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Author Topic: How do noise-cancelling headphones work?  (Read 7876 times)

Offline chris

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How do noise-cancelling headphones work?
« on: 25/10/2007 09:42:28 »
I have seen noise-cancelling headphones advertised in many places, usually glossy magazines given away free on noisy trains (now there's a captive market!), but how do they work? Are the really producing "anti-sound" at the right frequencies to counteract the sounds coming in from the environment?

Chris


 

Offline techmind

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How do noise-cancelling headphones work?
« Reply #1 on: 25/10/2007 10:09:43 »
I'm fairly certain the basic principle is that they have a small microphone on the outside of each earphone, and, as you suggest, they then "play" an amplified anti-phase version of this in the headphones.

If you want to cancel sound up to 22kHz (the maximum frequency on a CD), which has a wavelength of 15cm (lambda=330/f) then you ideally want the microphone and speaker substantially closer than this to maintain a known phase relationship. After all, the phase will change by 180degrees in 7.5cm. They may not need to cancel such high frequency sounds as the padding on the headphones (depending on design) will probably cut high frequencies fairly effectively anyway.

Digitial signal processing is probably needed to analyse any signals from the earphone which leak back into the microphone and to prevent feedback issues. It may also be needed to obtain a sufficiently flat frequency-response in the anti-phase sound.

(I write as a physicist with an interest in DSP, rather than someone who has detailed knowledge of state-of-the-art noise-cancelling headphones!)
 

lyner

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How do noise-cancelling headphones work?
« Reply #2 on: 25/10/2007 10:32:58 »
I am not sure (I am not an expert, either), but a microphone right by your ear canal might be better. A negative feedback system could then compare the wanted (electrical) signal with what you are actually being presented with and eliminate the difference. This would reduce the problem of phase shift. It would also help with the linearity and frequency response, at the same time.
 

Offline techmind

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How do noise-cancelling headphones work?
« Reply #3 on: 25/10/2007 13:45:17 »
a microphone right by your ear canal might be better
In principle you are right of course.
You probably effectively get this with miniature "in the ear" noise-cancelling 'phones.
How practical such an approach is for the larger type of headphones which cover the whole outer-ear I don't know...
 

lyner

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How do noise-cancelling headphones work?
« Reply #4 on: 25/10/2007 14:10:04 »
You would have to be sensing the sound level right next to 'the hole', of course. The feedback should then get the sound right just where it counts.
 

another_someone

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How do noise-cancelling headphones work?
« Reply #5 on: 25/10/2007 16:46:38 »
a microphone right by your ear canal might be better
In principle you are right of course.
You probably effectively get this with miniature "in the ear" noise-cancelling 'phones.
How practical such an approach is for the larger type of headphones which cover the whole outer-ear I don't know...

I cannot see this as being a problem outside of the ear, except that you will need multiple microphones in order to calculate the direction of the sound.
 

paul.fr

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How do noise-cancelling headphones work?
« Reply #6 on: 25/10/2007 23:10:36 »
this sounds like a Neil question!

anyway, this is what how stuff works has to say:

http://electronics.howstuffworks.com/noise-canceling-headphone.htm

One man's noise is another man's music, but no matter what your taste, ambient noise is the enemy. Luckily, there's a piece of audio equipment designed especially to maximize your listening experience, keeping ambient noise out without sacrificing your music's sound quality. That piece of equipment is the headphone, and in this article, we're going to look at how headphones, especially noise-canceling headphones, work.

On a 1978 flight to Europe, Amar Bose, the founder of Bose Corporation, put on a pair of airline-supplied headphones, only to find that the roar of the jet engines prevented him from enjoying the audio.He started making calculations right there on the plane to see if it was possible to use the headphones themselves as a noise-reducing agent. Bose introduced the first noise-canceling headphones a decade later.

Shopping for noise-canceling headphones?
Read headphones reviews and compare prices at Consumer Guide Products before you buy.

