Why do people on trains and buses have
to play their music so loud?
Look at it from their point of view: they're trying to enjoy a nice bit
of Beethoven or Schubert (as if!), but all they can hear is the
deafening throb of the engine—so they
turn the volume up as far as it
will go. But don't worry, because there is a solution to this problem
for both of you: noise-cancelling headphones. These amazing gadgets
block out the background noise, allowing people to listen to their
music without unwanted distractions. Since there is no competition
between music and noise, they can set their MP3 players to a much lower
volume, which is better for the people around them as well. Let's take a
look at how noise-cancelling headphones tell the difference between the
sounds you want to hear—and the ones you don't.
If you want to find out about how ordinary headphones work first, take a look at our separate articles on
loudspeakers and headphones.
Photo: My trusty Etymotic
ER-6 noise-cancelling earbud headphones.
These work by passive noise cancelling: each earpiece has a pair of (washable)
plastic suction cups that make a tight seal in your ear canal to stop noise getting in and music leaking out. If you prefer, you can
pull off the plastic cups and use disposable, soft foam ones instead.
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 canel. 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 cancelled 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®.
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 a 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 travelling 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 travelling, 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 travelling between a pneumatic drill and
you're 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.
So the original sound and the reversed sound would, in theory,
completely cancel each other out.
Don't believe it? Test it and see!
Do you believe this? It's true! We can prove it with a simple
sound-recording program called Audacity.
1. Take one pure tone
First, I've recorded two seconds of a pure tone at 440Hz (in music-speak, that's the A below
middle C). It sounds like this:
On an oscilloscope (a TV-screen used
for showing waves), it has an up-and-down wave pattern like this:
2. Make an inverted version of the same pure tone
Now I've used Audacity to invert the sound wave. It sounds exactly the same—like this:
but it looks like this:
Put the two sound waves one above the other and you can see that they are exact opposites:
3. Add the two sounds together
Now, using Audacity, I'm going to add the inverted sounds for two
seconds. And then, straight afterwards, I'm going to add the first wave
to itself for two seconds. What's this going to sound like? A bit like this:
For the first half, we get silence because
the two waves cancel out. For the second half, we get a noise that's twice as loud because
the two waves reinforce one another. And that's how noise cancelling
works!
With real noise reduction, it's never possible to exactly cancel the
two sounds out, so there's always some background noise left over. But
it's still a distinct improvement.
Try all this for yourself using the excellent,
free sound-recording program Audacity. Our MP3 sound clips are played here with the help of
the Audio Player Wordpress Plugin.
What 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 engines,
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?
Students trying to revise while other people play music often fret
about getting peace and quiet. Here's my foolproof solution to noisy
neighbours, 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 aeroplane 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 and
white/pink noise will cancel out virtually any background noise without
damaging your hearing. A pretty extreme solution, but it really does
work!
Further reading
Websites
Buying headphones: Our unbiased guide to things you need to consider before you buy.
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