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No. 551:
The Sounds of Silence

Today, we use noise to create silence. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

And in the naked light I saw
Ten thousand people, maybe more, ...
People writing songs that voices never shared,
No one dared
Disturb the sounds of silence.

The sound of silence is Paul Simon's wrenching metaphor for our inability to hear one another in the midst of our own noise. Now acousticians are transforming that metaphor. They are -- quite literally -- using noise to create silence.

Sound -- any sound -- is a pattern of density variations that travel like waves through air. If you add two sound waves together, those density variations add together. They can either reinforce or cancel each other.

Suppose we play two tones exactly out of phase. If we can do that, we leave the air absolutely undisturbed. We can use one sound to cancel another identical sound.

That idea is 120 years old. By the 1870s we'd canceled the sound of one tuning fork with another tuning fork. But what about irregular noise? How do you know what sound to play back?

The trick is to anticipate how a noise will sound in the next microsecond. Then you have to respond with an almost instant countersound. Doing that means solving a complex mathematical problem. And you must solve it constantly, with blinding speed. Even then, you can solve it only approximately.

That's called Active Noise Control or ANC. Until we had the newest microchips, ANC was pure science fiction. Now acousticians see it within grasp. They can mount a computer-fed microphone in a fan duct and reduce sound by 70 percent.

Magnetic Resonance Imaging is a wonderful new tool for medical diagnosis. But it sets up a noise that drives patients to distraction. You can already buy ANC to combat that noise.

Someday we'll be able to cope with more complex sound in more complex chambers. ANC inside an airplane, or in a forging shop, is still out of reach. Someday you might buy radio headphones that screen out barking dogs and crowd noise, but not for a while. Yet the potential payoffs are so great. Think how much fuel we'll save if, one day, we replace car mufflers with ANC.

So we are daring to disturb the sounds of silence after all. We're disturbing the noises that transmit nothing. The sad fact is that acousticians are doing only what so many of us do to each other. We silence each other with our own sound.

As we learn to fight noise with noise, we really do plumb Paul Simon's metaphor. And I remind myself that the closest friend is one with whom I can just sit -- in silence.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Alper, J., Antinoise Creates the Sounds of Silence. Science, Vol. 26, April, 1991, pp. 508-509.

Since this episode was first aired, Active Noise Control has become widely available.