Noise Canceling Headphone Solution Development

With the continuous advancement of technology and the continuous development of the consumer market for electronic products, the pace of human life is also accelerating. More and more people use headphones to listen to high-quality music. However, noise pollution in cities has become an unavoidable disturbance. If you use ordinary earphones in a noisy environment, you can only cover up the noise by increasing the volume. Not only will you not be able to enjoy wonderful music, but it will also have a great impact on your hearing. . How can we let the wonderful music relax your mood and relieve your fatigue while being free from the interference of external noise-noise-canceling headphones may become the best choice.

1. Types and hazards of noise

Sound pressure level is a unit that expresses the magnitude of sound amplitude, generally measured in decibels (dB). In the civil field, environmental noise is mainly divided into three categories: traffic noise, industrial noise and domestic noise. Among them, traffic noise comes from automobiles, trains, airplanes, ships, etc.; industrial noise mainly includes the noise of large-scale construction machinery, equipment noise in factories and enterprises, and construction site noise; common daily noise is the broadcasting and publicity of some street e-commerce companies , noisy speech in public places, and the music of square dances that have been common in recent years. In the military field, continuous noise mainly includes the noise emitted by various military vehicles, fixed-wing aircraft, helicopters, ship turbines, and various mechanical equipment. This noise is generally higher than that encountered in the civilian field. Noise The spectrum is richer. The above noises are usually above 90dB, and even some noises can be as high as above 130dB.

The harm of noise to human body can be summarized as follows:

(1) Affecting life and work: sleep is the most effective means to relieve fatigue. However, people cannot achieve deep sleep in an environment with strong noise, which often causes dreaminess and waking up. In addition, noise will affect everyone's normal work and conversation, causing people to be restless and difficult to concentrate, which not only reduces the quality of life, but also affects the quality of work;

(2) Hearing damage: The most obvious hazard of noise is the damage to human hearing. In a strong sound environment, people will feel uncomfortable in their ears. After being in a noisy environment for a long time, even in a quiet environment, they will still feel buzzing. If things go on like this, it will cause hearing loss and increase the hearing threshold. If you are in a high-intensity noise environment for a long time, your hearing will be irreversibly damaged, or even deaf;

(3) Nervous system damage: People who have been exposed to high noise for a long time often suffer from neurasthenia such as dizziness, headache, palpitation, and tinnitus;

(4) Other hazards: In addition, noise can also cause disorders of the cardiovascular and cerebrovascular systems and the digestive system. Some experts even believe that long-term high-noise environments are also an inducement for cancer.

2. Types of headphone noise reduction technology

Among the numerous noise reduction devices, there are two noise reduction methods: passive noise reduction (Passive Noise Control, PNC) and active noise reduction (Active Noise Control, ANC).

2.1 Passive noise reduction technology

Passive noise reduction equipment accounts for the majority of the share, and it can be seen everywhere in people's daily life, such as noise reduction panels commonly seen on both sides of expressways or expressways in cities, sound-absorbing wall design of anechoic rooms or KTVs, high-noise Polluted jobs require wearing soundproof earplugs or earmuffs, etc.

Among the many noise protection equipment, noise-canceling headphones are relatively effective and mainstream protection methods. The noise suppression method of general protective earphones is passive noise reduction, such as passive earplugs, earmuffs, etc. Its noise reduction mechanism is to rationally use the hard material of the outer shell of the product or the internal sound-absorbing material to prevent external sound from entering the ear canal by means of isolation and absorption. This noise reduction method has a particularly rich noise reduction effect on high-frequency components. Notably, this is the most commonly used method of noise suppression today. The passive noise reduction method is easy to implement, but for the noise below 800Hz and lower frequency bands, if you want to achieve the ideal noise reduction effect, you need to pay a high cost, and the noise reduction materials will also be very heavy. lost its practical value.

2.2 Active noise reduction technology

After long-term exploration, people have clearly realized that the fundamental way of noise control is to effectively control the sound source. Therefore, starting from the analysis of the sound mechanism of the sound source, active control of the sound source or near-field is the solution to noise control. Direction of development. Active noise reduction technology uses this technology to control noise. Active noise reduction technology makes up for the weakness of traditional earmuffs in low-frequency noise reduction, so that earmuffs can obtain a relatively uniform noise reduction effect in each frequency band.

