In the quest to muffle the mounting problem of noise pollution, engineers have come up with everything from better mufflers for lawn mowers to rubber surfaces for roads (the latter to reduce the din of highway traffic).
But, like trying to bottle a frog, the problem is tricky. Most of these passive methods help reduce, but do not eliminate, annoying noises. Now, however , a number of researchers are working on an idea that, in some applications, could virtually silence noise problems altogether. Their method: Fight noise with noise.
The idea of ''noise-wave cancellation'' has been around for almost 50 years. But scientists have only recently been tapping the potential of this still-embryonic technology as a result of advances in acoustic research and the advent of better microprocessors and sensors, which are used to help create the ''anti-noise'' waves.
The concept is simple enough. Two sound waves of equal and opposite intensity , when bumped together, will cancel each other out. That's the theory, at least. In practice it is a much tougher matter. Because sound waves travel in the three dimensions of space, and at different speeds, it's tough to produce opposite sound pressures that will cancel out all the noise in the area.
The whine from an industrial turbine, for instance, radiates off the machine in all directions. This means the motor would have to be studded with speakers, like cloves in a ham, to blot out the machine chatter.
This is where the computers and sensors come in. They measure the sounds coming off the motor and reproduce a counter concerto, piped through the speakers, to silence the noise. In the lab, British researchers have quieted a room where a turbine engine was whirring.
Noise is no small concern. Around many airports it is as testy an issue as a tax increase. Miles of sound barriers line traffic-choked highways across the country. In the workplace, noise is believed to cost industry millions of dollars each year in worker absenteeism.
Sound-wave cancellation will probably be used only to muffle one small part of these nuisances. Because the technology is complex and costly, it works best on sounds from ''concentrated'' noise sources - where the racket ricochets out from a single area, like an exhaust pipe or duct, and is thus easier to counteract. Antinoise piped into an air-conditioning shaft, for instance, could cut the groan of the system throughout a building.
''The technology is just starting to become feasible on a broad scale,'' says Richard Hayden, an engineer with Bolt Beranek & Newman Inc., a Cambridge, Mass., consulting firm.
Best bets for its use will be to combat low-frequency sounds made by heavy apparatus such as the whine of engines inside jets, the moan of heating and ventilating systems, the roar of industrial furnaces and power plant turbines, the throb of diesel trucks and locomotives. These are often the hardest to muzzle with conventional devices like mufflers and insulated barriers.
Still, many acoustics experts, such as Richard Lyon, a mechanical engineering professor at the Massachusetts Institute of Technology, contend the technology will find only limited use in the future. He thinks the best way to stifle sound is at its source by redesigning engines and machines to run more quietly to begin with. Automatic weigh-in for trucks
It looks like just a tar patch on the road, but overloaded trucks driving over it may hit a pothole of problems.
The device: an ''electronic mat'' capable of weighing trucks at speeds of up to 65 m.p.h. The sensor-studded mat, developed by a British company, is being tested in Arizona. If it's successful, it may be used at checkpoints throughout the US to help nab truckers violating weight regulations - and thus to help cut down on road wear.
Linked by wires to a portable computer, the rubber mat is stretched across the highway. When a rig zips over it, the $30,000 to $40,000 system measures its weight, speed, and direction. Few other low-cost, mobile systems exist to weigh vehicles at high speeds, says state transportation planner Louis Schmitt.