How Does Noise Cancelling Work?

Noise-cancelling technology uses microphones and speakers to reduce or eliminate unwanted noise by generating a sound wave that cancels it out. This phenomenon creates anti-noise, which is then compressed with a listener’s choice of audio in real time.

“Noise-cancelling technology … produces a sound wave that is 180 degrees out of phase with — or opposite of — the captured sound,” Shubham Munde, a tech research analyst at global market analysis firm Market Research Future, told Built In. “This means that the two waves effectively cancel each other out, resulting in a reduction in overall sound level.”

This “anti-sound wave” is then played through a speaker or set of headphones, in sync with any audio content a user may be enjoying, such as a song or podcast, according to Munde, who is also an information and communication technology domain expert and has contributed to reports involving low-noise amplifiers and noise-monitoring systems. The process is a bit like a math equation, where a positive integer cancels out a negative integer, with the remaining term being a listener’s desired audio content.

“So let’s say you’re in a coworking space with crowd noise, but you have an online meeting and only want the speaker’s voice present without the extra commotion,” Craig Fraser, co-founder and CEO at audio equipment manufacturer BEACN, told Built In. “A set of noise-cancelling headphones would minimize, or even eliminate, background noise so that all you hear is what’s going on in your meeting.”

How Does Noise Cancelling Work?

Noise cancellation is made possible by an effect known as phase cancellation. In order to cancel out a sound wave, another sound wave is created in its equal but opposite form. That is, each peak and valley, or compression and rarefaction, of an incoming audio wavelength is matched with an inverted version of itself at the same amplitude.

Typically, noise cancellation is facilitated through a system of tiny microphones located in and outside a device that “listens” for ambient sound. Processors then analyze the sound and produce neutralizing sound waves via built-in speakers.

During a process known as destructive interference, the unwanted, ambient input then combines with an outbound signal in the shape of its inverse to form a new wave. So when a listener is enjoying music or a podcast through an active noise-cancelling headset, they are hearing their audio content of choice play over a one-of-a-kind anti-sound wave, curated to their unique surroundings at any given time.

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Active Noise Cancelling vs. Noise Suppression

There are two main types of noise cancelling technology: active and passive. While active noise cancellation uses technology to completely eliminate sound, noise suppression is a form of passive noise cancelling, and is more about how the physical device muffles sound.

ACTIVE NOISE CANCELLING

Generally speaking, when people talk about noise cancelling, they’re referring to active noise cancelling (ANC). This digital solution requires electronics (and, in some cases, a software component) and is achieved by sending out sound waves to counteract incoming, unwanted sound, allowing listeners to enjoy audio content without being disturbed by external noise, such as chatter or traffic.

Audio tech brand Ultimate Ears estimates that active noise cancellation  can cut ambient noise by 30 decibels on average. Low-frequency, repetitive noise, such as lawnmowers, subway traffic and the constant murmur of a jet engine, can be reduced by much as 60 decibels.

One type of active noise cancelling on the rise is known as adaptive noise cancelling. This new approach builds on top of existing technology with machine learning and hybrid detection, surveilling signals both inside and outside of the headphone.

“Several ways to eliminate noise exist, especially with the growing popularity and feasibility of machine learning.”

Regular active noise cancelling focuses on the sound pollution that penetrates the headphone, whereas adaptive noise cancelling tech is equipped with external sensors that register and anticipate unwanted noise from outside the device. These devices are typically accompanied by some sort of audio control, like an app or mixer, to program customizable settings. When a user’s environment is particularly noisy, the advanced algorithms digitally adjust to strengthen noise cancellation. When it’s quiet, levels will lower instead. Additionally, these sensors also factor in leakage caused by hair, glasses, ear shape and head movement — hence, the “adaptive” element.

“Several ways to eliminate noise exist, especially with the growing popularity and feasibility of machine learning,” Fraser said. Some BEACN products, for instance, use adaptive noise cancellation methods that estimate the average background noise in real time in order to subtract it from the audio signal.

This method better maintains the fidelity of a vocal track while counteracting typical noise-polluting pests, such as computer fans and the occasional air conditioner kicking on, Fraser said.