Active noise reduction technology, also known as active noise control (ANC) technology, is an important research direction of modern noise control, because of its small circuit size, light weight, easy control, and ability to handle stable broadband noise It has been widely used and has been vigorously developed in the field of acoustics and has gradually matured in recent years. At present, there are endless companies researching noise-cancelling earphones, and noise-cancelling earphones are widely used in military and civilian fields to effectively protect people's health.

The principle of active noise reduction is to use the principle of interference and cancellation of sound waves to artificially generate a secondary noise in the characteristic space that is opposite in phase to the original noise and has the same amplitude and frequency, and offsets the original noise to achieve the purpose of reducing noise. Different from passive noise reduction, active noise reduction systems need to be equipped with power supplies, circuits, and related acoustic devices to reduce noise in an active way. As mentioned above, passive noise reduction is mainly effective for medium and high frequencies, and because low-frequency wavelengths are long, it is easy to achieve interference, so active noise reduction mainly achieves noise suppression in low-frequency bands.

3. Research history and current status of active noise reduction technology

German physicist Paul Leug first proposed the concept of noise reduction through the principle of acoustic interference in 1934, and applied for a patent for "electronic muffler". However, limited to the basis of electronic technology at that time, Paul Leug only demonstrated and expressed its basic principles, without any substantive experiments and products. In the next 20 years, active noise reduction technology has been stagnant and has not achieved substantial development.

Until 1953, with the rapid development of electronic technology, Harry Olson and Everet May of RCA Company in the United States started from Paul Leug's theory, carried out a new theoretical demonstration, and developed the "electronic sound absorber" for the first time. "Electronic sound absorber" is similar to the current active headrest system. It installs the active noise reduction system on the seat headrest of the aircraft or car occupants, and releases the reverse noise through the principle of active noise reduction. Reduce noise in a small area of the person's head. However, due to the narrow noise reduction frequency band, shallow noise reduction depth and small noise reduction range of the "electronic sound absorber" device, it has not been promoted and applied as a product.

During the same period, in 1956, still at RCA, W.B. Conover applied active noise reduction technology to the noise control of a large 15,000kVA transformer. Through demonstration and experimentation, there is an obvious noise reduction effect directly in front of the speaker. This effect is inversely proportional to the distance, that is, the farther the distance is, the smaller the noise reduction effect is. In addition, the noise reduction effect is related to the positive deviation angle. In general, the noise reduction effect is smaller. Through the test, the noise reduction can reach about 20dB at 30cm in front of the speaker.

Until the 1970s, no substantive results were applied in the research of active noise reduction technology. In the late 1970s, French Jessel, Mangiante and Canevet et al. proposed the JMC active noise reduction control algorithm based on the Huygens principle, which can be applied to free-field space. Through the demonstration test, it is finally applied to the noise treatment of large transformers. The JMC active noise reduction control algorithm points out that the noise radiated by any sound source can be controlled by secondary sound sources distributed on the continuous surface.

The theory of adaptive active noise control came into being as early as the early days of active noise reduction theory. Conover explained the principle of adaptive active noise reduction: monitor the noise reduction effect according to the human ear, and at the same time manually adjust the gain and phase control of the circuit to achieve the highest noise reduction. This is the original idea of adaptive active noise reduction. Due to the limitations of the development of electronic technology at that time, adaptive active noise reduction was not put into action. There are no related products or applications.

Until the early 1980s, with the development of high-speed signal processing technology, people applied the method of adaptive filtering to the active suppression of pipeline noise. In the early 1980s, Chaplin, J.C.Burgess, and C.F.Ross made certain achievements in the field of adaptive active noise reduction, especially in the aspect of pipeline noise reduction, and achieved satisfactory results.

In the mid-1980s, P.A.Nelson, S.J.Elliott and others from the University of Southampton in the United Kingdom conducted further in-depth research on active noise reduction technology in enclosed spaces and made innovative breakthroughs. The theory of intrinsic coherence is explained, the placement of microphones and secondary sound sources in the noise reduction system is demonstrated, and the design method of adaptive active noise reduction in the cabin is further studied.

