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 current pair of Etymotic HF5 noise-isolating 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.
What's the difference between active and passive 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, viscoelastic 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.
You can swap the foam for plastic flanges, as I've done, which are a bit less comfortable but
easier to clean and more durable.
Photo: For noise isolating earbuds to work properly, they have to make a tight seal with your ear canal to prevent ambient noise from getting in. But different people have different-sized ears, so how
can one pair of earbuds fit everyone? Quite a few manufacturers now attempt to solve the problem by supplying a choice of different tips for their noise-isolating earbuds—and these are the ones that come with the Etymotic HF5. Made from different shapes and sizes of plastic and memory foam, with and without flanges, they simply clip onto the ends of the earbuds.
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 (sometimes called
a "reference" 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®.
Animation: The basic idea of active noise cancelling: to cancel out a noisy sound wave (1), we add an inverted (anti-phase or 180° out-of-phase) version of the same wave (2). Ideally, with complete cancellation, there is nothing left to hear. (3).
Some forms of active noise reduction go even further by using multiple reference microphones.
Some also have a separate ("error") microphone to sample the final sound output and send feedback to the noise-reduction circuit. If the output is too noisy, the circuit can adjust how it's cancelling to try to improve its performance.
Advertisements for active noise-cancelling headphones tend to exaggerate their benefits: you imagine yourself on the plane or the train, drifting off to slumber in a wonderful field of perfect silence. Unfortunately, that's not what happens. They don't remove all noise; they're best for reducing continuous, low-frequency, constant-tone sounds. For general noise reduction, simple earplugs can be much more effective—and they're obviously much cheaper too.
Photo: Some manufacturers combine active and passive noise reduction in
the same headphones. These Audio Technica AN3 earbuds use active noise cancelling (when the power is
switched on), but you'll notice they have similar suction-type tips to noise-isolating earbuds, so they reduce noise even when they're
switched off. One of the big drawbacks of noise-cancelling headphones is that they need batteries to power
their "active" electronics; headphones like this can still eliminate background noise even when the batteries run flat (or are switched off).
Photo courtesy of Audio Technica published on Flickr under a Creative Commons (BY-ND-2.0) licence.
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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 disappears. 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!
Artwork: How active noise-cancelling works.
1) Ambient noise hits the outer case of the headphones.
2) One or more reference microphones pick up the noise and feed it to
an active noise cancellation (ANC) circuit, which is powered by a battery.
3) The circuit inverts the ambient noise waves from the reference
microphones and combines them to produce the maximum amount of
noise reduction.
4) A speaker driven by the circuit plays the inverted noise on top of
the ambient noise.
5) The inverted waves (purple) cancel out much of the original ambient noise (blue).
6) The greatly reduced noise that remains feeds into your ear.
7) An error microphone picks up the reduced noise so it can be measured
by the ANC circuit and used to improve overall noise cancellation.
Try noise-reduction for yourself!
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 above
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.
What are the best noise-cancelling headphones?
Photo: My old Etymotic ER-6 noise-isolating earbud headphones. The sound quality is great, but might not be to your taste if you like lots of bass. The flange-fitting, "suction" earpieces also take a bit of fiddly fitting in your ears—and some getting used to.
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?
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 flanged type that you can use and reuse endlessly.
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—and there are also some readymade apps that do the same thing
(search your favorite app store for "white noise"). Simply play the noise in your ears at reasonable volume and
it should cancel out most things. Because it's not music or speech, your brain
gradually tunes it out and you can concentrate on what you're doing.
Photo: Apps like White Noise for Android let you play a variety of natural, ambient sounds through your headphones, including waves crashing on the beach and rainstorms. Use an app like this as an alternative to white noise, if you find it too harsh and disruptive.
The ultimate solution I've found for really disruptive construction noise
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! Two things to remember. First, foam earplugs are viscoelastic, so you have to curl them up tightly
to squash them into your ear canal, then hold them there for 30 seconds or so while they
relax and seal themselves firmly in place. Second, WARNING: if you block noise this way, you will also
block sounds from things like telephones and (more seriously) fire alarms, smoke detectors, and carbon monoxide alarms,
and that could obviously be dangerous.
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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 (the earliest Bose patent for noise reduction that I've found was filed by Amar Bose himself in May 1980).
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: 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.
Artwork: If you believe Bose invented active noise cancelling quite recently, this artwork should convince you otherwise. It's Paul Lueg's conceptual representation of noise cancelling from his 1936 patent. Suppose a sound source (A) plays a sine wave (S1) in a long pipe (T), and that we sample it with a microphone (M), boost the signal with an amplifier (V), and play it back with a loudspeaker (L). Providing the distance between M and L is just right, we'll produce an inverted sound S2 that exactly cancels S1. As I showed above, we can do the same thing just by inverting S1. Artwork from
US Patent: 2043416: Process of Silencing Sound courtesy of US Patent and Trademark Office. The earliest Bose noise cancelling patent I've found (from 1981) references this Lueg patent as "prior art."
How does a noise-cancelling microphone work?
Noise-cancelling headphones are great, but they're not a perfect solution in every situation.
Suppose you're on the deck of an aircraft carrier guiding planes in to land and you need to
talk to the pilots who are flying in. If there's a microphone on your helmet, it's going
to pick up huge amounts of noise from the environment, making it very difficult for the person
at the other end to hear what you're saying. Noise-cancelling headphones won't help; what you need here is a noise-cancelling microphone.
Photo: A typical noise-cancelling microphone headset used by the US military.
Photo by Hailey Farrell courtesy of US Air Force and DVIDS.
In one popular form, it has two microphones inside that record from slightly different positions. The signal from one microphone is inverted and added to the signal from the other. When you speak, because your
mouth is right next to the microphone, your voice travels a slightly different distance to the two microphones so the signals reaching them aren't the same and there's little or no cancellation. However, more distant sounds will travel almost exactly the same distance to the two microphones.
When those sounds are inverted, they cancel out. That's how a noise-cancelling microphone removes distant
sounds in the environment around you without affecting the sound of your own voice.
Artwork: A noise-cancelling microphone has two recording elements (red). 1) Sound waves from your voice travel very different distances to the two elements (yellow and green) and make two different signals. When one signal is inverted and added to the other, there's still a distinct signal corresponding to your voice. 2) Sound waves from further off travel almost exactly the same distance to both elements (blue), making almost identical signals. When one signal is inverted and added to the other, they cancel out.
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Don't want to read our articles? Try listening instead
Actively reducing noise by ionizing air by Ecole Polytechnique Federale de Lausanne, Phys.org, 22 May 2023. Here's an entirely new spin on noise reduction that uses a very unconventional "loudspeaker."
Tuning Out the World with Noise-Canceling Headphones by Allison W. McCulloch, Ashley Whitehead, Jennifer N. Lovett, Blake Whitley. The Mathematics Teacher, April 2017. Why figuring out the science of noise canceling can be a good introduction to mathematical modeling.
Quiet Comfort: Noise, Otherness, and the Mobile Production of Personal Space by Mark Hagood. The American Quarterly, Vol 63 No 3, September 2011. A technical solution to a noisy world... or (as this author argues) "more to do with conflicts of sound, space, and self in an increasingly mobile modernity."
Patents
These give much more technical detail, but are harder for non-scientists to understand.
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