How active noise canceling headphones work. How does noise cancellation work in modern headphones?

Soundproofing is undoubtedly a useful thing. If there is a highway near your house, along which cars scurry back and forth day and night, then it is impossible to live in constant noise. And modern double-glazed windows help out in this situation, since they dampen most sounds. However, it turns out that living in absolute silence is an unbearable torment. If only there was a way of such intelligent filtering of sounds - when all loud sounds are extinguished - the noise of car horns, rumbling from a neighboring construction site - and the rustling of tree leaves, the chirping of birds would pass into the room.

Perhaps this problem will someday be solved by the system active noise cancellation. The idea of ​​acoustic engineers from Berlin Technical University consists of installing compact loudspeakers between the glass of window frames and extinguishing sounds penetrating from outside inside the frame with the same sounds, but emitted in antiphase. It is expected that their design will find application not only in residential and office buildings, but also in airplanes and cars.

Korean scientists have developed computer technology active noise suppression inside the car. Their version of the system provides a noise reduction of about 6 decibels, which is noticeably better than all alternative approaches. In fact, a sound wave is a wave of compression and rarefaction of air. If you use speakers to create waves of the same frequency and amplitude, but of opposite phase, they will cancel each other out. Noise in a car mainly comes from the noise of tires on the road surface, transmitted through the suspension and body. The difficulty in suppression systems lies in the need to position the speakers so that the sound wave from them interferes with the surrounding noise precisely in the places where people are located, and computer system managed to react to changes in ambient sound. Korean specialists placed four vibration sensors on the suspension, and two speakers on the floor behind the front seats. On-board computer analyzes the signal from vibration sensors and generates a signal sent to the speakers in order to minimize noise in the area around the head of the driver and the passenger sitting in the front seat.

Active noise cancellation reaches good results, in the presence of a point sound source, and when the system is located in the path of the point receiver. Therefore, the ideal result is achieved by a system located... in the headphones.

Headphones with NoiseGard system have built-in electret microphone capsules and circuitry feedback. The sound received by the headphone microphones contains both background noise and acoustic sound. The strongest noise interference occurs at low frequencies, so the midrange and high frequencies– which make up much of human speech – are filtered out of the signal. Then electronic circuits process the remaining ambient noise, reverse its phase 180°, and mix this phase-inverted noise with the original ambient noise. At the same time, the level of unwanted noise is significantly reduced, but the compensation does not affect audio frequencies human speech, and the clarity of their reproduction increases.

Passive noise canceling headphones are effective at canceling noise at mid and high frequencies, but their effectiveness decreases sharply in the low range. However, in closed-type headphones, noise reduction using NoiseGard technology, together with passive noise cancelers, reduces the noise level by more than 25 dB in the frequency range from 25 to 500 Hz. The total attenuation resulting from active and passive noise reduction is about 30 dB over the entire audio range.

A noise reduction of 10 dB is subjectively perceived as reducing the sound by half. Subsequent reductions in the noise level by 10 dB again result in 50% suppression of unwanted noise.

As technology develops, we can expect that natural sounds of nature will be heard in our apartments, and man-made noise will be cut off. IN smart home we can adjust the quality of noise reduction at our own discretion. Technology is at the service of comfort.

Today, the market offers a variety of headphone models, among which you can choose from cheap Chinese models to expensive products. Moreover, their cost largely depends on the technical component. Most closed-back headphones Active noise reduction technology is used. Rumor has it that it causes headaches when listening to music for a long time.

A little history

It is worth considering the history and principle of operation of the active noise reduction system. Music allows a person to relax, cope with strong emotions, it accompanies a person throughout his life. Manufacturers began to think about portable music in the middle of the last century, so in 79 Sony created a cassette player. Within a few years, this device was recognized as the company's greatest invention. In 84, the first CD player was created, and already in 98, MP3 players became available to users, which are still used today. The development of this industry cannot be characterized without mentioning headphones, because they provided good and high-quality sound.

The active noise reduction system began its history in the forties of the last century.. It was then that inventor Paul Luge patented a study that described the principle of operation of sine waves. In '50, Lawrence Vogel used a noise suppression system in a helicopter and airplane. With headphones this system began to be used in '57, then Willard Meeker was able to cut off noise from 50 to 500 Hz. Already in the 80s, headphones for aviation began to be produced, they used this technology.

