How Noise Cancelling Works: Using Sound to Erase Sound
Noise cancelling works like this: a mic picks up the noise, and a chip makes a sound wave whose peaks and troughs are flipped. The two waves meet in your ear and cancel out. The steady low roar of a train or a plane fades well. But high or sudden sounds, like voices or clinking dishes, are hard to time and mostly remain.
What you'll learn
- The difference between "passive" (blocking your ears) and "active" (cancelling sound with sound)
- How a mic, a chip and a speaker work together to make the opposite sound
- Why low sounds fade well while voices and high sounds remain (the speed of sound and delay)
- Transparency mode, wind noise, the "blocked ear" feeling, and your hearing
Prerequisites: None. Some numbers in dB (decibels) appear, and each one is explained when it comes up.
In everyday life
Many wireless earbuds and headphones have a "noise cancelling" button. Turn it on in a train, and the low roar of the ride suddenly drops. It gets quiet around your ears even with no music playing.
Yet you can still hear the person next to you talking, or the station announcements. Some sounds fade, and some don't. This article explains where that difference comes from, starting with what sound really is.
Sound is a tiny ripple in air pressure
Sound is air being squeezed slightly and then thinned out, over and over. This ripple travels through the air as a wave. Your eardrum, deep in the ear, moves along with the ripple, and you sense that as sound.
The number of ripples per second is called the frequency. Its unit is the Hz (hertz). Fewer ripples per second sound lower, and more sound higher. The rumble of a train or a plane engine is mostly low. The parts of a voice that matter for understanding words, and the clink of dishes, are higher.
Loudness is measured on a scale called dB (decibels). Compared with the faintest sound we can hear, each tenfold rise in sound intensity adds 10 dB. "Decibels explained" covers it in more detail.
The ripple is very small. Even a fairly loud 80 dB sound changes the pressure by only about 0.2 Pa (pascals, a unit of pressure). That is about 1/500,000 of the air pressure around us (about 101,300 Pa).
Two ways to cut noise: block it or cancel it
There are two main ways to reduce noise. The first is to block the sound's path with an object, like an earplug or an ear cup that covers the whole ear. This is called passive noise cancelling, or sound isolation. It uses no electricity.
Blocking works well on high sounds. Low sounds are harder to reduce, because they easily shake thin walls or earplugs and pass through. To block low sounds too, you would need a heavy, thick cover.
The second way is the star of this article: active noise cancelling, or ANC for short. It hits the noise with a sound that cancels it out. ANC handles the low sounds that blocking struggles with. Real products use both methods together.
How it works 1: adding a wave with flipped peaks and troughs
When two sound waves reach the same place, their pressure ripples add up. Where a peak meets a peak, the ripple grows. Where a peak meets a trough, they cancel each other out. Picture two sets of ripples meeting on a lake.
The point a wave has reached within one ripple (a peak, a trough, or in between) is called its phase. A wave with peaks and troughs swapped compared with the noise is an "opposite-phase" wave. Add an opposite-phase wave of the same size inside your ear, and the total ripple is close to zero.
The key point is that both the "size" and the "timing" must match to cancel the noise. If the opposite wave is a little late or too big, some noise is left over. If it is off by half a ripple, peak meets peak, and the noise gets louder instead.
How it works 2: mic → chip → speaker
Parts inside the earbud make the opposite wave, step by step, like an assembly line. First, a tiny mic on the outside picks up the noise. It turns the pressure ripple into a ripple in voltage. "What is MEMS?" shows how such tiny mics work.
Next, the voltage is turned into numbers. The circuit that does this is called an AD converter (ADC). Once the wave is in numbers, fixed steps of math can reshape it freely.
A DSP (digital signal processor) takes in the noise as numbers and calculates the opposite wave. A DSP is a chip that is good at repeating set patterns of multiplying and adding, very fast.
A DA converter (DAC) turns the results back into a voltage wave, which drives the speaker. The speaker plays the opposite sound together with your music. In this way, the opposite sound meets the noise leaking in from outside, right in your ear. "What is an analog chip?" explains in detail how circuits switch between voltages and numbers.
A common way to compute the opposite wave goes like this. Take the last few numbers from the mic, multiply each by a weight, and add them up. This method is called an FIR filter. How the earbud sits in your ear and the type of noise keep changing, so some designs keep adjusting the weights bit by bit. Here we keep the math simple and skip the formulas.
