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)