How does the noise reduction system work?

If we consider the operating principle of ASP, experts describe it quite simply. Every separate sound represents a wave, from which we can conclude that it can be reflected. And this inverted wave will completely cancel out the original one. If we take the example of headphones that use the system, microphones will be located inside. They are designed to measure the level of noise generated. Therefore the resulting sound signal is converted into an inverted wave, after which it is transmitted to the main speaker. Here, antiphase to noise functions, while the music is not damaged as a useful signal, due to this the noise is removed.

Effectiveness of the active noise cancellation system

First of all, it should be said that such a system does not work well over the entire frequency range, but in the interval from approximately 100 to 1 KHz. On this site good models headphones can provide noise reduction down to -30 dB. In the lowest sound range, up to 100 Hz, a person perceives sound vibrations not so much with the ears as with the body, and headphones, no matter what level of compensation they provide, will not save you from noise.

Above 1 kHz the system may even add a little noise. However, this is not a street hum, but a uniform, calm “hissing”, and also very quiet -
It can only be heard in a very calm environment.


In the range of 100-1000 Hz, subject to high-quality implementation, “noise reduction” is quite capable of significantly reducing sound pressure environment. With good noise-canceling headphones, it’s quite possible to feel like you’re in a quiet, deserted room while on a noisy street.

IN selected models The “noise reduction” works truly amazingly. This is, for example, the latest Bose line and older Sony models, especially the MDR-1000x: you put on headphones and at first you refuse to believe your ears.

Models such as Sennheiser Momentum 2.0 Wireless, Parrot Zik, and all kinds of Beats do a very good job of cutting out noise. At the same time, in many budget “no-name” options, “noise reduction” can be present only nominally, and the effect of its work tends to zero.

Why does my head hurt?

Many people believe that using such a system makes the music better and the listening experience more comfortable. But it is not so. People have begun to actively use such devices, as headphones with active noise reduction systems are becoming more and more popular. At the same time, the market offers products from different price categories, among them there are the usual plugs and full-size models. You need to understand that technology does not stand still, the system is being improved, and the sound quality of music is constantly improving. Manufacturers are also trying to promote devices with this technology by making various marketing decisions. But some buyers are forced to refuse such a purchase, despite the sound quality. This is due to the fact that they do not tolerate active noise cancellation. For some people, the body reacts to the absence of extraneous sounds and the crystal clear sound of music with a headache. The headphones seem perfect, but this effect can occur for several reasons.

1. The effect of rocking on a ship. If you are familiar with seasickness, if you have a hard time being on ships, then you may find yourself in the category of people who cannot tolerate ASN (active noise cancellation system). You may only represent 7 percent of all users, but chances are you're one of those people. The idea is that when you use these headphones, you are deceiving your brain, just like when you are on a ship. You walk down the street and see passers-by, noisy traffic, a sharp wind. The organs of vision transmit information to the brain that you are outside, but the organs of hearing do not transmit noise due to the use of the system. This is why the brain is confused, which is why malaise and headaches appear.
2. Monotonous stimulus. Having understood the principle of operation of the system, it is worth emphasizing that for every action there is a reaction. As a result of the system's operation, manufacturers are able to eliminate noise and allow the listener to be completely immersed in the music. But one must take into account the following feature: in this case the organs of sound will be affected by monotonous sound waves. A human brain configured exclusively to accept a variety of sounds. It is worth mentioning the peculiarities of the nervous system, which can react negatively to such pure sound. Due to the characteristics of the body, nausea and headaches may occur when using the system. However, it is impossible to predict who is predisposed to such consequences and who is not.
3. Silence is not the key to peace. Well-known manufacturers test the devices they create in special organizations. Research is carried out in special rooms, so-called absolutely quiet rooms. Almost everything in the rooms is blocked extraneous sounds. This creates ideal conditions for testing equipment. But experts are of the opinion that absolute silence does not contribute to a calm state of the body.