In order to understand headphones, you must first understand sound waves. You can check out How Speakers Work for some information, but we're also going to provide a brief introduction here.

When most people think of waves, they think of water waves, like you'd seen in an ocean or lake. A shallow water wave is an example of a transverse wave, which causes a disturbance in a medium perpendicular to the direction of the advancing wave. You can see this relationship in the illustration below. The illustration also shows how waves form crests and troughs. The distance between any two crests (or any two troughs) is the wavelength, while the height of a crest (or the depth of a trough) is the amplitude. Frequency refers to the number of crests or troughs that pass a fixed point per second.

A transverse wave causes a disturbance in a medium perpendicular to the direction of the advancing wave.

Sound waves have many of the same characteristics as water waves, but they are longitudinal waves, created by a mechanical vibration in a medium that produces a series of compressions and rarefactions in a medium. When you pluck a guitar string, for instance, it begins to vibrate. The vibrating string first pushes against air molecules (the medium), then pulls away. This results in an area where all of the air molecules are pressed together and, right beside it, an area where air molecules are spread far apart. As these compressions and rarefactions move from one point to another, they form a longitudinal wave, with the disturbance in the medium moving parallel to the direction of the wave itself.

Longitudinal waves have the same basic characteristics as transverse waves. A compression corresponds to a crest, and a rarefaction corresponds to a trough. The distance between two compressions, then, is the wavelength, while the amount the medium compressed is the amplitude. Frequency refers to the number of compressions that pass a fixed point per second.

Sound waves are longitudinal waves, created by a mechanical vibration that produces a series of compressions and rarefactions in a medium.

For sound waves, amplitude determines the intensity, or loudness, of the sound. Frequency determines the pitch, with higher frequencies producing higher pitch notes and lower frequencies producing lower pitch notes. The brain is able to interpret these characteristics of sound, but before that can happen, the sound waves must be detected by a sense organ. That, of course, is the ear's job. To learn more about how the ear detects and interprets sound, check out How Hearing Works.

Next, we'll look at how headphones take advantage of some of these same principles to help people hear music, books on tape or other recorded material.
« Last Edit: 25/10/2007 23:14:16 by paul.fr »
 

paul.fr

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How do noise-cancelling headphones work?
« Reply #7 on: 25/10/2007 23:15:21 »
Noise-canceling Headphones
Bose was the first company to introduce noise-canceling headphones.
Photo courtesy Consumer Products
Bose was the first company to introduce noise-canceling headphones.

Unfortunately for music lovers, many types of ambient sounds can interfere with or even block the sounds coming through their headphones. If you have ever tried to listen to a CD or MP3 player on a plane, then you know the problem well: The roar of the engines makes it difficult to hear what's being piped through the speakers -- even when those speakers are situated in or on your ear. Fortunately, noise-canceling headphones can provide a more enjoyable listening experience.

Noise-canceling headphones come in either active or passive types. Technically speaking, any type of headphone can provide some passive noise reduction. That's because the materials of the headphones themselves block out some sound waves, especially those at higher frequencies. The best passive noise-canceling headphones, however, are circum-aural types that are specially constructed to maximize noise-filtering properties. That means they are packed with layers of high-density foam or other sound-absorbing material, which makes them heavier than normal headphones. The tradeoff of all that extra weight is a reduction in noise of about 15 to 20 decibels (dB). But considering jet engines create 75 to 80 dB of noise inside the aircraft cabin, passive models have some serious limitations. That's where active noise-canceling headphones come in.

Active noise-canceling headphones can do everything that passive ones can do -- their very structure creates a barrier that blocks high-frequency sound waves. They also add an extra level of noise reduction by actively erasing lower-frequency sound waves. How do noise-canceling headphones accomplish this? They actually create their own sound waves that mimic the incoming noise in every respect except one: the headphone's sound waves are 180 degrees out of phase with the intruding waves.

