vibration, acousitcs

1. Noise-cancelling headphones
Two kinds of noise reduction
There are two ways to reduce the noise in your headphones, one simple and one complex.
Passive noise reduction (noise isolation)
The simplest kind is called passive noise reduction or noise isolation. The headphones are
designed so the earpieces fit snugly into your ears. No sound can escape to bother the people
around you and no background noise can get in either. The Etymotic headphones shown in our
top picture work this way. They have earbuds with large pieces of soft foam built around them,
much like foam earplugs. You wear them by squeezing the foam so it makes a perfect seal with
your ear canal. They also come with plastic reusable earpieces a bit like the ear plugs you can
use for swimming.
Active noise reduction
A much more advanced way of getting rid of the noise is called active noise reduction, and it's
used in the sophisticated noise-cancelling headphones that pilots use. Headphones like this have
a small microphone built into their case. The microphone constantly samples the background
noise and feeds it to an electronic circuit inside the headphone case. The circuit inverts (reverses)
the noise and plays it into the loudspeaker that covers your ear. The idea is that the noise you
would normally hear is canceled out by the inverted noise—so all that's left (and all you hear) is
near-silence or the music you want to listen to. Headphones that work in this way include the
Bose QuietComfort®, which uses a system called Acoustic Noise Cancelling®.
How active noise reduction works
Suppose you have the noise of a pneumatic drill (jackhammer) driving you mad. You put on
your noise-cancelling headphones, switch them on, and the drilling noise virtually disappear.
How does that work? We've already seen that the headphones superimpose a reversed version of
the drilling noise on top of the original noise, but why doesn't that simply make the noise twice
as loud?
Sound is energy traveling through the air in waves. Sound waves don't look like the waves on the
sea—indeed, you can't see them at all. If you could see sound traveling, you'd see it squeezing air

2. molecules together in some places and stretching them out in others. In other words, sound
travels by making the air pressure change. Now suppose there's a sound wave traveling between
a pneumatic drill and your ear. At any given moment, the air between the drill and your ear has
areas where the sound is compressed (compressions) and areas where's it's stretched out
(rarefactions). Suppose you could exactly reverse the sound made by the drill and superimpose it
on top. Now the original compressions would be replaced by rarefactions and vice versa. Two
waves that are precisely reversed in this way are said to be in antiphase. Adding an original
sound and the same sound in antiphase would, in theory, make the two sounds completely
cancel each other out—leaving nothing but silence.
How active noise reduction works
Suppose you have the noise of a pneumatic drill (jackhammer) driving you mad. You put on
your noise-cancelling headphones, switch them on, and the drilling noise virtually disappear.
How does that work? We've already seen that the headphones superimpose a reversed version of
the drilling noise on top of the original noise, but why doesn't that simply make the noise twice
as loud?
Sound is energy traveling through the air in waves. Sound waves don't look like the waves on the
sea—indeed, you can't see them at all. If you could see sound traveling, you'd see it squeezing air
molecules together in some places and stretching them out in others. In other words, sound
travels by making the air pressure change. Now suppose there's a sound wave traveling between
a pneumatic drill and your ear. At any given moment, the air between the drill and your ear has
areas where the sound is compressed (compressions) and areas where's it's stretched out
(rarefactions). Suppose you could exactly reverse the sound made by the drill and superimpose it
on top. Now the original compressions would be replaced by rarefactions and vice versa. Two
waves that are precisely reversed in this way are said to be in antiphase. Adding an original
sound and the same sound in antiphase would, in theory, make the two sounds completely cancel
each other out—leaving nothing but silence!
hat are the best noise-cancelling headphones?
As you might expect, it's a matter of preference. Passive, noise-isolating headphones tend to be
less expensive than active ones, though high-end headphones like those from Etymotic, which
have very high-quality audio performance, are still expensive (mine cost me something like $200
or £100 a few years ago, though the price has now dropped). The best thing you can do is try out
different headphones and see what suits you. Remember that active noise-cancelling headphones
are designed to reduce predictable, steady noises like airplane engine hum, not complex varying
sounds like voices, so they're not so good for cutting out the sound of people's inane chatter. If
that's the noise that's bothering you, you need a different solution...
How do you get rid of noise you can't cancel?

3. Students trying to revise while other people play music often fret about getting peace and quiet.
Here's my foolproof solution to noisy neighbors, mad parties, construction noise, and other
distractions that stop you working. If you're bothered by people's conversations or music, and
earplugs or noise-cancelling headphones don't help you, a really effective solution is to record
yourself an MP3 of white noise (steady noise like you'd hear from the wind or the sea) or pink
noise (a deeper version of white noise, like an airplane engine) and put that on your music
player. You can find plenty of samples on the Internet. Simply play the noise in your ears at
reasonable volume and it should cancel out most things. The ultimate solution I've found is to put
foam earplugs into your ears, put large headphones on top, then play the white or pink noise as
well. The combination of earplugs, headphones, and white/pink noise will cancel out virtually
any background noise without damaging your hearing. A pretty extreme solution, but it really
does work!
Photo: Earplugs like this are great for blocking out most unwanted noise, but the ultimate
solution is to wear headphones on top and play white or pink noise through them. The top plugs
are disposable soft foam ones and can be reused a few times before you throw them away. The
bottom ones are a heavier duty
Who invented active noise reduction?
Many people suppose that Bose, which sells the best-known brand of noise-cancelling
headphones, invented the technology—and did so relatively recently. In fact, as Professor Colin
Hansen (of the Department of Mechanical Engineering, University of Adelaide) points out in an
excellent introductory book on the subject, the technology is much older. Hansen traces it back
to experiments with telephones in 1878, and notes that the first patents were issued (separately)
to Romanian aircraft engineer Henri Coandă in 1932 and German physicist Paul Lueg the
following year (see his US patent number 2043416: Process of Silencing Sound of January 27,
1933, patented in the United States in 1936). Both men developed systems for cancelling out
sound waves by adding other waves in antiphase. Many others built on this work, including
synthesizer pioneer Harry F. Olson. Until the early 1990s, active noise reduction was little more
than a "laboratory party trick"; then, as Hansen notes, the science swiftly became a practical
technology, with a growing number of everyday commercial applications—the best known of
which are noise-cancelling headphones.

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