noise cancellation

Noise Cancellation- What is it, and How Does it Work?

Noise cancellation also referred to as Active Noise Control, is the process by which we reduce the amount of unwanted noise from entering an audio system or signal path. This is done through a variety of methods, which we’ll examine in this article. Noise reduction technology in the form of headphones was first introduced to the commercial market in the late 1980s. Today it is a common addition to many of our handheld communication devices, including smartphones, landlines, and laptops.

The ability to minimize ambient background noise or cancel it out entirely was first developed to eliminate noise from ducts. This technology was then implemented in communication systems used on aircrafts and for military purposes. At this stage, only a very narrow bandwidth of noise removal was possible; however, it was a revolutionary move in the industry.

Before we get into noise cancellation techniques and how it all works, we need to gain a fundamental understanding of sound transmission and some of the terminology we use to describe it:

How Our Auditory System Works and What is Considered ‘Noise’?

Sound can be defined as our reception of audio waves through our ears and brain. This is done through a process known as transduction – the conversion of sound waves into electrical impulses received and decoded by our brains.

Sound is transmitted from several source ‘devices’ including musical instruments, animals, wind, machinery, and the human voice. When something produces a sound, it generates fluctuations in the air pressure around it, and these vibrations travel through the air to reach our ears. The human ear then notices these air pressure movements through a series of bones and hairs within our ears, which eventually is converted into electrical signals that our brain can interpret as sound.

Sound is made up of what we call frequencies. These are identified in Hertz (Hz). The range of human hearing starts at around 20Hz and ends at about 20kHz. This range varies between people and can change over time as we age and are exposed to different sound pressure levels, which can gradually harm our hearing abilities.

Generally, as we age, we begin to lose some intelligibility of the higher frequencies. We very rarely only hear a single frequency at a time. We often hear a large range of frequencies and therefore use a decent chunk of our hearing range to interpret it.

As discussed, sound is transferred as a waveform and behaves as such. Lower frequencies possess a longer wavelength, as they complete fewer cycles per second, while higher frequencies complete faster cycles and carry a shorter wavelength. This wave, of course, features positive and negative excursions, as shown in the diagram below, which depicts a 50Hz waveform:

Now that we understand the audio waveform, we can move on to one more important concept that plays a vital role in the noise cancellation technology – Phase.

Phase is the relationship between audio waveforms. If two identical waveforms are played with one another, they create a louder sound –  known as phase reinforcement. If one of the waveforms is reversed before the waveforms are played together, they interact negatively and result in the listener only hearing the difference between them.

The perceived volume or level of a sound is another crucial concept to grasp. Due to evolution, our ears do not behave in a linear fashion, meaning different frequencies need to be played louder for us to perceive them as the same volume. Our hearing system has become sensitive to the frequency band within which the human voice occupies. This differs from person to person and between males and females. However, our voices inhabit the frequency band of around 85Hz to 255Hz – male and female combined.

The volume of a sound is expressed in Decibels (dB) and can be represented as a positive figure or a negative figure. We use two terms along with this:

  • Reinforcement or Amplification– making a sound louder
  • Attenuation – making a sound softer

To gain a stronger understanding of the decibel level, we can examine the below table denoting common sounds and where they fall on the scale, as well as the impact they may have on our hearing.

Now that we understand how we perceive various frequencies and pressure levels, we can jump into noise cancellation technology.

Noise Cancellation Basics

Noise cancellation can be simply explained as the process of reducing (attenuating) unwanted noise from a system, to the point of it being barely noticeable, if audible at all. Unwanted noise can be defined as anything ambient you ideally don’t want to record, such as wind, other musicians in the room or on the stage, noise from the audio system, or external factors such as generators and machinery. You take one of two directions to do this:

  1. From the source device, such as a microphone
  2. Within the listening medium, such as a pair of headphones

Each of these methods, and which one is utilized, depends on the task at hand, and each provide different results.

