Active Noise Cancellation – A Detailed Study
As a user of earphones, or headphones, you are probably aware of the disturbance caused by random ambient sounds. It causes distortions and diminishes the clarity of the audio you are listening to. It muffles dialogues, weakens the bass and reduces the overall impact of the audio. This is where noise cancellation technology comes in. It is a method of producing a second sound that nullifies the intruding first sound. In an ideal scenario, the second inverted sound wave completely cancels out the first sound and you hear no sound at all. But, it is not so easy to accomplish, and you end up hearing at least some ambient sound along with the audio.
Let us deep dive into Active Noise Cancellation Mechanism & Recent Developments :
Active and Passive Noise Cancellation :
Noise cancellation can be of two types – active and passive. Technically speaking, all earphones and headphones provide a certain amount of passive noise reduction. Earphones have custom-made in-ear silicone ear buds that plug the ear channel and block outside sound. Headphones, on the other hand, have ear muffs made of special high-density foam with noise-absorbing properties. The foam obstructs noise, especially higher frequencies, and prevents it from permeating the ear cups. Also, most headphones have circum-aural ear muffs that surround the ears completely, blocking ambient noise and locking music in. This is the reason why headphones are so heavy and chunky – they have to pack in all the material to keep noise out.
Passive noise cancellation can lower invasive sounds by 15 to 20 decibels, not more. While it might be enough for indoor listening, outdoors it is likely to be ineffective. The cacophony of public places like streets, train stations and airplanes is too great for passive noise reduction to handle. For that, you need the added sophistication of active noise cancellation (ANC) technology which is a whole different ball game.
Active Noise Cancellation – How it works
Active noise-cancelling headphones have the same construction as passive noise-cancelling headphones, except they have an extra noise-cancelling circuitry. Before we go into the nitty-gritty of active noise cancellation, let us understand how sound works.
Sound is a pressure wave. It travels in alternating successions of compressions (crests) and rarefactions (troughs). Active noise cancellation involves producing a second sound wave that resembles the first sound wave in amplitude but is inverted. The new sound is called antiphase and it is 180ᵒ opposite of the first sound. The antiphase merges with the original sound and forms a new wave by a process known as interference. The two waves cancel each other out and you end up hearing no sound. This phenomenon is called destructive interference and it is the basis of active noise cancellation technology.
Sound devices that have active noise cancellation blend two sets of sound waves – one coming in from the outside and the other produced by the device. The two waves have identical frequency and amplitude, but their compressions and rarefactions are in the opposite direction. This means the compressions of one wave align with the rarefactions of the other wave and in essence cancel each other out. This is the mechanism behind destructive interference. The following illustration explains how the two sound waves merge.
Active noise cancellation gadgets like headphones combat ambient noise using state-of-the-art analog circuits with Digital Signal Processing (DSP). They use advanced algorithms to judge the characteristics and behavior of the invasive sound which may be a mixture of aural and non-aural sounds. The algorithm then generates an exact replica (antiphase) of the sound by phase shifting or inverting the polarity of the first sound. The antiphase is amplified, and a transducer produces a wave that matches the incoming sound wave in amplitude. The new sound mimics the outside noise and blends with it, in the process lowering its impact and interference. The result – less disturbance, better audio clarity.
Parts of an Active Noise-cancelling Headphone
A noise-cancelling headphone comprises of the following basic components.
The microphone is a tiny device placed inside the ear cup of the headphone. It picks up invasive sounds from the background that cannot be stopped through passive means. These sounds infiltrate the noise-obstructing barriers of foam, fabric and leather. Hence, they need to be actively countered, and the microphone’s job is to ‘listen’ to these sounds, and relay the information to the circuitry for necessary action.
The electronic circuit or transducer inside the ear muff uses DSP technology to generate the antinoise signal. It studies the input from the microphone, does a quick calculation of the desired antiphase and generates it before the noise reaches the ear. The antiphase has the same frequency and amplitude as the penetrating sound but is 180ᵒ out of phase. It is almost like a fingerprint of the incoming noise.