Many headphones and earbuds that offer active noise cancellation have a feature known as transparency mode. This mode allows users to let outside noises in while wearing their headphones or earbuds, essentially turning the ANC mode off without having to physically remove the devices. By using transparency mode, sounds and music will still play through the device speakers as normal but without the noise cancellation effect, so environmental noises can be heard at the same time.

Noise Suppression

However, total noise elimination may not be ideal. Consider a road worker who needs to be cognizant while operating heavy machinery or a runner dashing through city streets. Even in terms of in-studio recording, Fraser noted that there’s benefits to lighter soundproofing methods, such as noise suppression.

“We aim to suppress the noise and not completely cancel it, as this can introduce other undesired sound artifacts,” he said. “We prioritize the quality of the voice over the effectiveness of the noise suppression.”

“It’s basically the equivalent of sticking your fingers in your ears, though probably a bit more comfortable.”

Noise suppression, a form of passive noise cancelling, takes place in a device’s physical design. Like earmuffs, this method obstructs ambient sound through the use of dense, audio-absorbent materials, including foam or rubber, and other factors such as size, diameter and shape, Munde said. For wearable tech, effectiveness is all about a custom fit to create the perfect soundproof seal capable of reducing mid- to high-frequency noises by about 15 decibels to 30 decibels.

Some, however, prefer using the term “passive noise isolation,” as the majority of these devices can only suppress noise versus entirely eliminate it. As tech writer Geoffrey Morrison points out in CNET: “It’s basically the equivalent of sticking your fingers in your ears, though probably a bit more comfortable.”

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What Noises Are ANC Best For?

ANC Is Effective for Low-Pitched Noises

Most active noise cancelling devices do well at blocking sound frequencies falling below 1 kilohertz (kHz). Low-pitched, often droning noises like motor rumbling and engine sounds tend to be found in this range. This makes ANC effective for blocking out low ambient noise when riding in a car, plane, train or similar vehicle.

ANC Is Effective for Repetitive Noises

ANC is also effective for blocking out repetitive, sustained noises. This could include sounds like the hum from an air conditioner or the ticking of a clock. Similarly for its ability to block out low-pitched noises, this makes ANC ideal for spaces like offices, classrooms or libraries where humming from overhead lights and electronics may be present.

ANC Is Less Effective for High-Pitched Noises

ANC has a more difficult time filtering out noises that go above the 1 kHz threshold. These can include sounds such as alarms, loud music or people having a conversation close by. If you’re traveling down a busy street or in a crowded room, you’re more likely to hear noises from these environments even if ANC is activated. 

ANC Is Less Effective for Unpredictable Noises

While ANC is best for constant, predictable noises, it becomes less effective when a sudden noise appears. Dogs barking, someone shouting or an object falling on the floor can still often be heard while wearing ANC headphones and earbuds.

Is Noise Cancelling Bad for Your Ears?

The short answer is no. There is no evidence that noise-cancelling technology is the cause of any adverse effects experienced by users.

That isn’t to discredit any discomfort felt by some users who have reported vertigo-induced dizzy spells while using active noise-cancelling headphones, as its low-frequency signals may trick the brain into thinking it’s in motion. Another psychosomatic effect that can occur is known as “eardrum suck,” which can mimic unease often accompanied with a sudden change in altitude.

In terms of general harm reduction, Amy Sarow, doctor of audiology and lead audiologist at online hearing-aid marketplace Soundly, noted that the key to responsible headphone-use comes down to listening volume. 

“Noise-cancelling technology on its own does not cause hearing damage,” she said. “However, noise-induced hearing loss can result when the listening volume exceeds a safe level.”

Both the U.S. Environmental Protection Agency and the World Health Organization consider 70 decibels, equivalent to a dishwasher or a washer-dryer unit, as a safe threshold for helping to prevent hearing loss. In other words, consider keeping the volume at 70 percent of the maximum sound bar when listening through headphones, said Sarow, who also sits on the Forbes health advisory board.

“Fortunately, noise-cancelling technology allows the listener to listen more easily at a safe level, as they don’t need to turn the volume up over the background noise sound,” Sarow added.

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