May 21, 2008
Sound Waves May Cause ‘Shadow Bands’
A new theory claims that so-called "shadow bands," which sometimes pass across the ground during an eclipse, might be produced by sound pulses.
These bands, which pass across the ground during totality"”when the Moon completely covers the sun, are considered by some to be caused by atmospheric turbulence. However, astrophysicist Dr. Stuart Eves says shadow bands could be caused by something called infrasound.
Before totality, these bands have been observed to pass over the ground in the direction of the eclipse.
After totality, they are seen spreading at an angle to the path of the eclipse.
The most common theory about the bands says that they result from the illumination of the atmosphere by the thin solar crescent a minute or so before and after eclipse totality. Light from a distant point can reach a particular place on the ground by a variety of paths, each one is bent in a different way as it passes through the atmosphere.
When the effects of all the paths taken through the atmosphere are taken together, the result is a ragged banded pattern of light and shade - shadow bands.
Dr. Eves' theory says the process is caused by infrasound, or sound with a frequency too low to be heard by the human ear.
"As the eclipse shadow moves through the atmosphere, the sudden disappearance of the Sun changes the Earth's temperature," Dr Eves, an astrophysicist who works for Surrey Satellite Technology Limited (SSTL), said.
Rapid cooling causes a difference in pressure, and the potential energy associated with this pressure escapes as high-density infrasound, Eves said.
The Moon's shadow is generally supersonic, similar to the sonic boom of a jet breaking the sound barrier. Sound pulses are created continuously along a "shock front" which moves ahead of the eclipse itself.
A pattern of peaks and troughs in the atmosphere causes a change in the speed and direction of light waves, which is responsible for generating the shadow bands on the ground.
"If proven, it would be a something of a revelation that eclipses are a sonic as well as an optical phenomenon," Eves said.
"None of the [existing] theories seem to take account of the fact that shadow bands change direction," he said.
After the eclipse, Dr Eves theorizes that the shadow bands would travel at angles in the same bay that waves diverge behind a ship.
However, Professor Barrie Jones, from the Open University in Milton Keynes, argues that sound traveled too fast to be responsible for the phenomenon.
"I'm not sure how infrasound could generate the bands - it's too fast," he said.
"Infrasonic waves in the atmosphere would move at the speed of sound, which would be something like 400m/s. Shadow bands move at wind speed, so they can be anything from stationary to a few meters per second."
"The [accepted] theory works, there's no need to seek an alternative," said Professor Jones.
Other scientists have proposed that gravitational effects may be responsible for the wild swings of Foucault pendulums during an eclipse.
Dr. Eves said this could be due to infrasound pulses, which cause the ground to vibrate.
Animals have been seen to exhibit unusual behavior. Birds have been observed roosting prematurely and showing apparent signs of distress or alarm.
Birds have auditory ranges that extend well beyond those of humans, and might be affected by low frequency sound pulses.