The antinoise signal generated by the circuitry is routed to the headphone’s speaker. It is played along with the soundtrack the user is listening to. The signal erases the noise without affecting the audio that’s playing. In other words, the antiphase has the ability to differentiate between the noise wave and the audio wave. This is how destructive interference counters noise inside an active noise-cancelling headphone.
Noise-cancelling headphones need energy to run their circuits. Normally, this power is provided by a rechargeable battery built into the device. It may be powered by USB or may need direct current from a wall socket.
The following illustration shows how a noise-cancelling headphone works.
Effectiveness of Active Noise-cancelling Devices
Active noise-cancelling machines reduce ambient sound by an additional 15-20 decibels over and above the 15-20 decibels reduced passively. They are especially effective in rooting out low frequencies that escape passive screening. They are adept at separating noise filtering in from the environment and the audio. The processor targets the noise wave while keeping the audio intact. All in all, active noise-cancelling devices reduce around 70% background noise, allowing the listener to enjoy the audio in peace.
Headphones with ANC can be used to fight fatigue. You can wear them even when you are not listening to anything just to block ambient noise. For instance, on your way back home from work, you can have them on to cut off the chaos and relax. Low frequency sounds typical of public places have an exhausting effect on the psyche. Prolonged exposure to them can tire you out physically and mentally. A good way to mitigate this would be to pop a pair of noise-cancelling headphones. Your mind will stay fresh and you won’t feel jaded.
Difference between Active and Passive Noise Cancellation
The fundamental difference between active and passive noise cancellation is that the former requires a power source while the latter does not.
Active noise control makes use of a noise-cancelling circuit to produce a second sound to counter the first sound. Passive noise control entails physical barriers in the form of around-ear ear cups, thick noise-isolating foam or in-ear ear plugs to block noise.
Active noise cancellation obliterates the disturbing sound and cancels it out. Passive noise cancellation merely reduces the volume or reach of the invasive sound using insulating material to block the sound.
Active noise cancellation mainly tackles lower frequencies like the drone of an aircraft. Passive noise cancellation counteracts higher frequencies like the sound of people talking, babies wailing, etc.
Challenges of Active Noise Cancellation
ANC is an emerging technology and has its own unique challenges. The biggest hurdle so far has been reducing audio loss. A side effect of ANC is that sometimes a tiny fraction of the audio also gets suppressed. There have been complaints of the audio sounding muffled and lacking in impact. Another issue is air pressure. Some headphone users have reported a change in air pressure inside the ear cups. This is despite the fact that there are vents for letting out air trapped in the cups. Currently, a lot of research and development is going on to come up with solutions to these shortcomings. Upgrades are being made to develop more effective and comfortable ANC gadgets.
Let us take a look at the latest research in this field.
Emerging Noise Cancellation Technology
The active noise cancellation technology discussed so far in this article involves a circuit producing a sound that mimics the noise. Now, researchers are working on a new kind of technology that makes use of wireless transmissions to tackle noise before it reaches us. This technique is based on the fundamental fact that wireless network signals travel a million times faster than sound waves. It harnesses the speed of wireless signals to improve the efficacy of noise cancellation devices.
Active noise-reducing headphones have inbuilt microphones that pick up background noise and relay the information to the DSP circuit which then generates the right antiphase to counter the noise. But there is a flipside to this method. Due to the close proximity of the microphone to the ear, the headphone has very little time to process the invasive sound, create the precise antinoise signal, and play it through the speakers before the noise hits the ears. Such a close computing deadline limits the effectiveness of noise cancellation and prevents the gadget from achieving its true potential. It also hampers its ability to screen higher frequencies. That is the reason why headphones have bulky ear cups. The foam in the cups absorbs the higher frequency bands that ANC cannot erase.
In the new wireless network technique, the goal is to do away with the sponge altogether and replace it with a wireless forwarding system that will provide early information about sounds that are about to reach us. This will give the ear piece a comfortable head start and allow it sufficient time to do the necessary calculations. However, according to Sheng Shen, a Ph.D. scholar in electrical and computer engineering at the University of Illinois at Urbana-Champaign, it is easier said than done. He says that getting rid of the sound-absorbing material is a major hardware challenge and finding an alternative is an uphill task.
