Why do ocean waves sound like this from a distance?

August 2, 2022 11:08 AM

I was staying in an apartment a couple of blocks from the ocean recently and noticed that when the city got quiet late at night I could hear the waves. But instead of the rhythmic sound at the beach, it was a steady roar, like white noise. What makes the sound steady farther away?
posted by pinochiette to Science & Nature (6 answers total)

Best answer: I think it's because there's not just one big wave at a time, but several up and down a stretch of coast, that if you're far enough away it blurs into a single sound. Like when you're in a sports stadium you can clearly hear the individual voices of fans around you, but from down on the field it just sounds like a roar of the crowd.
posted by The otter lady at 11:19 AM on August 2, 2022 [11 favorites]

Best answer: Two reasons that work in the same way. The first reason is that wave crests are fairly linear (linear here meaning specifically a straight line) but a coastline might not be linear over the same scale.

Second reason is that even if a coastline is linear, the line formed by a wave crest will rarely be perfectly parallel with the shoreline.

Both of these reasons mean that only one part of a wave crest will be crashing into a stretch of shore at any moment, but that the crashing wave crest will then travel down the beach. And as that wave crest is finished crashing, the next wave crest is already starting to crash farther up the beach. Once you are far enough from the beach, it all blends together.
posted by cubeb at 11:35 AM on August 2, 2022 [2 favorites]

Best answer: If you imagine the coast is straight line and the waves are coming straight on to the beach, then the wave would reach the coast all at once and you'd hear a rhythmic sound. But if the waves are coming at any kind of angle to the beach, then each wave will hit the coast at different times because they have to travel a little further or a little less. For any kind of significant angle when you're far enough away that you can hear the waves hit along 1000m of beach or so, the first wave hitting the furthest bit of beach is going overlap with the next wave hitting the closest bit of beach, so the waves are essentially constantly hitting the beach.

In the real world, the coast is much more complex, waves are uneven and can reflect, sound is blocked and reflected in some directions and it all just averages out to more or less constant sound.
posted by ssg at 11:35 AM on August 2, 2022 [4 favorites]

Best answer: The sound is reflecting off various surfaces (buildings etc.) on the way to you which muddles it.
posted by kindall at 12:51 PM on August 2, 2022 [4 favorites]

Best answer: If I'm understanding everyone's response correctly, up close the echo effect is very minor and your mind focuses on the sound of individual waves nearby, but as you move away from the coast, the massing of multiple concurrent waves, plus the decay of previous ones as the sound travels to you turns it into a roar.

I think you could study this effect by comparing on the beach vs across the street vs a block away.
posted by The_Vegetables at 12:55 PM on August 2, 2022

Best answer: Sound varies by the inverse square law: double the distance and the sound decreases by 4; double the distance again and the sound decreases by 4 again, for 16x decrease.

Have you ever been walking toward a crowd of people talking? Say, a sporting event, a conference during a break, a party, an expo? When you're a block away, all their voices are approx. the same distance away. When you get to the expo, and talk to one person, that voice dominates because it's closest to you. Same with each wave crashing.
posted by at at 1:40 PM on August 2, 2022 [5 favorites]