   If a person finds himself in such a room, he will be haunted by a feeling of anxiety. Within a few minutes he will begin to hear the squeak of a mosquito and the rumbling of his own stomach. Just five minutes of being in such conditions is enough for the body to go into a state of anxiety and restlessness. Studies have shown that a person cannot spend more than 45 minutes in such a room. While using the systems does not create absolute silence because music is playing, the principle of operation of the system affects the concept of absolute silence. After all, when using such headphones, a person hears only music. This is why people may experience headaches and nausea when using devices.

4. Negative bass effects. When creating music, all frequencies are divided into high, medium, low. The system used in headphones is designed to eliminate all low frequencies, they represent the noise of the street. There is a risk in the system of “suppressing” a useful sound signal, that is, music. Sound wave, which is created in antiphase, is saturated with low frequencies. As is known, they can affect the liquid, causing vibrations; accordingly, they can also affect the vestibular apparatus, which is filled with a special liquid - endolymph. For this reason due to powerful bass At a certain frequency, headaches may occur.
5. Design flaws can also cause headaches. If the headphone arch is too tight, it can cause irritation and headaches. This is a common problem over-ear headphones, which does not specifically apply to the active noise reduction system. The products must be held on the head, so they should not be too weak, otherwise they will not hold on. But they should not be too tight, because constant pressure may cause increased blood pressure. After just a few hours of listening to music in such headphones, your head will begin to hurt, and you may also feel nauseous.

What to Consider When Buying Active Noise Canceling Headphones

These headphones have a number of advantages that make them unique and modern. This is both comfort when listening to tracks and high level sound detail. At the same time, a safe volume level is observed that does not adversely affect the health of the hearing organs. But all these points are not a guarantee that the body will respond positively to ASP. If you don’t want to complain about a headache, you should first try the headphones of friends or acquaintances. This will eliminate the negative impact of the system on your body, on general state, this will require listening to music on headphones for about 2.5 hours.

What's the result?

Active Noise Canceling Headphones - handy tool for comfortable listening to music in a modern city. They are well suited for those who are used to not parting with their favorite radio or playlist, but at the same time experience discomfort from “droplets” in the ears. They will also appeal to those who work in noisy environments, walk a lot on the street or ride the subway. At the same time, you don’t have to constantly listen to music - you can simply turn on the noise reduction system and find yourself in silence, while being in the middle of a noisy metropolis.

In general, active development is predicted for the wireless headphone segment, and failure large manufacturers smartphones from a 3.5 mm audio jack should significantly contribute to this. “Noise reduction” in such conditions can become a standard option for high-quality wireless headphones. While the implementation similar system in a device that a priori has electronic filling and batteries, does not look too expensive

Airlines typically provide business class passengers with a “welcome kit,” which includes earplugs. Indeed, the monotonous hum of powerful aircraft engines can be unbearable; it prevents you from falling asleep or enjoying your favorite music. No plug headphones (or even closed ones) can cope with it - passive noise isolation does not do a very good job of preventing the spread of low-frequency sound that is characteristic of both airplanes and other modes of transport. But headphones with active noise cancellation can come to the rescue.

Bring down the noise

The principle of active noise cancellation itself has been known since the 1930s, but the first working prototypes, intended for airplane and helicopter pilots, appeared in the late 1950s. True, they turned out to be too cumbersome and were not widespread. It wasn't until the late 1980s, when electronic components became small enough, that Bose made the first practical active noise-canceling headphones.

The idea behind active noise cancellation is quite simple. Sound is a wave of rarefaction and compaction, and where there is a wave, interference can be used. By creating an inverted wave (exactly out of phase) in the right point(in the ear, as close to the ear canal as possible) and at the right time, you can completely extinguish the original one. This is exactly how the active noise reduction system works: there are microphones inside the headphones that measure the level background noise. This signal is inverted and transmitted to the speaker, which ensures the appearance of a wave in antiphase to the noise (when playing music, you also need to take into account the “useful signal” - subtract it from the measured noise).

