How Do Noise-Cancelling Headphones Work? (PNC & ANC)
Whether you're trying to rid of external noise while studying or listening to your favourite music in a crowded place, noise-cancelling headphones are a superb product to help you hear what you want to hear and eliminate what you don't. If you've ever used a pair of noise-cancelling headphones, you may have wondered exactly how they work. This article will explain just that.
How do noise-cancelling headphones work? Headphones utilize passive and active noise-cancelling (PNC and ANC). PNC works by simply blocking sound waves from physically entering the ear. ANC methods are much more technical and include real-time recording of external noise that is added to the audio signal out-of-phase to cancel out noise.
In this article, we'll discuss passive and active noise-cancellation in headphones and develop a deeper understanding of the technology behind noise-cancellation.
Why Is Noise-Cancellation Important?
Noise-cancelling headphones have been gaining popularity in recent years. There is obviously a market for these types of headphones. So what's so great about noise-cancellation, and why is noise-cancelling so important?
The cancellation of environmental noise is welcomed in three big ways:
- To protect our sense of hearing
- To block out annoyances in our environment
- To improve the listener experience
Let's talk about each in a bit more detail:
Noise-Cancellation Helps Protect Our Sense Of Hearing
This is perhaps the most critical function of noise-cancellation. Our sense of hearing should never be taken for granted. Once our hearing is damaged, there's no naturally healing that will return it to what it was.
Excessive sound pressure levels are damaging to our hearing. The louder the sound, the less time we can be exposed to it.
Here is a table with the safe listening times laid out per sound pressure level. These values are taken from the NIOSH National Institute for Occupational Safety and Health) and the OSHA (Occupational Safety and Health Administration).
| NIOSH Standard (dBA) | Equivalent Sound Pressure Level (at 1 kHz) | Maximum Exposure Time Limit | OSHA Standard (dBA) | Equivalent Sound Pressure Level (at 1 kHz) |
|---|---|---|---|---|
| 127 dBA | 127 dB SPL 44.8 Pa | 1 second | 160 dBA | 160 dB SPL 2.00 kPa |
| 124 dBA | 124 dB SPL 31.7 Pa | 3 seconds | 155 dBA | 155 dB SPL 1.12 kPa |
| 121 dBA | 121 dB SPL 22.4 Pa | 7 seconds | 150 dBA | 150 dB SPL 632 Pa |
| 118 dBA | 118 dB SPL 12.6 Pa | 14 seconds | 145 dBA | 145 dB SPL 356 Pa |
| 115 dBA | 115 dB SPL 11.2 Pa | 28 seconds | 140 dBA | 140 dB SPL 200 Pa |
| 112 dBA | 112 dB SPL 7.96 Pa | 56 seconds | 135 dBA | 135 dB SPL 112 Pa |
| 109 dBA | 109 dB SPL 5.64 Pa | 1 minute 52 seconds | 130 dBA | 130 dB SPL 63.2 Pa |
| 106 dBA | 106 dB SPL 3.99 Pa | 3 minutes 45 seconds | 125 dBA | 125 dB SPL 35.6 Pa |
| 103 dBA | 103 dB SPL 2.83 Pa | 7 minutes 30 seconds | 120 dBA | 120 dB SPL 20.0 Pa |
| 100 dBA | 100 dB SPL 2.00 Pa | 15 minutes | 115 dBA | 115 dB SPL 11.2 Pa |
| 97 dBA | 97 dB SPL 1.42 Pa | 30 minutes | 110 dBA | 110 dB SPL 6.32 Pa |
| 94 dBA | 94 dB SPL 1.00 Pa | 1 hour | 105 dBA | 105 dB SPL 3.56 Pa |
| 91 dBA | 91 dB SPL 0.71 Pa | 2 hours | 100 dBA | 100 dB SPL 2.00 Pa |
| 88 dBA | 88 dB SPL 0.50 Pa | 4 hours | 95 dBA | 95 dB SPL 1.12 Pa |
| 85 dBA | 85 dB SPL 0.36 Pa | 8 hours | 90 dBA | 90 dB SPL 0.63 Pa |
| 82 dBA | 82 dB SPL 0.25 Pa | 16 hours | 85 dBA | 85 dB SPL 0.36 Pa |
Earplugs and earmuffs help to greatly reduce the decibel sound pressure level at our ears, largely because of passive noise cancellation. These protective devices effectively insulate our eardrums for external noise and block some, but not all, of the loudness of the sound around us.
Active hearing protection is also available, which helps to target specific sound pressure levels above a certain threshold to cancel them out with an anti-noise signal. More on this later when we get into the deeper details on how active noise-cancellation works.
