How noise cancellation works in headphones and why it does not save from all external sounds

2024 Author: Malcolm Clapton | [email protected]. Last modified: 2023-12-17 03:44

Ruthless physics prevents noise cancellation systems from creating perfect silence.

In 2020, due to the transition to telecommuting, many people needed headphones to communicate with colleagues in online conferences and distance themselves from household members. Gadgets with noise cancellation have gained particular popularity: they helped to better focus on work due to the fact that they reduced the level of external noise.

But there have been such models for a long time: household headphones with a noise canceling system first came out as early as 1989. More recently, Apple has drawn attention to this feature with the release of AirPods Pro and then AirPods Max.

Many expect noise canceling headphones to completely "turn off" ambient sound, and are disappointed when they don't. To understand why technology doesn't mimic the Mute button, you need to understand how it works.

How active noise cancellation works

Active noise cancellation technology migrated to household appliances from the military: already in the middle of the 20th century, such systems were used to reduce the noise level in the cockpits of aircraft and helicopters. The first on-ear active noise canceling headphones were created for helicopters in 1957.

For home use, the technology was adapted by Bose - it was its headphones that became the noise-canceling pioneers in the consumer electronics market in 1989.

The principle of operation is as follows: the microphone system detects the noise level around the headphones, and then a signal with the same amplitude, but in antiphase, is reproduced in the speakers. As a result, the waves of noise and anti-noise mutually suppress each other. Of course, in order for an antiphase wave to be created, the headphones must be connected to some kind of power source, which is why such a system is called active.

Implementing it in practice is not as easy as it seems. It's all about the very nature of sound. Generating a signal in antiphase is a difficult task: it must be completed quickly and accurately. Fast, because the noise can change, for example, in a cafe, either cups are knocking on saucers, or someone is being congratulated on their birthday, that is, a fickle sound background and you need to adjust to it. Exactly, because the deviation between phase and antiphase can lead to unpleasant distortion.

This is why Active Noise Cancellation is best at dealing with monotonous low-frequency sounds like the hum of aircraft engines or ventilation. They have a more or less constant amplitude, and the wavelength at low frequencies is quite large and forgives some signal discrepancy in phase and antiphase. In the case of a cafe, the basis of the noise is created by medium and high frequency sounds, for which the wavelength is much shorter and the amplitude changes more sharply - they have higher requirements for an antiphase signal.

Modern active noise cancellation systems do not yet have enough computing power to silence the conversations of strangers and the sudden rumbling. But they handle monotonous background noise well.

How passive noise canceling works

In addition to active noise cancellation, there is also passive noise cancellation. This can be said to be soundproofing: you put on headphones - and everything around becomes muffled. It does not require electricity and depends only on how tightly the device fits to the ears and head. If there is any gap, the noise will be heard.

That is why in-ear headphones come with several pairs of earpieces, different in size: everyone's ears are different, and sometimes it is not easy to find the perfect ear pads. If they do not sit very firmly, the gadget will not provide the proper level of sound insulation.

But perfectly matched nozzles effectively dampen exactly the very noise that active noise cancellation cannot cope with: mid and high frequencies, sharp, sudden sounds. Because this scheme does not rely on the speed of the electronics, but simply muffles everything around, although not to complete silence.

Passive noise cancellation also plays a significant role in full-size headphones with large cups. The tighter the ear pads are pressed against your head, the less noise you hear and the less people who are nearby will know about your music. But if you can change unsuitable attachments for in-ear headphones, then nothing can be done with full-size ones that do not fit well on the head.

And all the same, perfect silence with such a gadget cannot be achieved. The reason again lies in the nature of the sound and in the materials. Sound propagates wherever there is a medium. Physical obstacles can prevent him, so you can simply cover your ears with your hands and drown out the world around.

However, in order to achieve complete sound insulation, special sound-absorbing materials must be used.

Remember the big construction headphones worn to protect against harsh sounds like a jackhammer? There is no electronic filling in them - they simply cover the ears, creating a physical barrier to noise and freeing up your hands. At the same time, they still do not give complete silence: the jackhammer is audible, albeit dull.

