Passive Vs. Active Noise-Cancelling Headphones

If you've been searching the web or looking around your local shop for headphones, you've likely heard about active noise-cancelling (ANC) headphones and perhaps about the lesser-marketed passive noise-cancelling (PNC) headphones. Whether you're in an office, on a plane, or in a noisy apartment, noise-cancelling headphones can provide the isolation required to focus on the audio and not the exterior environment.

What are the differences between passive and active noise-cancelling headphones? PNC headphones hinder external noise from reaching the eardrum by physically blocking sound waves similar to earplugs. ANC headphones require power and act to produce and introduce an anti-noise waveform into the overall audio signal that virtually eliminates the exterior noise at the listener's ear.

One of these noise-cancelling methods is pretty simple, while the other is quite the feat of engineering. In this article, we'll discuss passive and active headphone noise-cancellation in greater detail, noting the differences and going over the pros and cons of noise-cancellation in general.

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What Is Passive Noise-Cancellation?

Passive noise-cancellation is actually quite simple to explain. It is the physical blocking of sound from reach the ear canal.

Earplugs are a great example of simple and effective passive noise cancellation. They effectively stop sound waves from reaching our eardrums and thereby reduce our sensitivity to environmental noise and protect our hearing from excessive sound pressure levels.

Earplugs act to reflect and absorb the sound waves at our ears and reduce their effects on our hearing.

Many headphones are designed to do the same thing. Let's have a look at a simple illustration:

The headphones in the above picture are the AKG K72.

As we can see, the sound waves from outside the headphones hit the ear cups and bounce off. Alternatively, the sound waves could hit the earcup cushion and be absorbed.

This passive noise-cancellation happens in closed-back circumaural (over-ear) headphones and in some well-fitted earphones (like custom in-ear monitors).

Open-back and supra-aural (on-ear) headphones are inefficient at blocking noise passively. The same goes for many earbuds on the market.

Note that high frequencies are relatively for the headphone body to reflect and/or absorb compared to lower frequencies.

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What Is Active Noise-Cancellation?

Active noise-cancelling is much more complex than its passive counterpart.

Let's begin by stating that ANC is, well, active! That means that ANC systems require power to function. This power is provided by batteries (either external/replaceable batteries or built-in rechargeable batteries).

We'll get to the active components in short order. First, let's have a look at a simple illustration of active noise-cancellation in a headphone:

The headphones (or half thereof) in the picture above are the Audio-Technica.

As seen in the above picture, a microphone picks up noise and converts it into an audio signal. This audio signal is then essentially phase-flipped by the ANC circuit to become an “anti-noise” signal. The headphone driver turns the anti-noise signal back into sound to cancel out the environmental noise.

The driver also produces sound based on the applied audio signal from the connected audio source.

Sounds complex?

Let's explain it further by looking into the three types of active noise-cancellation:

• Feed-forward
• Feedback
• Hybrid

Feed-Forward Active Noise-Cancellation

Feed-forward ANC headphones are designed with microphones to the exterior of their earcups/housing. Environmental noise is picked up at the exterior of the headphones before it reaches the listener's ear. Therefore, the derived anti-noise signal must be processed and delayed accordingly.

The anti-noise signal is effectively a time-delayed noise signal that is lined up in opposite polarity to nullify the actual environmental noise. Once properly set up, it is combined with the intended audio signal to drive the headphone speaker.

Capturing exterior noise before it hits the eardrum allows ample time for the ANC circuit (the amplifier, delay/phase shift circuit and summing amp) to produce the proper anti-noise.

The time between capturing the sound and the signal reaching the ear allows high-end circuits to cancel frequencies up to 1-2 kHz. This is something that feedback ANC systems, which we'll get to shortly, have difficulty doing.

Furthermore, the additional time helps to cancel transient noise better.

The main disadvantage of feed-forward systems is that they have no way of self-correcting. Because the mic is outside the body of the headphones, it does not detect the resulting sound from the headphone speaker/driver to know exactly how the ANC circuit's “anti-noise” signal is doing.

So if feedforward ANC headphones are positioned incorrectly, the cancellation may do more harm than good.

Another weak point in feedforward design is wind noise. While the passive noise-cancellation inherent in ANC headphones will block out much of the wind noise in outdoor environments, the outside mic won't. With wind noise in the anti-noise signal and no way of self-correcting, reverse-phase wind noise will effectively be introduced to the headphone signal.

Feedback Active Noise-Cancellation

Feedback ANC headphones have their microphones placed within the headphones. The microphone picks up the noise within the ear cups and the sound produced by the driver. It is also much closer to the ear than if it were outside the ear cups.