Noise-Cancellation Helps Block Out Environmental Noise
This is an obvious point, but noise-cancellation cancels noise.
Why is this important?
Well, beyond hearing protection and improved audio listening, noise-cancelling can be used to increase focus and productivity in noisy environments, improve sleep, and block out droning noise like airplane engine hum.
In fact, Amar Bose initially invented the first active noise-cancelling headphones to block out the annoying low-end rumble he was experiencing during his flights.
This benefit is worth noting since noise-cancelling hearing protection and headphones are not always used in loud situations or when listening to music and other audio.
Noise-Cancellation Improves The Listener Experience
Noise-cancelling headphones are probably best known for improving the listener experience by nullifying much of the external noise in the listener's environment.
With noise-cancelling headphones, this is achieved through both active and passive means.
Passive noise-cancellation is achieved with closed-back circumaural (over-ear) headphones and with snug-fitted earphones and in-ear monitors.
Active noise-cancellation technology is typically reserved for closed-back circumaural headphones but can also be used in earphones and is even included in some closed-back supra-aural (on-ear) headphones.
To learn more about closed-back headphones, check out my article The Complete Guide To Open-Back & Closed-Back Headphones.
What Is Passive Noise-Cancellation?
Passive noise-cancellation is actually quite easy to conceptualize. It is essentially the physical blocking, reflecting and attenuation of sound waves and sound pressure level through the body of the headphones/hearing protection device.
If we are to think of earplugs, we can easily envision passive noise-cancellation. The earplugs, if properly worn, effectively close off the eardrum from the external air and, therefore, the sound waves in the environment.
The sound waves are either absorbed into or reflected off of the earplugs. Of course, earplugs do no cancel out all noise, especially in louder environments, but they do provide ample attenuation of sound pressure at the wearer's eardrum.
Snug-fitted earphones and in-ear monitors operate in the same fashion.
The same principle applies to earmuffs. The earmuffs completely surround the ear and do not let sound waves reach the eardrums via the air. Sound waves are reflected off of the outer shell of the earmuffs or otherwise absorbed by the softer parts of the headphones.
Closed-back circumaural headphones block external noise in the same fashion as earmuffs.
Here is a simple picture to illustrate passive noise-cancellation:
Passive Noise-Cancelling Headphone Examples
To learn more about passive noise-cancelling headphones, let's have a look at a few examples:
Sennheiser HD 280 Pro
The Sennheiser HD 280 Pro is a popular pair of prosumer-grade headphones. The closed-back circumaural (over-ear) design of these headphones yields great passive noise cancellation.
The Sennheiser HD 280 Pro is also featured in the following My New Microphone articles:
• Top 5 Best Moving-Coil/Dynamic Headphones Under $100
• Top 55 Best Closed-Back Headphones Under $100
• Top 5 Best Circumaural (Over-Ear) Headphones Under $100
Sennheiser
Sennheiser is featured in My New Microphone's Top 13 Best Headphone Brands In The World.
AKG K72
The AKG K72 is another pair of closed-back circumaural headphones that block out noise passively.
AKG
AKG is featured in My New Microphone's Top 13 Best Headphone Brands In The World.
Decibullz Custom Molded ES In-Ears
The Decibullz Custom Molded ES In-Ears is a great example of passive noise-cancelling earphones.
These earphones are produced with Decibullz propriety ES (easy-shape) system. They can be quickly shaped to the contours of your ears for a perfect, comfortable, secure fit that acts to block out noise better than looser fitting earphones.
What Is Active Noise-Cancellation?
Active noise-cancellation is much more involved than passive noise-cancellation. It utilizes active circuitry (requiring power) to effectively produce an “anti-noise” signal in the headphone driver that cancels environmental noise by means of phase cancellation.
An active noise-cancelling circuit in headphone design includes a microphone to effectively measure the external noise; an “anti-noise” circuit that delays the noise signal in order to cancel the actual noise; an amplifier that adjusts the anti-noise signal to match the actual noise level; and a summing amp that combines the intended audio with the anti-noise signal.
Here is a simplified illustration of active-noise cancellation:
There are three distinct active noise-cancelling circuit designs used in headphone design. They are:
Before we get into the different active noise-cancelling designs, let's have a look at the general (and commonly listed) pros and cons of active noise-cancelling headphones:
| Pros Of ANC Headphones | Cons Of ANC Headphones |
|---|---|
| Can help boost concentration | Can reduce your alertness to your environment |
| Improves phone conversations | Often ineffective to noise above 1-2 kHz |
| Can help protect against hearing damage due to lower listening levels and by attenuation loud noise | Active: relies on batteries |
| Can help with sleep | More expensive |
| On/off switch | Bit heavier |
| Improves audio clarity in noisy environments | May cause distortion to intended audio source due to anti-noise signal summation |
| Sensitive to electromagnetic interference (due to microphone) |
Feed-Forward ANC
Feed-forward active noise-cancelling works with a microphone placed on the outside of the headphones.
