Not even laser imaging can tell you as much about your material surroundings as rain can.
by Jay Richardson
Sunday 10th April 2022
Most of the time, when you’re listening to something, it’s making sound by itself—a car engine, say, or a blender, or a barbershop quartet. Inanimate objects do not produce sound of their own accord. Moving objects almost always do—cars that travel too quietly to be safe for pedestrians even have to make their own sounds artificially at low speeds—but if they’re not moving, you usually can’t hear them. All of that changes, though, when it starts to rain.
The way that objects respond to rain reveals a huge amount of information about them. Sure, traditional imaging techniques like light detection and ranging (LIDAR) can map 3D space with astonishing accuracy—and sonar maps places you can’t hear when it’s raining, like the sea bed. Sonar even reveals surface characteristics like hardness, unlike LIDAR, although it’s a little less accurate. But they output 3D maps, so they’re fundamentally visual imaging techniques.
Rain, however, matches sonar for the human ear: it’s as if millions of tiny drumsticks are tapping on every surface open to the sky, the way that ASMRtists tap on objects to see if they make “good sounds”. I suppose rain resembles millions of tiny ASMRtists falling from the sky and tapping on everything they can find, although that image is a little more disturbing. Anyway, rain doesn’t make sound on its own: it ‘sounds out’ everything it falls on.
Rain reveals objects almost as much as touch can, if you have the patience to listen closely enough. It hints at what they’re made of—as in this recording, with the microphone under a bucket, the raindrops making dull thuds as they land because the thin plastic flexes a tiny bit on impact.
Rain also reveals an object’s angle and shape. In the next recording, it lands on a tarp on the ground. Because of the tarp’s flat and irregular shape, it’s already slightly wet as the rain lands, and so you get the faintly metallic, high-pitched sound of water landing in water, together with the occasional light tinkle as it runs chaotically over the surface or drips onto the ground. Again, the tarp flexes slightly on impact, dulling the sound slightly and lowering its pitch.
You can also tell if you’re in an open space by listening to the rain, because of the way it mixes and arrives at your ears from a long way off, as a hazy, white-noise, broadband hiss. This sounds very distinct from the characterful and variegated raindrops that you’ll hear landing close at hand. Often you can hear both in the same recording, like a landscape painting with perspective.
Of course, you’d only ever hear that sound if you went out in the open: otherwise, you’d get the much cosier sound of rain landing on a surface. Sound always embeds you at a particular listening point—in this case, under shelter. Essentially, rain sounds can tell you whether or not there’s something nearby protecting you from the rain.
Evidently, rain comes in many levels of intensity, raindrop sizes and so on, so the sound depends a little on the rain itself. Further, a lot of surfaces sound quite different when they’re wet, so rain introduces a kind of observation bias of its own—if you’ve ever been caught out in a sudden downpour, you’ll remember the distinctive sound it makes falling on dry surfaces for the first few seconds. But still, in any kind of rain, the actual sound source is the object on which the rain falls.
Rain sounds are more than pure background noise, independent of what the rain falls on and the listener’s position. More importantly, rainfall conveys sonic information that you can’t get visually—like the hardness of a pavement, the water saturation of a field, or the hollowness of a window pane and whether it’s made of glass or plastic. It truly sounds out the world.