Blizzard -> Tank

Growing up, I thought adults who said “nor’easter” were bad at saying “northeaster”. I was wrong.

Just about all nor’easters—a sort of winter tropical storm, for those not from New England—count as blizzards. But a blizzard is more than a bad snowstorm. It’s a winter storm with high winds and enough snow in the air to keep visibility fairly low for at least three hours. Since light bounces off snowflakes, the more snow in the air, the closer something has to be for most its light to get to you without being bounced all over the place first. Whether that snow came from the clouds or it was picked up from the ground by wind doesn’t come into play in defining the storm.

Snow muffles sounds just as it bounces light, but for a different reason. The coordinated vibration of molecules that forms a sound jiggles water molecules in the snow around and becomes random vibrations (heat). (Also, winter storms tend to have funny temperature distributions that refract sound upwards and away from our ears.) During a blizzard, with howling winds and sound-absorbing snow everywhere, you can only hear really loud stuff from afar—loud stuff like thundersnow.

But sounds dissipate with distance even without snow. Some of the coordinated vibration of a sound wave always turns into heat as sound travels through the air. Higher frequencies are especially susceptible: They start with more rapid vibrations at the outset, so it’s easy for those to get out of sync and become random noise. Also, it takes more energy to sustain more rapid vibrations, so the sound runs out of energy sooner. This means treble dissipates too quickly to be heard from a great distance. The farther you are from something, as a result, the lower-pitch it tends to sound. (Side note: This is also why people who live upstairs from you always tend so sound like they’re stomping and grunting rather than squeaking.) (Second side note: This is tangentially related to something I talked about for the Museum of Science [24:17 in that video is the question and then my answer], where you can tell the angle of a lightning bolt from the sound of the thunder.)

What’s true for natural processes is also true for natural processes that humans have a hand in—there’s nothing to be gained in separating us from nature, after all. So if you’re far from an outdoor concert, you’ll hear more bass drums, low voices, and low guitar chords. And if you’re close to a tank when it fires out of the big barrel, you’ll hear a wide spectrum of noises. But if you’re a ways away, you’ll mainly hear a boom. Hopefully you’re far away in the right direction.

Bike -> Drums

When you learn to ride a bike, adults tell you it’s easier to stay upright the faster you’re moving. I suppose some kids take this as license to rocket downhill, but I was never one of those kids. I always heard it as one of those lies adults tell you. I’ve never liked going particularly fast, and I always balanced just fine.

What I didn’t know at the time is that bicycles can balance even without a rider—and generally the faster they go, the better they balance. Now, there are arguments about exactly why bikes balance, with factors including the conservation of angular momentum, which makes it hard to twist a rotating object (like the wheels); a sort of drag on the front wheel that turns it in the direction the bike tilts, keeping it under the frame; and the distribution of mass in the frame, which forces it to fall toward the front wheel rather than sideways, even when the frame tilts. As interesting as that argument is, it won’t get us to drums. For that, we need the bike to fall.

You realize how heavy bikes are when one falls on your leg, but you also realize how loud they can be. They sure can clang, especially if they land on pavement. That clang comes from vibrations in the metal frame that start as soon as they’re hit by something hard like a rock. The more rapidly the metal vibrates, the higher-pitch the resulting sound. But you can’t easily change how quickly something vibrates by hitting it a little differently, at least not if you abstract away from the real world (as physicists are wont to do). Everything around us has its own resonance frequency—its own natural rate of vibration where it’s easiest for the atoms and molecules in the structure to exchange energy back and forth. Something’s resonance frequency depends on a bunch of stuff, and the actual sound you hear when something’s hit depends on even more stuff. But let’s keep abstracting away from that mess to find a good rule of thumb: If you have a bunch of tubes of the same kind of metal, longer tubes will make lower pitches when struck. The bulk of the frame will have a lower-pitch clang than the adjustable tube that the seat sits atop.

This is also true for the air inside the tube. If you blow over the top of a tube, longer ones will make lower notes than shorter ones. Pipe organs make music this way, as do flame-fueled Rijke tubes.

The source of all these sounds is microscopic vibrations, whether in the air inside the tube or the tube itself. All sounds are produced this way, even without a tube in sight. Sounds come from vibrations. And the rule of thumb holds true more widely, too: More material generally makes lower pitches because it’s hard to get a lot of stuff moving (as long as you keep everything else the same). The less energy it takes to get something moving in the first place, the more of that energy can go into vibrations.

This is why we can hear the difference between kinds of drums—say, between snare and bass. The snare drum is tiny and makes a high-pitched sound; the bass drum is much larger and makes a correspondingly lower-pitch sound. Admittedly, the drum heads might sometimes be made of different material, and also a snare often has stuff at the bottom to shake the sound up a bit, but that’s really neither here nor there.

And that’s the thread of continuity between bicycles and drums.

As a side note, I’m going to kick off this blog by doing one post a day for the first week, then I’ll settle down into the weekly schedule.