How does Active Noise Cancellation manage to reduce the sound from outside and thus ensure quiet and undisturbed music enjoyment? And is absolute silence actually possible? GIGA editor Stefan explains how noise reduction works technically with headphones and whether there is a risk of damage to health.
Active noise canceling technology: wave crest meets wave trough
The technical basis for active noise cancellation (ANC) is the anti- noise principle. This means: Sound is actively “extinguished” with sound. Unlike earplugs, which simply slip into the ear and block sound by isolating it – this is a passive process.
What does the sound look like that we can hear as tone, music, speech or noise? A vibrating guitar string is a good example: Here we hear the sound produced and also see what produces it. What we hear is the vibrating movement of the string, which is transmitted to the surrounding air and thus reaches the human ear in the form of vibration. The brain interprets these fine vibrations as sound. We can therefore perceive vibrations – or, to put it more precisely, pressure and density fluctuations in the air – as sound.
The term "sound wave" sums it up, it is a wave that is transported by the air and which continues to increase. You already know that from somewhere else: If you throw a stone into a pond, you can wonderfully observe how the waves generated spread out. They look like rings of increasing size from above, and a regular pattern from the side – an up and down of mountains and valleys. A single wave always has a highest and a lowest point.
You throw the stones away from each other in the same pond. What happens when two waves meet? The wave rings spread out, meet in the middle and… – what happens next cannot be said in general terms, because it depends on the exact point in time.
A particularly high wave crest could form if the moment is right and two wave crests add up. When two wave troughs unite, a particularly deep valley is formed. The third case is the basis for active noise reduction : A wave crest meets a wave trough and both cancel each other out. We know that from mathematics: +1 and -1 add up to 0. Applied to sound, this means that if two uniform sound waves meet at the right moment, they can cancel each other out and there is silence. In this case, natural science, more precisely physics, speaks of destructive interference .
Headphones with Active Noise Canceling work according to this principle and are therefore equipped with microphones . These constantly absorb the noises from the environment. The built-in electronics process these signals at lightning speed – the faster the better: it reverses the sound waves, an opposite polarity signal is generated and passed on to the headphones and “mixed” with the music, if you are listening to it. If this anti-noise hits the eardrum at the same time as the original signal – i.e. the actual ambient noise – they cancel each other out and you hear … nothing.
The noise-cancelling headphones must not only be able to play music, but also generate an opposite polarity image of the ambient noise and mix it in with millisecond precision. The better everything works, the stronger the noise reduction effect.
Active Noise Cancelling: How Does It Sound?
Active noise canceling is particularly suitable for switching off low frequencies. The technology can show its strengths with the monotonous humming in the airplane. In fact, the principle also comes from aviation, where pilots have the problem that the engine noise drowns out the radio traffic. With active noise suppression, you can hear the voice of your conversation partner much better in the headset and you don't have to turn it up so loud.
In an experiment, we recorded how strong noise canceling works (it's best to put on headphones when you watch the video):
At high frequencies, the method is less effective due to interference, and the principle also works less well with very loud noises. But something else helps there: shielding through a good fit and complete closure of the ear. As with hearing protection on the construction site: The noise of the jackhammer does not penetrate to the ear, the energy of the sound wave apparently dissipates at the hearing protection housing. Physically correct is: It is converted into thermal energy. If good passive shielding and effective active noise canceling are combined , the noise-reducing headphones are particularly good.
But: Even the best headphone model can not ensure absolute silence . You would have to measure each ear individually and adjust the electronics accordingly. Part of the ambient noise is also picked up by the skull bone and thus transported to the eardrum. The headphones cannot intercept and hide this. Anyone who experiences active noise canceling for the first time will still be amazed. You can still perceive your surroundings – but much quieter, as if you had locked a door behind you. Then turn on the music and the experience is perfect.
Is Noise Canceling Harmful to Health?
ANC can cause a feeling of pressure in some people – this is not particularly pleasant. But: Can noise-cancelling headphones actually harm your health? No , the opposite is the case, as Dr.-Ing. Hannes Seidler (Technical University of Dresden, Medical Faculty Clinic and Polyclinic for ENT Medicine Ear Research Center Dresden) explained at our request. Here is his detailed answer:
What does Active Noise Cancellation do? Every sound is transmitted as a pressure wave in the air and arrives at the ear in the same way. If there is a wave that swings up and down, I could create a second wave that swings exactly the opposite way. If both overlap correctly, the deflections compensate each other and there is no more sound. So rest.
Ideally, it would work like this. Unfortunately, our noise sources are mostly point-like and therefore emit circular sound waves. If I overlay two of these formations, what you can see on a lake after two stones have fallen into the water results: When the two circular wave fields intersect, calm spots remain on the water and elsewhere particularly pronounced wave crests remain. Translated to acoustics, this means that a reduction in noise at one point is bought at the cost of an increase in noise elsewhere.
Headphones with active noise canceling contain microphones, special amplifiers and loudspeakers in order to use this process to reduce the external noise so that significantly less of it arrives at the entrance of the auditory canal. In the case of low and middle tones, the reduction can be achieved in the entire space of the headphone capsule (the reason is the sound wavelength in air to the size of the headphone). In the case of high-pitched sounds, it can actually get louder in some places in the earphone cup. That's why the fit of the headphones is important. Then these louder sounds do not reach our auditory canal and thus the inner ear. There is no hearing damage either.
Hearing damage can only occur if the music actually desired is listened to too loudly. This is exactly what you no longer need because the outside noise is quieter. So: Headphones with noise canceling help to avoid hearing damage!
Noise canceling headphones
You can always find the latest recommendations for headphones with noise canceling in our buying guide:
Numerous manufacturers offer active noise-cancelling headphones – both classic on-ear headphones with a headband and compact in-ears for plugging in. The models are usually a bit more expensive than conventional headphones and either additional batteries, a built-in rechargeable battery or a connection via Lightning or USB Type-C are required.
Here is a list of popular ANC headphone models that GIGA has tested. Click on the name to open the corresponding test report.
Apple AirPods Pro Beats Solo Pro Bose Headphones 700 Bose QuietComfort 35 II Bowers & Wilkins PX5 Sony WH-1000XM4 Sony WF-1000XM4
In practice, no ANC headphones achieve the state of absolute silence, a little ambient noise always gets through. Above all, (children's) voices and very loud noises push even the best noise-cancelling headphones to their limits. For the (home) office, but also in the university library, ANC headphones are a blessing and help to work in a concentrated manner.