The microphone (typically an electret mic) effectively picks up the external sound and noise at the outside of the headphone body. This “noise signal” is then processed and summed with the intended audio at the headphone driver.
For more info on electret mics, check out my article The Complete Guide To Electret Condenser Microphones.
The mic captures noisy sound waves well before they reach the listener's eardrum, relatively speaking. Therefore, the active noise-cancelling circuit has ample time to process the mic's signal into an anti-noise signal.
These ANC systems amplify, delay and sum the anti-noise signal with the intended audio to produce appropriate noise-cancellation.
The time allotted to the feed-forward circuit allows for cancellation up to 1-2 kHz and an arguably better cancellation of transient noise.
The downside of feed-forward ANC circuits is that they have no way to self-correct if they're ever off in terms of cancelling important frequency bands. This is due to the microphone being placed on the exterior of the headphone ear cups and not being able to “hear” the sound that results from its effect on the signal.
The pros and cons of feedforward active noise-cancellation can be summed up in the following table:
| Pros | Cons |
|---|---|
| Simpler than feedback and hybrid systems | Active circuit relies on power to function |
| Great at reducing noise up to 1-2 kHz | Limited frequency cancellation band |
| Ample time to produce the anti-noise signal with proper phasing | No self-correcting |
| Does not cancel noise introduced within the ear cups | |
| Will actually introduce wind-noise to the signal when trying to reduce wind noise |
Note that each ear cup in a pair of ANC headphones will have its own active noise-cancelling circuit. The environmental noise will be different at each ear cup due to phasing and amplitude differences in the sound pressure level.
Feed-forward systems are designed to effectively cancel noise. However, there are some drawbacks. If the headphones are positioned incorrectly, the noise is coming in at an odd angle, or the headphones are misused in another fashion, the feed-forward ANC circuit may become ineffective. What's worse is that the feedforward circuit may even amplify the noise at some frequencies.
Another con, as we mentioned briefly, is that the feed-forward design will be ineffective against wind noise. The mic will pick up wind noise in an outdoor environment, while the passive isolation of the headphones will naturally block out the wind noise. In practice, this actually results in wind noise being added to the signal and getting heard by the listener since the anti-noise signal has nothing to cancel out inside the ear cup.
Feedback ANC
Feedback active noise-cancelling works with a microphone placed on the inside of the headphone ear cup.
The microphone (also an electret mic) captures the sound within the ear cups of the headphones. In other words, it picks up the noise within the earcup along with the actual sound of the intended audio signal.
With this design, the ANC circuit can constantly readjust its anti-noise signal to cancel the noise more effectively.
So the audio signal from the audio source is sent to the headphone driver, which converts it into sound waves. The feedback microphone is located inside the ear cup in close proximity to the driver.
Any difference between the audio source and microphone's audio could be considered noise. The ANC circuitry then produces a differential “noise signal” and offsets it so that it can be sent to the driver in opposite polarity as an “anti-noise signal.”
The source audio and anti-noise signal are summed together to cancel the noise within the ear cup without affecting the sound source. The microphone then picks up the new sound and is able to constantly adjust to provide the best anti-noise signal.
The mic effectively gets immediate feedback as to the effectiveness of the ANC circuit.
The pros and cons of feedback active noise-cancellation can be summed up in the following table:
| Pros | Cons |
|---|---|
| Self-correcting | Active circuit relies on power to function |
| Cancels inherent noise inside the ear cup | More complex and expensive than feed-forward systems |
| Wide frequency cancellation band | Ineffective above ~1 kHz |
| Effective at cancelling wind noise |
Again, each ear cup in a pair of ANC headphones will have its own active noise-cancelling circuit. This is because most audio is recorded in stereo, so the two drivers will produce different signals.
Unlike feed-forward systems, a feedback ANC circuit will remain fairly effective if the headphones do not fit perfectly. This is because the noise is measured within the ear cups.
However, some disadvantages of feedback ANC systems are that they have difficultly cancelling high-frequency noise. The short wavelengths reflect around the ear and ear cup in such ways that make it difficult for the stationary microphone to capture all different phasings of the high-frequency sound waves.
Though rare, an imperfect feedback ANC design may actually result in microphone feedback within the headphone. This is due to the close proximity of the mic and headphone driver. If the anti-noise circuit does not line the anti-noise signal up perfectly, feedback could ensue.