The main problem with active noise reduction systems is related to inaccurate phase matching. Even a small phase shift results in serious distortion. Therefore, active noise reduction works well with monotonous and low-frequency noise (the wavelength of low-frequency sound is long, and therefore the phase difference is small) and poorly with high-frequency and sharp irregular noises - screams, speech, music. The latter have to be dealt with using passive noise insulation. Often in headphones with active noise reduction you can hear a quiet hiss - this is precisely a consequence of the inaccuracy of the phase shift of high-frequency noise.

No confusion

The creators of headphones with active noise cancellation also sought to get rid of the ever-tangling and inconvenient wires. All these devices work on Bluetooth protocol, which allows you not only to transmit high-quality stereo sound (although, of course, audiophiles will not agree with this) using the A2DP profile ( Advanced Audio Distribution Profile) and new improved codecs AAC and AptX, but also control the player of a mobile device (for this, the AVRCP profile (Audio/Video Remote Control Profile) is used). Headphones are also wireless headsets, supporting Hands-Free Profile (HFP) and Headset Profile (HSP), which allows you to make and receive calls when connected to smartphones. You just need to take into account that noise reduction only works in one direction, that is, your interlocutor will hear all the noise around you. By the way, all headsets in our test are equipped with NFC identification, so connect them to mobile devices that support this technology is very simple - just touch the point with the NFC icon on the headphones with the same smartphone.

And that's all wireless headphones are equipped with wires - according to airline rules, use wireless devices prohibited during the flight. Just plug the wire into the corresponding headphone jack, and Bluetooth will turn off automatically. In addition, most models, even when the batteries are completely discharged, continue to work like regular wired headphones.

How we tested

A smartphone was used for the test Samsung Galaxy Note 3 (SM-N900). Sound processing in headphones (equalizer, effects) was forcibly disabled using proprietary utilities or control keys. The player was set to the Flat equalizer profile. The headphones were tested in wireless and wired modes. Operating time is based on wireless music listening with noise reduction enabled.

Nokia Purity Pro by Monster (BH-940)

Bluetooth: Ver. 3.0 NFC: yes Operating time: 24 hours Noise reduction: 6/10

Ergonomics? When unfolded they turn on automatically. Sensors turn on noise canceling as soon as you put the headphones on and automatically turn them off a few minutes after you take them off. Controls - mechanical buttons. In general, except small size buttons, the headphones did not cause any complaints, everything is quite convenient. For wire connection, a standard 3.5 mm three-pin connector is used.

Sound I would like to call the sound spectacular: the bass is elastic and intelligible, the drums are biting, like a slap in the face, the voices are clear, without distortion. Increasing the volume does not cause distortion. Unfortunately, all this works only while the headphones are connected to the player by wire. When playing via Bluetooth, interference appears, and the crystal brightness gives way to a primitive “blurredness”. A good compromise is a wire for home, Bluetooth for a noisy street.

Pros: comfortable ergonomics; excellent sound over the wire; big time battery life

Minuses: lack of a proprietary application for management and configuration; lack of battery charge indication (on a smartphone or headphones); average level of noise reduction

Sennheiser MM 550-X Travel

Bluetooth: Ver. 2.1 NFC: yes Operating time: 8 hours Noise reduction: 5/10

Ergonomics This is one of two models in our review that have a dedicated audio pass-through button. When pressed, the speakers switch to transmitting sound from external microphones. The quality of sound transmission in headset mode is frankly poor, since the microphones are too sensitive and transmit even the slightest noise, and they are audible not only to the interlocutor, but also to you (noise reduction does not work).

Sound These headphones have very high quality speakers. For example, cymbals sound more natural on them than on any other, but only if this moment Only the cymbals play. But as soon as the musicians give it a blast, the sound is filled with distortion in such quantities that it becomes difficult to recognize the singer’s voice. It’s a pity, but the headphones from the famous company demonstrate complete impotence when listening to almost any music, both wirelessly and with it.

Pros: good ergonomics; built-in surround sound processor; removable battery

Minuses: poor sound quality (both as headphones and as a headset); lack of a proprietary application for management and configuration

Creative AURVANA Platinum

Bluetooth: Ver. 3.0 NFC: yes Operating time: 12 hours Noise reduction: 8/10

Ergonomics The headphones have three noise reduction modes: “Airplane”, “Street” and “Office”, they are switched manually (from other manufacturers - automatically). They are comfortable as a headset, although the microphone placement may not be the best - the interlocutor heard interference from surrounding noise. It's also worth noting that noise cancellation turns on and off independently of the headphones, which can drain your battery quickly.

