How come yelling on a mountain can start an avalanche?
How come yelling on a mountain can start an avalanche? asks reader Paul Grinde.
Scream and bring down a roaring cascade of snow? It seems plausible, especially since a single person skiing in the wrong place can trigger an avalanche above. But researchers say that a human voice doesn’t have the power to move mass quantities of snow.
A powder snow avalanche
According to snow researchers, the idea that avalanches can be triggered by ordinary noise is a (popular) myth. At an International Snow Science conference in Switzerland this year, researchers presented a paper comparing the human voice to aircraft noise, sonic booms, and deliberate explosions.
All sounds produce pressure waves in the air. But even the loudest scream produces a pressure wave just one-tenth the amplitude, or magnitude, of a passing plane. The pressure wave produced by a sonic boom — created by a plane flying faster than the speed of sound — has an amplitude 100 times as large as a scream. A detonation of explosives produces pressure waves more than seven times as strong as the boom’s.
Scientists say that whether a snow mass shifts depends on the pressure wave amplitude. They note that a short-lived pressure wave must be at least as strong as that produced by a sonic boom to trigger an avalanche. So even a low-flying jet — let along a human scream — won’t start an avalanche.
The avalanche myth may have its origins in the idea that a human voice, singing the right note, can cause a glass to shatter. That idea, it turns out, is true.
Resonance is the phenomenon behind glass-breaking. Have you ever swung on a swing, and noticed that if you pump your legs in the right rhythm, the swing will go higher and higher? Your rhythm has exactly matched the natural rhythm, or frequency, of the swing, and the swing swings higher and faster.
Something similar happens when a musical note shatters an empty wine glass. A thin, lead-crystal wineglass has its own natural frequency, which we can hear by lightly tapping the glass with a spoon. A sound of the same frequency will make the glass vibrate. And if the right sound is also loud and long, the glass may shatter.
But while an amplified voice singing the right note was known to break glass, no one had absolute proof that an un-amplified voice would also work. Enter the show “Mythbusters,” which used a crate of wine glasses to demonstrate that the human voice can shatter glass. Watch the “Mythbusters” wine glass episode here. (But don’t try this at home!)
While a wine glass poses the risk of flying shards, resonating bridges can be even more dangerous. In England in 1831, marching cavalry soldiers were crossing a suspension bridge in Manchester. The bridge began to sway, and the troops automatically matched their marching to the motion. Like a swing powered by pumping legs, the bridge’s swinging steadily increased. As the bridge oscillated, a bolt broke, and the soldiers fell into the water below. These days, soldiers are instructed to march out of time with each other when they cross a bridge — just in case.