To get maximum sound insulation in these headphones, you need to add more absorbent material, which will make them noticeably heavier and more voluminous. Now imagine that you will have to connect all the necessary electronics to the already gaining weight headphones. The model will become very heavy.

Of course, manufacturers are improving both the materials from which the headphones are made and their electronic filling, but they are still far from creating ideal silence. Now they are trying to strike a balance between the comfort of use and the efficiency of the offered functions.

What affects the quality of noise reduction

There are many companies on the market offering active noise canceling headphones, but how do you know who has the best?

You can take a look at the characteristics. For example, the more microphones working in the noise reduction system, the more accurately the ambient hum level will be read and the more correctly the antiphase signal will be built. The more powerful and modern the processor, the faster the system will respond to changes in the noise around. In theory.

Some companies even offer custom noise reduction algorithms, such as Sony, Bose, Samsung, Apple. Headphones equipped with such systems, in addition to microphones that listen to the noise outside the device, also have microphones that pick up the noise inside.

During calibration, the headphones reproduce test signals, and the same internal microphones read the response. Based on the calibration results, the noise reduction algorithm corrects its work: it is now adjusted to the geometry of specific ears and head.

Passive sound insulation also affects the operation of the active noise reduction system. If the headphones are loose and pass through noise, the antiphase signal will not work as effectively. Most full-size active noise canceling headphones feature large, soft ear cushions that hug the curves of your head for maximum sound isolation.

How not to fall for the tricks of marketers

The problem with noise canceling headphones is that companies are lying: in commercials, when noise canceling is turned on, the party is replaced by a ringing silence, the place of the pounding train wheels is taken by serene alpine meadows, and the baby screaming in the next seat dissolves into the haze.

But due to the physics of the noise reduction process itself, it is unrealistic to achieve such an effect. Initially, by the way, the Bose company honestly called its headphones Noise-Reducing - "noise reducing", but then, like others, switched to the term Noise-Canceling - "noise canceling".

As a result, users, hoping for silence, go down in such headphones in the subway, turn on noise cancellation … The howling of the wind in the tunnel disappears, but the students who talk about passing the session with Stanislavsky's anguish do not. And the way someone coughed nearby can be heard. Dull, but still audible.

Where are my promised silence, meadows and alpine freshness? In dreams. Because technology has not yet reached this level.

At the same time, in marketing materials, developers are very fond of bragging about the fact that their system provides noise reduction, for example, up to -15 dB. The first thing we pay attention to is the word "before". The second is that it is not specified in what conditions these −15 dB can be achieved. That is, there are some figures, but how to compare them with reality is not very clear.

Another trick is the substitution of concepts. It happens that in the description of headphones, noise cancellation is called a system that dampens noise during voice transmission during calls. That is, your interlocutor hears you more clearly. But it does not affect the suppression of external noise when listening to music.

What is the bottom line

Noise canceling technology in the form in which it is presented by the advertising departments of headphone manufacturers does not exist. Moreover, the effect of it is individual and depends on the physical parameters of a particular listener. For some, full-size headphones provide adequate passive sound isolation so that active noise cancellation works well. And in another person, for example, due to a different structure of the cheekbones, the headphones will not fit so tightly and the algorithm will be ineffective. So everything needs to be tried on.

Models with active noise cancellation are suitable for those who travel a lot in transport: on buses, trains and airplanes. The main feature of such headphones is the ability to muffle the monotonous low-frequency hum, and it is he who most often accompanies on trips. They differ in high-quality active noise cancellation, for example:

• Sony WH ‑ 1000XM4,
• Soundcore Liberty Air 2 Pro,
• Sennheiser Momentum True Wireless 2,
• Marshall Monitor II ANC,
• Samsung Galaxy Buds Pro.

Those who are trying to escape from noisy offices and households should pay special attention to the fit of the headphones and look for models that fit as closely as possible to the head or to the ear canal. These headphones reduce the level of all noise around, including voices. But you can't choose them according to advice: you need to try them on.

If you come across a model that fits snugly and snugly and comes with Active Noise Cancellation, you can rejoice: you get the best of both worlds. But you still can't achieve perfect silence.