Sound The developers wanted to transport the listener to an imaginary listening room, secluded, safe, and cozy. Electronics are always involved in sound processing (with or without wires), filling it with a slight reverberation to simulate space. This approach is good for acoustic blues or chamber pieces. Music with a powerful rhythm section is drowned in loud, inaudible bass, “blurred” by reverberation. The sound is interesting, but not for everyone.

Pros: good noise reduction (both passive and active)

Minuses: sound quality is average; buttons are difficult to find; lack of a proprietary application for management and configuration

Sony MDR-ZX750BN

Bluetooth: Ver. 3.0 NFC: yes Operating time: 13 hours Noise reduction: 6/10

Ergonomics The ear pads of the headphones are small in size, so it would be more correct to classify them not as closed, but as over-ear. However, they are in no way inferior to their competitors. The control buttons on the bottom end of the headphones are convenient, although they are not always easy to find. The voice quality in headset mode is excellent. Noise reduction has three modes, which turn on automatically after analyzing the surrounding sound environment. You can turn on noise reduction manually, but only in “street” mode.

Sound Wow! With a small-looking driver, these headphones are an uncompromising choice for bass players. The built-in amplifier is configured for powerful, loud, but very intelligible bass. At the same time, high frequencies are reproduced quietly and very carelessly. Sony can be safely recommended to fans of dance music, electronics, hardcore, as well as those who want to memorize the parts of Duff McKagan or Flea. There is no point in connecting a wire - without a built-in amplifier, the headphones are practically powerless.

Pros: good wireless sound; good speech transmission as a headset

Minuses: buttons are difficult to find; lack of a proprietary application for management and configuration

Parrot Zik

Bluetooth: Ver. 2.1 NFC: yes Operating time: 6 hours Noise reduction: 8/10

Ergonomics? Philippe Starck design, white leather and gold plating. And the noise reduction level is very good. For control, the manufacturer left two small mechanical buttons recessed into the body - power and noise reduction; other commands are issued using touchpad. It's actually not very convenient. The sensors automatically pause the music when you take off your headphones. When used as a headset, the interlocutor can clearly hear the surrounding noise, despite the many microphones, which, in theory, should neutralize this effect.

Sound The sound of these headphones is tempting to call unnatural. The built-in amplifier, which does not turn off, enhances the highest and lowest frequencies, muffling the mids. This is a long-known way to make the sound a little more cheerful, but a sophisticated listener cannot buy this trick. However, Parrot also has strong point: This is the only model that is completely free of interference when listening via Bluetooth.

Pros: excellent noise reduction; convenient utility for management and configuration

Minuses: short battery life; average sound quality; uncomfortable touch control

Samsung Level Over

Bluetooth: Ver. 3.0 NFC: yes Operating time: 15 hours Noise reduction: 8/10

Ergonomics Two mechanical buttons, power and noise reduction, the rest is controlled using the touch panel, which, in our opinion, is not very convenient due to vulnerability from accidental touch. The model works well as a headset. There is a proprietary utility for managing headphones with many options for customizing and processing sound. For some Samsung smartphones Galaxy available additional features (voice control). By the way, this is the only model in the review that has a hard case included.

Sound This model impresses with its power reserves. She can easily throw a disco even in a subway car. It has a catchy sound, although less honest than Monster. Adding to the fun is the slightly enhanced upper mids, which add punch to the drums. Overall, this is a universal, comfortable, well-protected model for all occasions. There is no point in using a wire; without “safety electronics” the sound of the model is much duller.

Pros: excellent noise reduction; convenient utility for management and configuration; good sound; hard case included

Minuses: inconvenient touch controls

Plantronics Backbeat Pro

Bluetooth: Ver. 4.0 NFC: yes Operating time: 24 hours Noise reduction: 7/10

Ergonomics The volume is adjusted using a ring (it’s hard to imagine a more intuitive way). It's hard to miss the control buttons. Sensors automatically pause when you take the headphones off and resume playback when you put them on, gradually increasing the volume. This is the second model in the review that has a dedicated pass-through button. But it works in a very interesting way: the music does not turn off completely, but is only muted, the speakers switch to external microphones, and the quality of sound transmission is very close to “live”.

