Noise Cancellation and the Engineering Behind It

First things first – What is noise?

Noise is defined as unwanted sound. Other definitions of noise include sounds that don’t make sense. This obviously depends on the listener; you can’t expect someone who only understands English to understand French!

In more specific terms, noise may be essentially just another sound wave that your speakers/headphones play along with the original sound notes.

Sound travels in waves, and you might’ve already known this, and every wave has an amplitude and a frequency. The amplitude corresponds to how loud or intense a sound is, while the pitch depends on the frequency. And like music, noise also has both of these things.

Your speakers or headphones have audio drivers, which are just the small speakers inside the housing that process both amplitude and frequency signals to produce sound. And if the audio file that you’re playing has background noise signals, or any other type of noise, these get played as well.

Noise leads to distortions at higher volume levels, and this is the cracking sound you may have heard when playing loud music. To remove this noise, a higher-quality audio can be played; one simple example is to increase the quality on YouTube. Higher quality audio has a higher bitrate (measured in kilobits per second, or kbps), and this reduces the chances of noise signals being generated, especially at higher noise levels.

Noise Cancellation

Surrounding Noise and Passive Noise Cancellation:

This is something that you might encounter with earphones or headphones. You put the headphones on and play high quality sound, but let’s say you have a blender running in the background. The noise from the blender will surely be audible even with the headphones on full volume, and can be unpleasant for most people. Especially when your entire purpose is to block out surrounding noise.

Most headphones and earphones have noise-absorbing materials on the ear cups to provide a firmer fit in your ear and to make sure that noise from the surroundings stays in the surroundings. The noise-cancelling effect that these cups produce is called passive noise cancellation, and is basically just how well the headphones act as earplugs.

However, these cups are not very effective for cancelling out noise from the surroundings, especially those noises that are high-pitched and have a frequency of below 1 kHz.

Your blender will definitely fall into this category since the blades are whirring quite fast! (Frequency is just speed divided by time). This is why, even with loud or good-quality earphones, you still end up hearing the blender.

For cancelling noise frequencies below 1 kHz, there’s a nifty bit of tech called Active Noise Cancellation, and we’ll tell you how this works.

Active Noise Cancellation

While the name may make you think of earplugs or cotton (both of which are commonly used for passive noise cancellation), active noise cancellation technology is an auditory phenomenon used to cancel out low-frequency noise in the 30-to 1500-Hz spectrum.

Boat engines, which makes the headphones excellent for aircraft rides (except on take-off and landing, in accordance with current airline restrictions), vacuum cleaners, jack hammers, boat motors, lawn blowers, and so on, come into that range.

Active noise-cancellation technology offers two advantages. First, it reduces all unpleasant noise, which is reason enough to wear a pair that offers ANC. Second, because it eliminates noise altogether, it allows you to listen to music at appropriate and safe levels.

Noise-cancellation technology uses “anti-noise” to cancel unwanted frequencies. An anti-noise is just a noise wave that’s the mirror image of the noise wave being targeted for reduction.

Tiny microphones inside the headphones gather up ambient noise signals and relay them to a microprocessor circuitry located in the companion control enclosure. This is more like a frequency splitter, or a music filter to basically separate noise from music.

The CPU then executes a series of algorithms to forecast what the noise waveform will look like a millisecond before it hits the phone.

Another microprocessor then adds a signal that is 180 degrees out of phase with the noise. These mirror-image impulses are referred to as anti-noise waves, and they are subsequently transmitted back to the headphones. When noise waves and anti-noise waves collide, they cancel each other out (something referred to as “super-positioning”), lowering certain low-frequency disturbances.

To understand the issues that restrict the use of active noise cancellation, it is necessary to first explore the physical mechanisms responsible for noise reduction accomplished by an “anti-noise” source. Keep in mind that a typical single-channel active noise cancellation system includes the following components:

  • a microphone reference sensor to sample, or detect the disturbance to be cancelled,
  • an electronic control system to process the reference signal and generate the anti-noise signal.
  • a loudspeaker driven by the noise control signal to generate sound-cancelling anti-noise, and
  • An error microphone provides information to the controller so that it can adjust itself to minimise small variations within the system.

All this is present, along with a basic functioning music system as well. Think of the above components as an addition, which adds noise-cancellation as a feature!

Structure of Active Noise Cancellation Systems

The Basic Structure of Active Noise Cancellation Systems:

There are two sorts of active noise control systems to consider: adaptive filtering (feedforward or feedback) and waveform synthesis (a type of feedforward control that is only suitable for periodic noise). All this can be pretty confusing, but we’ll try to keep it easy.

Adaptive Filtering:

Incoming noise signals are detected by a reference sensor (often a tiny microphone) and an electronic controller filter them to create the output signal opposite to the noise signal. This signal is then used to play the final music through a loudspeaker, in this case.

The error sensor measures the controller’s effectiveness and gives a signal to the control algorithm for use in changing the controller output accordingly. This is a fail-safe mechanism that keeps on ensuring that the noise signal generated is always of the right kind.

For any noise frequency range control, the controller’s processing time must always be less than the time it takes for the noise signal to reach from the reference sensor microphone to the controller itself. It makes sense if you remember that sound travels as frequency and amplitude, and frequency is just an inverse of time.

Waveform Synthesis

Like the name suggests, this type of active noise cancellation also generates a new signal to cancel out noise, in the same way we’ve discussed above. The mechanism of both sensing and controlling is, however, entirely different.

Waveform synthesis uses sensors to measure ‘droning’ noises from fans and other equipment that is essentially rotating. Consider the blender noise that we talked about earlier. The tachometer is connected to the rotating shaft, and this measures RPM values. These RPM values are compared against set values in an algorithm, and a waveform is generated, which is inherently the opposite of the noise signal.

However, this piece of technology is used in buildings inside ducts that house fans to reduce noise, and does not apply to headphones at all. We thought it would be cool to mention that active noise cancellation has many different applications!

To put things in retrospect:

Using noise-cancelling headphones is definitely a premium convenience. In the day-to-day world, airline cabin noise is reduced by up to 95%, car noise is reduced by up to 80%, and train and subway noise are reduced by up to 70%. Lawn mowers may be up to 60% quieter, while vacuum cleaners can be 40% quieter.

All this means you can enjoy your music more, and headphones now do more than just convert an electrical signal into an audio waveform for personal listening; they also isolate you from the world!

Avatar for Jamie K. Martin

Jamie K. Martin holds a degree in Audio engineering from Husson University, Bangor. Martin spends most of his time testing and trying the technology he writes about to ensure that he provides first-hand information to our customers from all walks of life.

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