Noise cancellation that occurs at the source device, such as a microphone, is a common practice used in live music performance recording or concerts, telecommunications, or filming a television show or movie. As for recording and performance arts, you can bring about noise-cancellation using a series of methods. 

Microphone positioning is critical when capturing a performance of any kind, be it a musical instrument or human voice. Without going into too much detail, microphones possess different polarity patterns. Therefore you will capture any sound entering this magnetic field, but not anything outside of this field. 

Using this knowledge, engineers can position microphones, instruments, or artists in specific directions to reduce the amount of unwanted noise entering the microphone’s field and maximize the amount of the desired content to enter this field. You can also execute noise reduction in the form of additional equipment such as noise gates and by using higher-quality cables and equipment.

Some of these methods are used on our cellphones to reduce uninvited sounds from entering the microphones on our devices when on a phone call or recording a voice memo. As our phones have more than one onboard microphone, a phenomenon known as phase can be utilized to combat noisy environments and improve voice quality on our mobile phones, gaming headsets, and laptops with onboard microphones. 

This is done by using phase cancellation techniques by inverting one of the audio signals, thus filtering out everything except your voice from the system, clarifying the audio signal heard on the receiving end of your phone call.

The other direction of noise cancellation occurs on the listening or receiving end of the audio signal. We’ll examine noise-canceling headphones as an example here.

Noise cancellation headsets make use of one of two types of noise cancellation technology: Passive Noise Cancellation or Active Noise Cancellation.

Passive Noise Cancellation:

Passive noise cancellation is made possible through the headset’s physical features, such as extra thick padding used around the ears to increase the amount of isolation between the listener and their surroundings. This is commonly seen on construction sites used by workers to preserve their hearing when working with loud machinery and sounds all day. 

Passive noise cancellation is, in a way, always present in most over-ear style headphones and can make a great difference when used to combat soft to medium intensity sounds, such as background talking in the office or the sounds of a car or bus. These types of headphones prevent those around you from hearing too much of your music.

Active Noise Cancellation:

Active Noise Cancellation on headphones uses the same phase cancellation techniques we discussed earlier and often employs another noise reduction method. When you’re using your noise-canceling headphones in environments with a consistent noise in the background – such as a car, train, plane, air conditioning units, or power generators. The headphones use a built-in microphone to analyze these sounds and produce a phase-reversed version of these sounds to cancel them out effectively. 

Many modern noise cancellation headsets use both active and passive noise control technologies to maximize unwanted sound reduction, creating a pleasant and distraction-free listening experience. In addition to the discussed noise cancellation techniques, added noise cancellation functions can be found on noise cancellation headsets and other audio devices that you can use instead of, or in collaboration with the abovementioned methods:

Adaptive Noise Cancellation:

This form of noise removal technology utilizes onboard microphones in very much the same way that standard Active Noise Cancellation works. However, this method allows for the system to digitally adjust to your surroundings as they change.

Adjustable Active Noise Cancellation:

Here, the listener is given the freedom to control the amount of noise cancellation that is applied to the incoming audio signal, therefore governing the amount of background noise allowed into the system. This is incredibly useful if you’re using noise cancellation headphones in an environment where you may want to be able to hear just enough ambient noise to remain aware of your surroundings.

Transparency Modes:

This function allows the listener to quickly and easily remove all noise cancellation methods in place, and convert noise cancellation headphones into a standard set at the flick of a switch, without the need for removing the headphones or turning the music off. 

Adjustable Transparency Modes:

Similar to the above, this mode allows the listener to control the amount of background noise that is amplified and fed into the audio system.

Noise cancellation technology, as we have seen, has many uses. Whether you wish to minimize background noise in your headphones, or simply eliminate the ambient noise of your surroundings to obtain a quieter environment, there are a wide variety of noise cancellation headphones, in-ear earbuds, and other technologies on the market as it grows in popularity. 

There is also software available for smartphones and computers that execute the same noise cancellation methods to obtain clearer communications, this is aimed towards those who spend lots of time learning, teaching, or participating in meetings online or over video or voice calls.

Written by
Matthew Cox
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