No noise is good news.

You're cruising down the street and your favorite song comes on the radio. But just then, traffic horns blare, a helicopter hovers overhead, and some guy with a jackhammer is tearing up the road beside you. "Argh!" you shout, adding to the clamor. "There's just too much noise to hear anything here!"

Fortunately, there may soon be a solution: more noise! Though it sounds crazy, if you add the right noise to another noise, the two noises can wipe each other out.

Here's how it works. Noise is sound, of course, and travels through the air as a wave of vibrations. And like other sound waves, noise sound waves have a certain amplitude. The amplitude, or height of the waveform, determines the intensity, or loudness, of the sound--just as the wavelength determines the frequency, or pitch.

Now suppose two sound waves are heading into your ear, and each sound wave has the same amplitude and frequency. But let's say one wave's peaks are arriving at your eardrum at the same time as the other wave's valleys. When that happens, the peaks and valleys cancel each other out. What you get is level ground--no sound (see diagram, below). When one sound wave is noise, the canceling sound wave is called anti-noise.

Engineers are now designing equipment that uses antinoise to silence loud machinery. The basic idea is this: A microphone takes a sample, "a kind of snapshot of the noise," explains Steve Goodrich, an engineer with Active Noise and Vibration Technologies. Then a computer chip determines the amplitude and frequency of the noise, and tells a small speaker to broadcast a matching anti-noise signal.

Factory workers and pilots are already wearing headsets that flood their ears with anti-noise to cancel the sounds of machinery and propellers. Goodrich says that anti-noise devices work best with steady, regular noise like that from motors, fans, or generators. Guess that means you can't use it to silence car horns or your neighbor's opera music. But who knows what the future will bring?