Similarly, ANC can accidentally filter out the sound waves of the intended audio if the processing circuits are not designed properly. This is particularly true with low-frequency long-waveform sounds which may result in a thinning of the headphone sound.
Hybrid ANC
Hybrid active noise-cancellation combines the two methods mentioned above with a microphone to the headphone earcup's interior and exterior.
Hybrid systems portray many of the pros of both feed-forward and feedback systems while mitigating some of the cons. The pros and cons of hybrid active noise-cancelling headphones can be summed up in the following table:
| Pros | Cons |
|---|---|
| Wide frequency cancellation range up to ~2 kHz | Active circuit relies on power to function |
| Self-correcting | Expensive |
| Cancels noise within the ear cup | |
| Effective at cancelling wind noise |
Again, each ear cup will have its own ANC circuit due to differences in the left and right audio channels and the variation in sound pressure level at the exterior of the two ear cups.
Adaptive Vs. Fixed Noise-Cancellation
Other terms you may hear regarding noise-cancelling headphones are adaptive and fixed noise-cancelling.
As the names suggest, adaptive noise-cancelling with adjust to the noise in the environment and attempt to cancel it out accordingly. Conversely, fixed noise-cancelling allows for an unchanging amount of noise-cancelling.
So, in other words, adaptive and fixed noise-cancellation are just ways of describing active and passive noise-cancellation, respectively.
Active noise-cancellation continuously measures the exterior noise with a microphone and uses this noise signal to affect its noise-cancelling. Passive noise-cancellation has no way of altering its noise-cancellation automatically based on the noise itself.
Note that feedback noise-cancellation is arguably more adaptive than feedforward due to its constant feedback loop of “listening” to its effect on the audio signal. Feedforward ANC is adaptive in the way that its microphone adapts to the changing environmental noise but is nowhere near as adaptive as feedback ANC.
Active Noise-Cancelling Headphone Examples
To learn more about ANC headphones, let's have a look at some examples:
Bose QuietComfort Series 35 II
The Bose QuietComfort Series 35 II is a popular pair of hybrid active noise-cancelling headphones. They are Bluetooth wireless, closed-back and circumaural in design and even have additional microphones for telephony.
The Bose QuietComfort Series 35 II is also featured in the following My New Microphone articles:
• Top 5 Best Wireless Headphones Under $200
• Top 5 Best Closed-Back Headphones Under $200
• Top 5 Best Moving-Coil/Dynamic Headphones Under $200
• Top 5 Best Noise-Cancelling Headphones Under $200
Bose
Bose is featured in My New Microphone's Top 13 Best Headphone Brands In The World.
Beats By Dre Solo Pro
The Beats By Dre Solo Pro is another pair of hybrid noise-cancelling headphones. Like the Bose model listed above, the Solo Pros are Bluetooth wireless and have a closed-back circumaural ear cup design.
Honeywell Howard Leight Impact Sport
The Honeywell Howard Leight Impact Sport is a pair of earmuffs that double as noise-cancelling headphones. They amplify ambient sounds up to a safe level of 82 dB while instantaneously cancelling out sounds above 82 dB.
An external 3.5mm audio jack allows these earmuffs to double as stereo headphones when connected to an MP3 player or other audio sources.
It has a Noise Reduction Rating (NRR) of 22 dB.
The Differences Between Passive And Active Noise-Cancelling Headphones
The differences between passive and active noise-cancelling headphones can be summed up in this short table:
| Passive Noise-Cancellation | Active Noise-Cancellation | |
| Requires Power | No | Yes |
| Cancellation Method | Blocked out through physical means | Cancelled via electrical means |
| Price | Lower | Higher |
For a more in-depth article on the specifics of passive and active noise-cancellation, check out my article Passive Vs. Active Noise-Cancelling Headphones.
Related Questions
Can you listen to music with noise-cancelling headphones? Many of the popular noise-cancelling headphones on the market are designed to cancel extraneous noise while listening to music. Note that ANC may naturally muffle the sound and actually thin out the audio in some circumstances but for the most part, it will improve music listening.
Related article: Do Noise-Cancelling Headphones Work With Or Without Music?
How do I remove background noise from a recording? Removing background noise from a recording can be tricky, though there are digital plugins/software that can help. Low-end rumble can be reduced by a simple high-pass filter above the frequency. Other de-noise plugins are available from UAD, Waves and many other software companies.
Related articles:
• Top 7 Best Noise Reduction Plugins For Your DAW
• 15 Ways To Effectively Reduce Microphone Noise
Choosing the right headphones or earphones for your applications and budget can be a challenging task. For this reason, I've created My New Microphone's Comprehensive Headphones/Earphones Buyer's Guide. Check it out for help in determining your next headphones/earphones purchase.
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