-foam rolls ashore. Now, the smaller bubbles tend to generate more energy at higher frequencies (higher pitch) than do the larger ones, in the same way that a short violin string vibrates at a higher pitch than a long one (all other conditions being equal). The result is a delicious broadband whooosh of sound whose predominant frequency slides upward as the foam comes ashore.
But that’s not all. Because the noise source (the foam) is distributed over a wide area (the foam extends from the wet sand at your feet all the way out to where the waves first break, as well as across the width of the entire beach), each little bubble-pulse of sound arrives at your ears at a slightly different time, resulting in a wildly complex pattern of constructive and destructive interference around your ears. Moreover, some of the foam-sound travels directly to your ears, while some is reflected off the wet sand. This travel-time difference produces a “comb” filter effect, accentuating some frequencies at the expense of others. As the foam moves ashore, this time-delay decreases, causing the overall pitch to slide upward.
(You can hear comb-filtering at work the next time you’re outdoors as a plane flies overhead. If you lean your head down to the ground (or simply squat slightly) you should hear the average pitch of the plane’s noise rise as your ears moves closer to the ground. By moving your ears closer to the ground, you decrease the time delay between the sound coming directly from the plane and that reflected off the ground.)
Finally, remember that all these minute sound sources are in motion relative to you, the listener. At first, when the wave breaks, the bubbles are moving towards you quite rapidly. But as the water rolls up the shore, its energy dissipates and its (and the bubbles’) forward movement slows. In the same way that the pitch of a train whistle decreases as the train flies past, we’d expect the pitch of the foam to decrease as the waves’ forward movement slows. Although this Doppler effect should lower the pitch, the relative velocity of the foam is so low that the effect is probably negligible. So of the two competing effects — comb-filter and Doppler — the former wins: the pitch rises.
P.S. The moving pitches in this recording are 100% natural. No variable bandpass filters were used. It’s the real deal.
I live near a beach in Wanganui, New Zealand. I’m usually up before 5:00 AM when the city is very quiet. I’ve noticed on windless days that there are large differences in how loud the surf sounds. With a bit of trouble I can look at a patch of surf just beyond the beach itself. It isn’t obvious that the sound correlates with the apparent height of the waves. I’ve wondered how the noise level is driven by such factors as wave direction, height, and tide level, but have no way at the moment to quantify any of this. I expect this has been well worked out–probably years ago by a physics student who surfs–but can’t find anything on the web. Are you aware of anyone who’s worked out the physics?
Liked your recording and comments on source for the sound components.
Bob Hays, PhD, Biology (ret).
@Bob: Great question! I noticed this same phenomenon some years ago when I lived in the woods, about 2 km from a big highway. On some mornings the traffic noise was terribly loud, as if the road were right in my backyard. On other mornings it was eerily quiet, as if someone had moved the road a few km farther away overnight. It was very strange.
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.