In the 1990s, PA.Nelson and S.J.Elliott set out to conduct research on active noise reduction in a sealed cabin, and finally completed a 6-way active noise reduction system in the cabin of a BAE748 twin-propeller propeller aircraft. The research and development, and achieved a certain effect, in which at 88Hz and the 2nd and 3rd harmonics, the environmental noise was reduced by 13dB, 9dB, 6dB respectively, and the results were consistent with the theoretical analysis. This development is the highest-level development and test since the principle of active noise reduction technology was proposed, which shows that this technology can already be applied in engineering practice.

After 2000, with the continuous development of electronic technology, active noise reduction technology has also been greatly developed. For example, SongY and Gong Y applied feedback active noise reduction technology in headphones; Gonzalez A and Ferrer M passed The active noise reduction application eliminates the noise of the car engine; Castane-Selga R and Sanchez Pena apply the active noise reduction technology to the motorcycle helmet, which has a significant effect on eliminating the wind noise generated when the motorcycle is running at high speed; Kochan K and Sachau D applied space active noise reduction technology to military transport aircraft, which effectively suppressed the cabin noise and improved the comfort of the aircraft.

4. Working principle of active noise reduction

The principle of active noise reduction technology is to use the principle of superposition and interference cancellation of sound waves in space. The noise reduction system picks up environmental noise, and through circuit processing, actively generates another noise, which is equivalent to the original noise in frequency and amplitude, but opposite in phase. This actively generated sound wave and the original noise sound wave cancel each other out in space, so as to achieve the purpose of reducing noise. Since the low-frequency sound has a longer wavelength, its interference in space is easier to achieve. Therefore, active noise reduction has a better low-frequency noise control effect, and is usually used as compensation for passive noise reduction in the low-frequency band. Therefore, this technology is mainly used for protection against broadband noise or low frequency noise.

5. Classification of active noise reduction systems

According to the working principle and structural characteristics of the active noise reduction system, it can be divided into two basic types: feedback type and feedforward type.

5.1 Feedforward Noise Canceling Headphones

Among them, the feed-forward active noise reduction earphones place the microphone away from the secondary sound source, and are composed of an external microphone, a secondary sound source, earphone structural parts, and a noise reduction control circuit. The sound hole picks up the ambient noise outside the earmuffs, as shown in Figure 2-2. The noise signal picked up by the external microphone is sent to the secondary sound source through the ANC control circuit without a feedback loop. Its response parameters are often fixed, and it is impossible to adaptively adjust parameters such as gain and phase control according to the time-varying environmental noise, so its noise reduction performance is not stable, and it has a good noise reduction effect for some steady-state noise, and its application is relatively difficult. Generally, it is only used in some low-end earplug products. The main reason is that the earplugs are small in size, and the internal layout of the feed-forward noise reduction is easy to design and implement.

As shown in Figure 2-3, it is a feedforward active noise reduction design. The small hole outside the earplug in the figure is the sound hole of the microphone that picks up the external environment noise.

5.2 Feedback Noise Canceling Headphones

Feedback active noise-canceling headphones are composed of an inner microphone, a secondary sound source, headphone structural parts and a noise-reduction control circuit. The inner microphone faces inside the earmuffs and is generally placed at the entrance of the ear canal. After the noise of the cover is sent to the ANC noise reduction processing control circuit for processing, the secondary sound signal with opposite phase, equal amplitude and equal frequency is sent to the secondary sound source, and the reverse noise is released through the secondary sound source to achieve noise reduction. Noise effect, as shown in Figure 2-4. The internal microphone is generally placed near the secondary sound source, and it will pick up the noise near the secondary sound source, and its noise reduction system will form a feedback loop to perform adaptive adjustment of the noise reduction parameters. Therefore, the microphone is usually placed close to the secondary sound source. The location of the source can more accurately reflect the noise near the auditory organs. There is a feedback loop, and the effect of noise reduction is better, but the structure, circuit and debugging work are relatively complicated. In addition, due to the existence of the feedback loop, when the feedback control system is not designed properly, instability will occur, such as howling self-excitation, which is the weakness of this type of active noise reduction earmuffs.