If you look at the illustration below, you can see how this works. Notice that the two waves -- the one coming from the noise-canceling headphone and the one associated with the ambient noise -- have the same amplitude and frequency, but their crests and troughs (compressions and rarefactions) are arranged so that the crests (compressions) of one wave line up with the troughs (rarefactions) of the other wave and vice versa. In essence, the two waves cancel each other out, a phenomenon known as destructive interference. The result: the listener can focus on the sounds he wants to hear.

 Of course, several components are required to achieve this effect:

    * Microphone - A microphone placed inside the ear cup "listens" to external sounds that cannot be blocked passively.
    * Noise-canceling circuitry - Electronics, also placed in the ear cup, sense the input from the microphone and generate a "fingerprint" of the noise, noting the frequency and amplitude of the incoming wave. Then they create a new wave that is 180 degrees out of phase with the waves associated with the noise.
    * Speaker - The "anti-sound" created by the noise-canceling circuitry is fed into the headphones' speakers along with the normal audio; the anti-sound erases the noise by destructive interference, but does not affect the desired sound waves in the normal audio.
    * Battery - The term "active" refers to the fact that energy must be added to the system to produce the noise-canceling effect. The source of that energy is a rechargeable battery.

Using these components, noise-canceling headphones are able to provide an additional reduction in noise of 20 decibels. That means about 70 percent of ambient noise is effectively blocked, making noise-canceling headphones ideal for airline and train travel, open office environments or any other location with a high level of background noise.

While noise-canceling headphones do a good job distinguishing between the audio a wearer wants to hear and the background noise he or she wants to keep out, some people say that they compromise sound quality by muffling sounds. Users can also experience a change in air pressure, although ports built into the ear cup are meant to vent air trapped behind the speakers.

In spite of these tradeoffs, many people would never go back to normal audio headphones. That's because noise-canceling headphones do more than reduce noise. They also help alleviate fatigue when traveling, which can result from exposure to low-frequency noise for an extended period of time. You can even use noise-canceling headphones if you don't want to listen to another audio source but do want to cancel out background noise. And a little bit of quiet can be music to anyone's ears.
 

Offline chris

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How do noise-cancelling headphones work?
« Reply #8 on: 25/10/2007 23:34:36 »
Thanks Andrew et al. for the assistance.

Chris
 

Offline nikomaster

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How do noise-cancelling headphones work?
« Reply #9 on: 04/11/2007 11:36:07 »
Is an electronic Filter a DSP based filter.
 

Offline nikomaster

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How do noise-cancelling headphones work?
« Reply #10 on: 04/11/2007 11:56:54 »
Being more specific DSP Digital Signal Processing takes the digitial information previously converted from analog, and applies a series of filters , those filteres are designed to elimate that noise,so the filtered signal is returned to the analog state and to the headphone speakers.
 

another_someone

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How do noise-cancelling headphones work?
« Reply #11 on: 04/11/2007 13:47:03 »
Being more specific DSP Digital Signal Processing takes the digitial information previously converted from analog, and applies a series of filters , those filters are designed to elimate that noise,so the filtered signal is returned to the analog state and to the headphone speakers.

Electronic filtering, whether digital or analogue, will not cancel existing noise (great for removing noise from an electronic transmission of sound, but not for removing noise from a non-electronic environment).

What I suspect you are trying to say is that the electronic filtering tries to avoid cancelling what is not noise.  I would expect this to be so, and it would almost certainly be DSP, since the analogue circuitry these days is too complex and too expensive and too large to fit into the available space.
 

lyner

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How do noise-cancelling headphones work?
« Reply #12 on: 04/11/2007 18:08:04 »
As they say "first identify which is the noise - then you can eliminate it".
If the noise (interference) comes from outside - you can identify it and eliminate it to give yourself (almost) silence inside your ear canal, by cancellation.
Analogue or digital processing are  alternatives for achieving it.
 

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How do noise-cancelling headphones work?
« Reply #12 on: 04/11/2007 18:08:04 »

 

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