What is Noise Cancellation

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Noise cancellation headphones, whilst becoming increasingly common, are still poorly understood. The basic concept that anti-sound is created to neutralise incoming noise is not wrong but it really leaves more questions than answers.

A product that claims to have noise cancellation may or may not offer a real benefit to the user. Why is this? The noise cancellation system aims to create “anti-noise” to reduce the apparent noise level at the ear, however an inevitable consequence of creating the anti-noise is that some frequencies will also be boosted in level. If the noise cancellation is designed poorly the added noise will be worse or equal to the noise reduction, in which case the design has failed. The user may hear a “difference” when the noise cancellation is switched on, but that difference doesn’t necessarily mean the noise cancellation is a net benefit. With careful design the added noise will be insignificant and the user will benefit from a quieter environment. Many producers take advantage of the consumers limited understanding of noise cancellation to sell products that claim to have noise cancellation (along with vague claims of dB reduction or % reduction) when in fact the end user doesn’t really get the benefit they expected.

Noise Cancellation Level Explained

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Noise cancellation products often claim “18dB reduction” or “reduces up to 90% of background noise” but what does this really mean?

Firstly watch out for the language tricks like “up to”, this necessarily implies there may be much less noise reduction and the stated value is only a possible maximum.

The common practice in the industry is to quote noise reduction level that is the maximum level of reduction at a single frequency point, it does not mean that all sounds are reduced by this amount!

A typical high-end noise reduction headset will offer around 18dB reduction at 200Hz, but as the frequency goes up the reduction level reduces and by the time it reaches, say, 600Hz for an over ear headphone, the noise reduction has reduced to 0dB (similarly as the frequency goes down, below 200Hz, the level of reduction also drops off to 0dB).

Now this bring us to the next type of claim, that being the % reduction. This is really the most deceptive way of specifying amount of noise reduction. This is often simply a mathematical trick of converting the dB number into a % number, there may also be other methods of calculating a % unfortunately no one tells you how they arrive at the figure.

So does an 18dB NC headphone reduce noise by 18dB? No, it reduces SOME noise by 18dB. Does a “reduces up to 90% of background noise” headphone really eliminate 90% of noise? No, it reduces SOME noise frequencies by 90% only. So far there is no international standard for specifying the noise cancellation level of a headphone, the consumer must decide based on reviews, brand reputation or personal experience.

Ideal Headphone Frequency Response

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What is the ideal headphone frequency response. Well no one really has a perfect answer to that. One thing we do know is that a “perfectly flat” response –  a well established goal for loudspeakers – is not suitable for headphones and earphones.

The problem is firstly made difficult by various ways to measure headphone and earphone responses. In other words, it depends on the test jig.

There are some characteristics about the outer ear that mean we get a different experience when the sound arrives from the free field compared to being delivered in a closed earphone cavity, despite the two having a very similar response shape, perhaps some subtle cues coming from reflection around the pinna or an inconsistency with the vibrations ariving through the body. “the missing 6dB” is a phrase used sometimes to highlight the fact that headphones generally need a strong bass response to sound good, perhaps to compensate for the lack of vibration coming through the body as would be the case for sound from a loudspeaker. The answer really is to sit down with a good parametric EQ and fiddle with it until you get the sound you want. Then that response can be implemented into the headphone.

Factory Quality Control

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The thing to remember with an OEM assembly factory is they make the maximum profit by doing the minimum possible. So if a buyer asks a factory to do their own “standard” QC (Quality Control) checks, chances are the factory will default to doing essentially nothing. By doing absolute minimal or no quality checking the factory reduces their labour cost and the product failure rate to extract the most profit out of the 15-20% margin they have on the BOM. Put in a Wavefounder Production Intel Solution and take control of the product quality.

Feedback vs Feedforward noise cancellation

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Feed-forward means that the noise is measured at some “remote” location and that the measurement is used to create an anti-noise signal inside the headphone or earphone. The designer must decide in advance what happens to the noise as it moves from the remote location into the headphone and setup some signal processing based on this prediction.

Feedback means the noise is measured at the same place as the anti-noise signal is produced, i.e. Inside the headphone. There is no need to guess what will happen to the noise in future, it is being instantaneously measured and corrected.

Both styles of noise cancellation have advantages and disadvantages. For feed-forward there are problems such not being able to deal with noise getting into the headphone (e.g. Through the body) that the remote measurement does not know about, it is susceptible to wind noise pickup, that is noises measured by the remote sensor that do not enter the headphone. Feedback presents much more difficult design processes. We prefer feedback, take on the design challenge in the lab and let the user enjoy the performance advantages!