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Tsunamis of Sound

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You don't see sound coming at you, but if you could it would take on all sorts of shapes. A sonic boom would be a slab in the sky approaching you at 770 miles per hour. A lightening bolt would launch jagged waves as scary as its thunder will sound: a ragged cylinder filled with chaotic disturbances, stretching from ground to sky, expanding at 770 miles per hour, silent until it overtakes you.

The sound from a distant rock concert would launch skyward and also toward you at 770 miles per hour, intermingling pulsations of color (I imagine short wave lengths as blue, long wavelength as red) passing harmlessly by. Some of the waves would be only a foot or so from crest to crest. Others, perhaps from the bass drummer, would be more like ocean waves, 50 feet from crest to crest, low undulations bringing sound at the bottom of your hearing range.
If you could see sound traveling, you'd soon notice other waves coming at you with even longer wavelengths: like ocean swell 100 feet or 1000 feet from crest to crest, or like tsunami waves, miles from crest to crest. These are the "tsunamis of sound," hugely long waves. These pass by you silently at 770 miles per hour.

It would be a thing of beauty and wonderment to be able to see such choreography of wave propagation. You couldn't take your eyes off the sky.

The long wavelength tsunamis of the sky -- so-called "infrasound" waves that we can't hear -- what causes them? A big one went by you recently, silently of course. You know about the meteor that caused it, the one that blew up over Russia. It caused the largest such shock anywhere in the world in over a century. You likely weren't anywhere near, but the infrasound it generated traveled as a pulse around the world, right through your living room, even your body. Infrasound stations as far away as Antarctica picked up the blast. (Listen to the meteor's infrasound, sped up so you can hear it).

If you were close to the event, the shock was of course more powerful and it carried more than infrasound. Many people were struck by flying glass and felt a strong pulse as it passed by. The total energy of the explosion was greater than the recent nuclear test by North Korea, which by the way, like all nuclear tests, was picked up on bomb sensing infrasound sensors thousands of miles away.

Not much earlier, on April 12, 2012, the Sutter's Mill meteorite in the Sierra Nevada broke the world speed record for meteors, clocked in at 60,000 miles per hour. Still, the shockwave it caused reached the ground traveling at a leisurely 770 miles per hour. The energy released when it blew up, the equivalent to one kiloton of TNT, was estimated by the infrasound it gave off.

Sonic boom shockwaves don't travel supersonically, except very close to their source. You can see the shockwave from an atomic blast (which was initially traveling VERY fast near the fireball) reaching the camera position a few miles away traveling at the normal speed of sound, 770 miles per hour, visible because of the dust kicked up on the ground.

Even though all sound waves travel at the same speed through the air, infrasound takes the prize for sheer stamina, loosing less energy per mile traveled than higher frequency, shorter wavelengths do. Eventually all that remains of a big explosion is infrasound, traveling for hours and sometimes days on end through the atmosphere.

When Krakatoa blew up in 1883 in the Dutch East Indies, windows broke 15 minutes later some 200 miles away. Almost four hours later and more than 2500 miles away, a loud boom was heard on Rodriguez Island, across the Indian Ocean. A few more hours and only the infrasound remained, but that kept going for seven more days, traveling over five times around the globe. It was so "loud" (even though inaudible since we are not sensitive to it) that ordinary barometers recorded the pulses at weather stations. The pulses kept coming back to the same stations after they'd traveled around the globe. Krakatoa is the largest explosion on earth in recorded history.

Explosions aren't the only source of infrasound. Ocean waves crashing on shore, wind oscillating in the lee of buildings or turbulence near mountains in a breeze produce infrasound, as do thunderstorms, avalanches, waterfalls, tornadoes, and earthquakes. Infrasound microphones can detect tornadoes forming minutes before they touch down; infrasound detectors now locate avalanches in Colorado remotely.

Elephants produce and hear infrasound, using it for communication, as was discovered by Katy Payne in 1998 observing elephants in the Washington Park Zoo, Portland, Oregon. An elephant's call is audible to humans for about 800 meters, but the infrasonic component travels 10 kilometers or more where it can be picked up by another elephant (they can hear well below our threshold of 20 Hz). Clearly, elephants are taking advantage of infrasound's long range.

"Microbaroms," known also as the "voice of the sea," are caused by waves colliding in the open ocean, head-on collisions typically found at the edge of storms. The collisions cause the sea surface to rise and fall like a giant loudspeaker, launching infrasound, which can then be heard round the world by sensitive infrasound microphones. Their frequency is about 0.14 Hz, or about 1 oscillation every 7 seconds.

The sounds can be heard for hundreds of miles, that is if you are a pigeon. Cornell's Jonathon Hagstrum may have solved the longstanding mystery of how homing pigeons find home, even if driven hundreds of miles in a random direction in a dark cage. Homing pigeons can hear down to 0.1 Hz (one vibration every ten seconds!) or even lower, and may listen to the "voice of the sea" or other infrasound cues hundreds of miles away.

Infrasound has many other facets. It is plausible that infrasound is generated before major fault lines shift, spawning damaging earthquakes. This could explain why some animals seem to know that a quake is imminent. Infrasound has been said to induce feelings of awe or fear in humans, to make people uneasy, as if odd or supernatural events are taking place. Infrasound can apparently cause changes of blood pressure and heart rate. These are not widely accepted claims yet; more (and carefully controlled) work is needed.

It's hard to focus attention on something that can't be seen, can't be heard, and does no damage, even if it is ubiquitous and important. So next time you look up at the sky, imagine waves and spherical shells of color traveling faster than a passenger jet crisscrossing each other, the long wavelength waves mixed in with the short, piling together like a chaotic three-dimensional sea, passing over you, beside you, and right through you.