How it works 3: the outer mic and the inner mic
There are two ideas about where to put the mic. One is to place it on the outside of the earbud. It hears the noise before it reaches your ear and makes the opposite sound in advance. This is called the feedforward method. It acts ahead on a prediction, without checking the result.
The other is to place a mic inside the ear, right next to the speaker. This mic hears what is left after cancelling. The system keeps correcting the opposite sound to make what is left smaller. Measuring the result and correcting is called feedback control, and in ANC this is the feedback method.
Each has strengths and weaknesses. The outer mic can act ahead, but it can't tell whether the noise was really cancelled in your ear. The inner mic can check the result. But if the correction is made too strong, the system becomes unstable, with a steady high-pitched squeal. That is why some products combine both. Apple, for example, says it uses both outward-facing and inward-facing mics to make the opposite sound. "Open-loop vs closed-loop control" compares the two kinds of control.
How it works 4: finishing the math before the sound arrives
The hardest limit in ANC is time. Sound travels through air at about 343 m per second at 20 °C. In 1 millisecond (one thousandth of a second), it goes about 34 cm. In 1 microsecond (one millionth of a second), it goes about 0.3 mm.
In an earbud, the outer mic is only a few cm from the eardrum. Say it is 3 cm, and the sound travels in a straight line. Then the sound covers that distance in about 90 microseconds. In the feedforward method, the whole chain must finish in this short time: ADC → DSP → DAC → speaker. If it is too slow, the opposite wave arrives late and can't cancel well.
That is why ANC chips keep the delay from input to output especially short. For example, a chip from Analog Devices (ADAU1787) has a shortcut path for ANC. In a setting that skips the math, the delay from input to output is 5 microseconds. In that time, sound moves only about 1.7 mm. Chip datasheets sometimes call this kind of delay group delay.
By the way, the delay of music sent from your phone over Bluetooth is a separate matter. ANC does all its math inside the earbud, so radio delay does not affect it. "How does Bluetooth work?" explains the Bluetooth delay.
Why low sounds fade but voices and high sounds remain
The same delay affects low and high sounds very differently. One ripple of a low 100 Hz sound takes 10 milliseconds. One ripple of a high 5 kHz (5,000 Hz) sound takes 0.2 milliseconds. Suppose the opposite wave is 0.1 milliseconds late. At 100 Hz, that is a shift of just 1% of one ripple. At 5 kHz, it is a shift of half a ripple, so peak meets peak and the sound gets louder.
Working it out shows that with a 0.1 millisecond delay, the size of a 100 Hz ripple drops to about 1/16. In sound intensity, that is about 1/250 (about −24 dB). At 1 kHz, the ripple only shrinks to about 0.6 times its size (about −4 dB). Here we assumed the opposite wave was exactly the right size, with delay as the only error.
Place matters too. The distance sound travels during one ripple is called the wavelength. At 100 Hz, the wavelength is about 3.4 m. At 5 kHz, it is about 7 cm. A low sound with a long wavelength ripples almost the same way everywhere around your ear, so it can be cancelled all at once. A high sound with a short wavelength changes when you move just a few mm. That makes it hard to cancel across the whole inside of the ear.
Sudden sounds are hard too. The clink of dishes or the slam of a door starts all at once and contains many high sounds. In contrast, steady sounds like an engine or the rumble of a moving train are easier to predict a little ahead. The weight adjustments can also keep up. For these reasons, high sounds are left to blocking by the ear tips or ear cups, not to ANC.
Transparency mode: letting outside sound in
Some ANC products also have the opposite mode, which makes outside sounds easier to hear. Makers call it transparency, ambient sound, or similar names. The sound picked up by the outer mic is not cancelled. Instead, the speaker plays it as it is.
Even with earbuds in, you can hear station announcements or someone calling you. Apple describes this mode as letting outside sound in, so you can hear what's going on around you. Still, take care when walking along a road or riding a bike, so you don't miss sounds around you.