Figure 2-5 shows AKG's K495NC feedback active noise-cancelling earphones. The arrow in the figure indicates the inner microphone, which is placed at the entrance of the ear canal to pick up noise signals in the earmuffs.

In addition, in recent years, with the continuous development of electronic technology, the composite active noise reduction system using the combination of feedforward and feedback has gradually become the focus and direction of everyone's research. The compound active noise reduction earphone is composed of an outer microphone (reference microphone), an inner microphone (error microphone), a secondary sound source, earphone structural parts and a noise reduction control circuit.

As shown in Figure 2-7, Sennheiser’s PXC450 composite active noise-canceling earphone is shown. The small hole in the left picture of Figure 2-7 is the external microphone for feed-forward noise reduction, the lower left is the internal structure, and the right picture The arrow points to the internal microphone for feedback noise reduction, which is placed at the entrance of the ear canal to pick up noise signals in the earmuffs.

With the continuous development of electronic technology, due to the structural size limitation of earplugs, except for some low-end noise-canceling earplugs on the market that cannot use feedback or compound noise reduction, and still use feed-forward structure to achieve active noise reduction, most Most active noise-canceling headphones adopt a feedback or compound active noise-canceling system structure to achieve better product noise-canceling performance. Table 2-1 shows the performance comparison of the three noise reduction structures.

6. Design points of noise-canceling headphones

Active noise-cancelling headphones are mainly composed of a power supply, a control circuit, a pickup, a speaker, and an ear cavity, among which the control circuit is the core component of the headset. In the product design process, the main factors affecting noise reduction technology are as follows:

(1) Control circuit: The control circuit realizes noise cancellation through three main processes of measurement, reverse control, and sound superposition, and measures the phase and amplitude of the noise signal picked up by the pickup. After reverse processing, it is compared with the sound from the speaker (called If the amplitude of the noise suppression signal is not exactly the same as that of the original signal or the phase is not exactly 180° different from the original signal, it will only weaken the noise and cannot achieve the effect of complete cancellation.

(2) Frequency response of electro-acoustic devices: Electro-acoustic devices include pickups and speakers, and the spatial sound transmission path between electro-acoustic devices inside the earmuffs undergoes complex processes such as electro-acoustic conversion and acoustic-electric conversion, and the diaphragm This mechanical vibration system composed of coils and coils has its own vibration mode, and it may have multiple natural frequencies. The frequency response may show peaks at each natural frequency of the system, and the phase response may appear at each natural frequency of the system. All frequencies are down 90 degrees. At this time, it is manifested as waveform distortion and time delay in the time domain, and the amplitude-frequency function is not flat in the frequency domain. Therefore, loudspeakers and microphones with relatively flat amplitude and frequency and slow change in phase frequency are required.

(3) Earphone cavity design: The function of the earphone cavity structure is mainly to eliminate acoustic short circuit, suppress acoustic resonance, broaden the frequency response range, and reduce distortion. At the same time, it is also related to the material of the ear cavity. The lower the vibration, the lower the interference; the thicker the material, the better the playback effect on the sound, and the better the low frequency effect. Therefore, it is necessary to consider comprehensively when designing the ear cavity in order to achieve the desired effect.

Summarize

To sum up, the active noise reduction headphone system is generally composed of a microphone, a secondary sound source, ear cavity structural components and a noise reduction circuit. Each of the above parts has a greater impact on the final active noise reduction performance of the headphone. Care should be taken when designing.

The above are the details of the noise-cancelling earphone solution introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. for you. If you have the development needs of TWS noise-cancelling headphones, you can trust us. We have rich experience in customized development of electronic products. We can evaluate the development cycle and IC price as soon as possible, and can also calculate the PCBA quotation. We are a number of chip agents at home and abroad: Songhan, Yingguang, Jieli, Ankai, Quanzhi, realtek, with MCU, voice IC, Bluetooth IC and module, wifi module. Our development capabilities cover PCB design, single-chip microcomputer development, Bluetooth technology development, software customization development, APP customization development, WeChat official account development and other hardware and software design. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS earphones, the development of Bluetooth earphone speakers, the development of children's toys, and the development of electronic education products.