In this system, the anti-noise signal is being captured and processed instantaneously, and constant self-correcting is taking place.

Basically, the mic signal is continually being compared to the intended audio signal from the sound source. The differences between these two signals are considered “noise,” and the ANC produces an anti-noise signal to cancel the noise via phase cancellation.

The source and anti-noise signals are summed together at proper levels to produce a clean, noise-free signal.

Compared to feed-forward systems, feedback ANC systems work on a broader range of frequencies. Feedback ANC is also more effective if the headphones are not worn correctly since the system is practically independent of the outer environment.

A con of feedback ANC systems is poor high-frequency cancellation due to the short wavelengths of these sounds and the way in which they reflect/bounce around the space between the ear and headphone cup.

Poorer feedback ANC designs may actually result in feedback between the microphone and headphone driver similar to how microphones feedback with loudspeakers and monitors, though this is rare.

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.

Hybrid Active Noise-Cancellation

A hybrid ANC system, as the name suggests, combines principles of feed-forward and feedback active noise-cancellation. This requires multiple microphones on the inside and outside of the headphone body.

Hybrid systems are much more complex to design perfectly and do cost more. Still, high-quality hybrid systems do a great job of combining the benefits while cancelling (pun intended) much of the cons between the two systems.

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The Differences Between Passive And Active Noise-Cancelling Headphones

The differences between passive and active noise-cancelling headphones can be summed up in the following 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

This short table may seem simple, but it sums up what we've discussed thus far in the article.

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Similarities Between PNC and ANC Microphones

A major similarity between passive and active noise-cancelling microphones should be discussed here. This similarity is in form factor.

Closed-back circumaural (over-ear) headphones are the best for passive noise-cancellation when fitted correctly. This same form factor is also used for active noise-cancelling headphones. Remember that active noise-cancellation builds upon the attenuation provided by passive means.

There are some active noise-cancelling earphones on the market. These earphones are designed to seal the ear canal and block sound waves from reaching the eardrum. In this way, the ANC circuit once again builds upon the passive attenuation of the earphones.

This may seem obvious but is worth a mention in this article.

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Pros & Cons Of Noise-Cancelling Headphones

Noise-cancelling headphones are designed to provide an excellent feature: noise-cancellation. That is the biggest advantage of active noise-cancelling microphones.

However, with any strengths comes weaknesses. Let's go over the pros and cons of noise-cancelling headphones.

We'll start with the best type of passive noise-cancelling headphones: the closed-back circumaural type.

Pro & Cons Of PNC Closed-Back Circumaural Headphones

Circumaural closed-back headphones, when fitted and designed correctly, provide the most attenuation to external sound.

Other pros include:

• Extra comfort (depending on pads) vs. supra-aural (on-ear) headphones and earphones
• Less bleed (sound escaping) to the environment
• More bass response (pro or con)

However, their cons include:

• More bass response (pro or con)
• Heavier weight
• Narrower stereo field
• Trap heat around the ears
• Typically more expensive

Pro & Cons Of PNC Sealed Earphones

When fitted and designed correctly, earphones that seal the listener's ear canals also provide attenuation of external sound. This is often the case with in-ear monitors.

Other pros of IEMs or sealed earphones include:

• Lightweight (compared to headphones)
• Less bleed to the external environment
• Clearer sound due to the coupling of the IEM diaphragm and the eardrum

And cons include:

• May produce unwanted pressure in the ear canal
• Should be custom fitted for performance and sanitary reasons
• Expensive

Now let's talk about the pros and cons of active noise-cancelling.

Pro & Cons Of Active Noise-Cancelling Headphones

The pros and cons of active noise-cancellation can be summed up in the following table:

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)

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Related Questions

Is active noise-cancelling worth it? ANC circuits cost money to incorporate in headphone designs and, therefore, ANC headphones are typically more expensive. Personal needs vary, so ANC may be worth it some and not to others. Note that ANC does not perfectly shut out all noise, and it may cause interference in the intended audio. It's also worth noting that ANC circuits can be turned off.

For tips on determining whether a particular pair of headphones is worth it to you, check out my article Are Expensive Headphones (Or Cheap Headphones) Worth It?

Can noise-cancelling headphones be used without music? Yes, because the noise-cancellation circuits in headphones are independent of the audio source. The ANC circuit will still capture exterior noise and produce an “anti-noise” signal to be played by the driver. Note that the cancellation will likely sound less effective since there's no music to aid in drowning out external noise.

To learn more about active-noise cancelling and music, check out my article Do Noise-Cancelling Headphones Work With Or Without Music?