Shen along with his colleagues are currently working on a noise cancellation system of physically separated parts connected via wireless network. The system, called MUTE, comprises of external microphones placed near the source of the sound. Since wireless signals travel faster than sound, the sound picked up by the microphone reaches the circuit well in advance giving it enough time to develop the antiphase. This means the MUTE system has information on ‘future sounds’ and has several milliseconds – which is far greater than the tens of microseconds conventional ANC headphones get – to do the math and devise the right antinoise signal in time.
The advantage of this lead means the DSP processor has more time at its disposal to work on other complex sounds. Early lab test results indicated that the new method might potentially outshine the scope of high-end commercial ANC headphones currently sold in the market. The initial version of the MUTE system is a simple contraption consisting of basic microphones, DSP circuits and speakers. But judging from the prospects of the technology, the engineers hope to develop MUTE into a wearable device. They envision a lightweight conical speaker fitted into the ear canal held in place by a behind-the-ear frame. The contraption would not cover the ear like normal headphones nor would it completely plug the ear passage like earphones. The ear passage would be left mostly free, ensuring air circulation and balanced air pressure. Right now MUTE can handle sounds between 0 to 4 kHz. Researchers are working on developing a faster digital signal processor that could go beyond 4 kHz.
MUTE, however, has its limitations. The biggest one is that it only works indoors. The other is that it can only screen a single dominant sound source at a time. The engineers aim to increase the potential of the system to multiple sound sources. For that they will need multiple wireless microphones at different locations, each mic serving as an Internet of Things (IoT) relay. They will also need to devise source separation algorithms for differentiating between different sounds. The dependence on multiple external microphones also affects MUTE’s portability. It might therefore be better suited for stationery settings with the mics permanently fixed to their positions. This could be a good way to combat noises in homes and offices during sleep, study, work and other activities that require a silent environment.
Devices that employ Active Noise Cancellation
Noise cancellation is used in a number of modern consumer, medical and industrial gadgets. Here’s a look at the most widely used applications.
One of the earliest applications of noise cancellation was in the aviation sector. It was used to reduce cockpit and cabin noise for the safety and comfort of the crew and passengers. Pilot headphones have the most advanced kind of ANC technology. It helps the pilot hear instructions clearly over the radio, something that is very crucial and has zero room for error.
Hearing instruments employ Speech Priority Noise Reduction to separate speech from background noise. Noise is singled out and acted upon to prevent it from interfering with the speech. It helps the user understand speech better as the distracting and confusing sounds are removed. Hearing aids use noise cancellation to improve signal-to-noise ratio, listening comfort, listening effort and voice clarity.
Headphones and earphones
Headphones and earphones are the most commonly seen noise-cancelling gadgets. They use the technology to elevate the audio playing through their speakers. They make use of soundproofing material as well as active noise cancellation to combat disruptive environmental sounds. Earphones generally have in-ear noise-cancelling earbuds that block the ear canal to cut off noise. Headphones have large ear muffs that cover the outer ear to lock the audio in and noise out. These are passive means of noise reduction. To actively cancel noise, earphones and headphones rely on the standard components namely microphone, ANC circuit, speaker and battery.
Smart phones are another category of consumer appliances that use active noise reduction. In addition to the main microphone that picks up the user’s voice, the phone has a few more which it uses to tackle background sounds in noisy environments. The data relayed from these microphones is applied to cancel out noise coming in from the surrounding. The same data is used to erase the noise that’s picked up by the voice microphone the user speaks into. The information gathered by the ancillary microphones is also used to reduce interference from howling winds and adjust call volume based on the level of chaos in the environment.
Smart home assistants
Multi-microphone based ANC technology has another up-and-coming application – smart home assistants. Gadgets like Amazon Echo and Google Home are fitted with several omni-directional microphones for better speech detection from a distance. It suppresses the sounds in the room so the user’s voice commands register more clearly.
Noise cancellation is gaining mileage in an increasing number of sectors. A lot of funding is being funneled into its advancement. Whether it’s music, entertainment, aviation, defense, consumer electronics or medical, ANC technology has proved to be highly resourceful. It is definitely a technology to watch out for in the coming years.
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