Sound The element of these headphones is complex arrangements. Enhanced high frequencies enhance the clarity of background instruments. It's interesting to listen to them, the volume reserve is pleasing, although the insufficiently powerful amplifier produces unintelligible, albeit loud, bass. Headphones are suitable for lovers of jazz, blues and classical music. Listening to rock and electronics on them is a bit boring. We did not feel any difference between listening with and without wire.

Pros: excellent ergonomics and controls; good sound; battery charge indication; long battery life

Minuses: A hard case would be nice to have included

This example demonstrates the use of adaptive filters to attenuate acoustic noise in active noise reduction systems.

Active noise cancellation.

Active noise control systems are used to attenuate unwanted noise propagating through the air using electroacoustic devices: measuring devices (microphones) and signal exciters (speakers). The noise signal usually comes from some device, such as a rotating machine, and it is possible to measure the noise close to its source. The purpose of an active noise reduction system is to create an "anti-noise" signal using an adaptive filter that will attenuate noise in a specific quiet area. This problem differs from conventional adaptive noise reduction in that: - the response signal cannot be immediately measured, but only its attenuated version is available; - when adapting, the active noise reduction system must take into account the secondary signal propagation error from the speakers to the microphone.

The tasks of active noise reduction are discussed in more detail in the book by S.M. Kuo and D.R. Morgan, "Active Noise Control Systems: Algorithms and DSP Implementations", Wiley-Interscience, New York, 1996.

Path of secondary spread.

The secondary propagation path is the path that the anti-noise signal travels from the speaker output to the error-measuring microphone located in the quiet area. The following commands describe the impulse response of a speaker-microphone path with a limited bandwidth of 160-2000 Hz and a filter length of 0.1 s. For this active noise reduction task, we will use a sampling rate of 8000 Hz.

Fs = 8e3; % 8 KHz N = 800; % 800 counts at 8 kHz = 0.1 second Flow = 160; % lower cutoff frequency: 160 Hz Fhigh = 2000; % upper cutoff frequency: 2000 Hz delayS = 7; Ast = 20; % suppression 20 dB Nfilt = 8; % filter order % Create a bandpass filter to simulate a band-limited channel% bandwidth Fd = fdesign.bandpass("N,Fst1,Fst2,Ast" ,Nfilt,Flow,Fhigh,Ast,Fs); Hd = design(Fd,"cheby2" ,"FilterStructure" ,"df2tsos" ,... "SystemObject" ,true); % Noise filtering to obtain channel impulse response H = step(Hd,); H = H/norm(H); t = (1:N)/Fs; plot(t,H,"b" ); xlabel("Time, s" ); ylabel( "Odds values"); title( "Impulse response of the secondary signal propagation path");

Determination of the secondary route of spread.

The first task of an active noise reduction system is to determine the impulse response of the secondary propagation path. This step is usually performed before noise reduction using a synthesized random signal played through the speakers in the absence of noise. The commands below generate a random signal lasting 3.75 seconds, as well as a microphone-measured signal with an error.

NtrS = 30000; s = randn(ntrS,1); % random signal synthesis Hfir = dsp.FIRFilter("Numerator" ,H."); dS = step(Hfir,s) + ... % random signal passed through the secondary channel 0.01*randn(ntrS,1); % microphone noise

Create a filter to evaluate the secondary propagation path.

In most cases, to adequately control the algorithm, the response duration of the filter estimating the secondary propagation path must be shorter than the secondary path itself. We will use a 250 order filter, which corresponds to an impulse response of 31 ms. Any adaptive FIR filtering algorithm is suitable for this purpose, but the normalized LMS algorithm is usually used due to its simplicity and robustness.