Wind noise and that "blocked ear" feeling
With ANC on outdoors, you may notice a buffeting rumble from the wind. When wind hits the mic directly, the swirling air shakes the mic, which picks it up as a loud low sound. This ripple is not the noise reaching your ear. So making an opposite sound for it can add noise instead. Sony also offers settings and tips to reduce wind noise when using ANC.
Some people feel a blocked ear, as if the air pressure has changed, when they turn on ANC. But ANC cancels only the tiny ripples of sound. As we saw earlier, they are about 1/500,000 of the air pressure. ANC does not create a pressure that keeps pushing on the eardrum. This feeling differs from person to person, there is little research on it, and its cause is not yet confirmed. If it bothers you, turn ANC down or take a break.
A short history of noise cancelling
The idea of erasing sound with an opposite sound is much older than earbuds. Paul Lueg in Germany came up with a way to cancel sound with an opposite wave, for example inside a pipe. He was granted a US patent in 1936. But the parts of that time could not turn it into a product.
In the 1950s, Lawrence J. Fogel in the US took the research further. His goal was to make radio messages easier to hear in the noise of airplanes and helicopters. His patent (US Patent 2,966,549) was filed in the 1950s and granted in 1960. Its title is "Apparatus for improving intelligence under high ambient noise levels."
The US audio company Bose brought the idea to products. According to the company's timeline, founder Amar Bose was bothered by cabin noise on a flight in 1978 and decided to study it. In 1986, the company supplied prototypes to the pilots of Voyager. Voyager was the plane that attempted a nonstop flight around the world without refueling. Bose launched an aviation headset in 1989 and consumer headphones in 2000. Since then, chips have become smaller and use less power, so ANC now fits even in earbuds.
Common myths
Myth: Noise cancelling erases every sound around you
In fact, it works best on steady low sounds, and voices, high sounds and sudden sounds remain. Sony's help guide also says it works on low sounds like airplanes, trains and air conditioning, but less on higher sounds like human voices. If you expect everything to vanish, you may think the earbuds are broken when high sounds remain.
Myth: Playing an opposite sound is bad for your ears
In fact, the opposite sound is about as loud as the noise it cancels, so the total sound reaching your ear gets smaller. A 2022 study found that turning on ANC lowered low sounds near the eardrum. With in-ear earbuds, the volume people chose also went down. The WHO also recommends using noise cancelling in noisy places, so you don't need to turn up the volume.
Myth: On a loud train, just turn the volume up to drown out the noise
In fact, the strain on your ears builds up with both loudness and listening time. The WHO (World Health Organization) gives a guide: at an average volume of about 80 dB, you can listen safely for up to 40 hours a week. By the same idea, if the sound intensity doubles (+3 dB), the time halves to 20 hours. Using ANC to cut the noise and keeping the volume down protects your ears better than turning it up to drown the noise.
By the numbers
| Item | Number |
|---|---|
| Speed of sound in air (20 °C) | About 343 m per second (about 34 cm in 1 millisecond) |
| Wavelength / one ripple's time, 100 Hz sound | About 3.4 m / 10 milliseconds |
| Wavelength / one ripple's time, 5 kHz sound | About 7 cm / 0.2 milliseconds |
| Example delay of an ANC chip (ADAU1787) | 5 microseconds (time for sound to move about 1.7 mm) |
| Maximum active reduction (1992, US Air Force evaluation) | 15–22 dB (100–250 Hz) |
| Pressure ripple of an 80 dB sound | About 0.2 Pa (about 1/500,000 of air pressure) |
How much ANC reduces noise varies a lot with the product and how well it fits your ear. In the 1992 evaluation in the table, ANC alone cut noise by up to 15–22 dB, but high sounds barely changed. A drop of 20 dB means the sound intensity falls to 1/100. A rule of thumb says that when intensity falls to 1/10 (−10 dB), a sound seems about half as loud. To read dB numbers, see "Decibels explained," mentioned earlier.
Learn more
- Terms: feedback control / FIR filter / group delay / delta-sigma ADC (an ADC common in audio that turns signals into numbers in fine steps)
- See the differences in a table: FIR vs IIR filters / Open-loop vs closed-loop control
- Try the numbers: dB and dBm converter / ADC SNR and ENOB calculator
- Related explainers: "What is an analog chip?" / "What is an op-amp?" (how negative feedback works) / "How does Bluetooth work?" / "What chips are inside a smartphone?"