The Hush of Forbidden Bands

Those tall concrete walls that line many a stretch of urban highway don't do a very good job of confining noise to the roads, and they're ugly to boot. Now physicists have created a much more efficient and elegant device for blocking sounds: An array of wooden or steel cylinders dangling from a frame. The structure, which looks like a giant wind chime without the chime, can block sound waves of certain frequencies from passing through. The mass and density arrangement of the cylinders creates a "band gap" in the structure--a zone of forbidden energies that are completely inaccessible to intruding waves from the outside, say researchers in the 15 June issue of Physical Review Letters.

Forbidden bands are common, for instance, in semiconductors, in which ions in the crystal create energy states that electrons cannot occupy. In the late 1980s, physicists showed that some materials chalk out similar regions for light--making a material such as gallium arsenide opaque to light of only certain frequencies. Jose Sánchez-Dehesa of the Universidad Autonoma de Madrid and his colleagues set out to see if this so-called photonic band-gap theory held up in acoustic waves, too.

The scientists found their inspiration in a Madrid art museum: A sculpture composed of vertical bars of varying lengths that band-gap theory suggested would be the right shape and size for blocking sound. Preliminary tests at the museum were encouraging, so they built their own sculptures in the lab, arranging up to 500 wooden or steel rods, about a meter long, as a cube or a triangle. Next they placed a loudspeaker on one side of an arrangement and a receiver on the other to measure how much sound passed through. Just as theory predicted, the cubic sculpture muffled sounds around 1500 Hertz, by as much as 20 decibels. Surprisingly, however, the triangular sculpture blocked a range of frequencies that, on paper, should have freely passed through. The team called this range of frequencies "deaf bands," which have never been observed before with sound waves.

Similarly shaped sculptures lining the sides of highways could potentially squelch the roar of traffic more effectively than slabs of concrete, which only dampen sound waves. But for now, physicists are just excited about breaking new ground. "This is the first time someone has seen a band gap for acoustic waves," says Mihail Sigalas, a physicist at the Ames Laboratory in Iowa. He is currently working on calculations that describe what happens after removing a few of the cylinders from the sculpture. In that case, the sculpture could be used as a filter, he says, allowing only one frequency among the forbidden ones to pass through.