M = 250; muS = 0.1; hNLMS = dsp.LMSFilter("Method" ,"Normalized LMS" ,"StepSize" , muS,... "Length" , M); = step(hNLMS,s,dS); n = 1:ntrS; plot(n,dS,n,yS,n,eS); xlabel("Number of iterations"); ylabel( "Signal level"); title( "Identification of a secondary propagation path with the NLMS algorithm"); legend( "Expected signal","Signal Output","Error signal" );

Accuracy of the resulting estimate.

How is the impulse response of the secondary path accurately estimated? This graph shows the coefficients of the real path and the path calculated by the algorithm. Only the end of the resulting impulse response has inaccuracies. This residual error will not harm the performance of the ANC system while it is working on its selected task.

Plot(t,H,t(1:M),Hhat,t,); xlabel("Time, s" ); ylabel( "Odds values"); title( "Determination of the impulse response of the secondary propagation path"); legend( "Valid", "Evaluated" , "Error" );

The main path of signal propagation.

The noise path to be suppressed can also be described using a linear filter. The following commands generate an impulse response of the noise source-microphone path with a limited bandwidth of 200-800 Hz and a response time of 0.1 s.

DelayW = 15; Flow = 200; % lower cutoff frequency: 200 Hz Fhigh = 800; % upper cutoff frequency: 800 Hz Ast = 20; % suppression 20 dB Nfilt = 10; % filter order % Create a bandpass filter to simulate impulse response with % limited band Fd2 = fdesign.bandpass("N,Fst1,Fst2,Ast" ,Nfilt,Flow,Fhigh,Ast,Fs); Hd2 = design(Fd2,"cheby2" ,"FilterStructure" ,"df2tsos" ,... "SystemObject" ,true); % Noise filtering to obtain impulse response G = step(Hd2,); G = G/norm(G); plot(t,G,"b" ); xlabel("Time, s" ); ylabel( "Odds values"); title( "Impulse response of the primary propagation path");

Noise suppression.

A typical application of active noise cancellation is to muffle the sound of rotating machinery due to its irritating properties. Here we will artificially generate noise that could come from a conventional electric motor.

System initialization.

The most common algorithm for active noise reduction systems is the LMS algorithm with additional filtering of the filter output signal before generating an error signal (Filtered-x LMS algorithm). This algorithm uses secondary propagation path estimation to calculate an output signal that is destructive to unwanted noise in the error measurement sensor area. The reference signal is a noisy version of the unwanted sound measured close to its source. We will use a controlled filter with a response time of about 44 ms and an adjustment step of 0.0001.

% FIR filter used to model the primary propagation path Hfir = dsp.FIRFilter("Numerator" ,G."); % Adaptive filter implementing the Filtered-X LMS algorithm L = 350; muW = 0.0001; Hfx = dsp.FilteredXLMSFilter("Length" ,L,"StepSize" ,muW,... "SecondaryPathCoefficients" ,Hhat); % Noise synthesis using sine waves A = [.01 .01 .02 .2 .3 .4 .3 .2 .1 .07 .02 .01]; La = length(A); F0 = 60; k = 1:La; F = F0*k; phase = rand(1,La); % random initial phase Hsin = dsp.SineWave("Amplitude" ,A,"Frequency" ,F,"PhaseOffset" ,phase,... "SamplesPerFrame" ,512,"SampleRate" ,Fs); % Audio player for playing the results of the algorithm Hpa = dsp.AudioPlayer("SampleRate" ,Fs,"QueueDuration" ,2); % Spectrum Analyzer Hsa = dsp.SpectrumAnalyzer("SampleRate" ,Fs,"OverlapPercent" ,80,... "SpectralAverages" ,20,"PlotAsTwoSidedSpectrum" ,false,... "ShowLegend" ,true);

Simulation of the developed active noise reduction system.

Here we will simulate the operation of an active noise reduction system. To highlight the difference, noise reduction will be disabled for the first 200 iterations. The sound on the microphone before suppression represents the characteristic “howl” of industrial motors.

The resulting algorithm converges approximately 5 s (simulated) after turning on the adaptive filter. Comparing the spectra of the residual error signal and the original noisy signal, it can be observed that most of the periodic components were successfully suppressed. However, the effectiveness of stationary noise reduction may not be uniform across all frequencies. This often happens in real systems, used for active noise control tasks. By listening to the error signal, the annoying “howling” is significantly reduced.

for m = 1:400 s = step(Hsin); % generation of sinusoids with random phase x = sum(s,2); % noise generation by adding all sinusoids d = step(Hfir,x) + ... % noise propagation through the primary channel 0.1*randn(size(x)); % addition of noise accompanying the measurement process if m<= 200 % disable noise reduction for the first 200 iterations e = d; else % enable noise reduction algorithm xhat = x + 0.1*randn(size(x)); = step(Hfx,xhat,d); end step(Hpa,e); % output signal reproduction step(Hsa,); % spectrum of the original (channel 1) and attenuated (channel 2) signals end release(Hpa); % mute speakers release(Hsa); % disable spectrum analyzer Warning: The queue has underrun by 3456 samples. Try increasing queue duration, buffer size, or throughput rate.

A sound wave is a wave of compression and rarefaction of air. If you use speakers to create waves of the same frequency and amplitude, but of opposite phase, they will weaken each other. This is the principle of operation of ANC (Active Noise Control), shown in Figure 1. Active noise reduction is a technology that can significantly reduce noise levels, especially if the sound source is well localized. ANC shows even better results if the noise spectrum has periodic components.

The innovative company Promwad is developing embedded, scalable active noise reduction systems for various applications.

Areas of application of ANC systems

• Ventilation
• Quiet server cabinets
• Windows and slopes
• Cars and trucks

Figure 1 - ANC operating principle

Ventilation devices, hoods, compressors

One obvious application for active noise cancellation is ventilation- ventilation devices, hoods, compressors. Mechanical ventilation systems are noisy, which can have a negative impact on people who spend long periods of time in such areas. An example of such a space would be “clean” rooms where people have to work for long hours at a time. The principle of active noise reduction was proposed a long time ago, in 1936 by P. Leug, but at that time it was not technically possible to apply the ANC system in the modern sense, and until recently the problem of noise from ventilation was solved only by installing sound-absorbing structures, sound screens and various resonators . We are currently developing a scalable ANC system for ventilation.

This audio excerpt shows the result of an ANC system simulation. At first it is turned off, the noise of the fan is clearly audible. Then the system turns on, and the noise weakens - periodic components disappear from the spectrum. In the presented example, passive sound insulation was not modeled, which can further improve the result.

Quiet Server Enclosures

Quiet Server Enclosures- another popular product where the ANC system can be successfully used in conjunction with passive sound insulation means. This symbiosis of the two principles is most effective because noise is attenuated across the entire frequency range: ANC is most effective in the low-frequency region, and passive sound insulation is most effective in the mid- and high-frequency region. Generally speaking, passive sound insulation can be effective in the low frequency range, but the thickness of the soundproofing material must be at least half the wavelength. For example, for a hum with a frequency of 50 Hz, effective noise insulation requires a layer of material about 3 meters thick, which is an unrealistic requirement for a server cabinet. And the ANC system is much more compact, and also does not interfere with the air flow for ventilation of the cabinet contents.

Double-glazed windows and slopes

A promising area of ​​application for ANC is double-glazed windows and slopes. If the house is located near a highway, then constant noise can adversely affect the health of residents. Therefore, our immediate plans include adapting ANC for installation in double-glazed windows and window slopes. The popularity of such windows is difficult to overestimate - it’s worth imagining a summer night when you can’t open the window because of the noise on the street, and you don’t want to sleep with the air conditioner on.

Noise reduction in cars

Development ANC for use in cars, cars and trucks- one of our immediate goals. Noise in a car mainly comes from the noise of the tires on the road surface and is transmitted through the suspension and body. The difficulty in suppression systems lies in the need to position the speakers so that the sound wave from them interferes with the surrounding noise precisely in the areas where people are located. We plan to develop a system both for implementation by large automakers and for customizers.

Specifications:

• Number of compensating speakers: 1-8
• Number of microphones/non-acoustic sensors: 2-16
• Operating frequency range: 20 Hz - 1000 Hz
• Periodic Attenuation Level: 25 dB

Do you want to implement noise reduction technologies in your project?
with us, we will answer your .