Design and development of noise reduction headphones

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, and more and more people use h

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, and more and more people use headphones to listen to high-quality music. However, urban noise pollution has become an inevitable interference. When using ordinary earphones in a noisy environment, you can only cover up the noise by increasing the volume. This will not only prevent you from enjoying beautiful music, but also have a great impact on your hearing. . How can I relax my mood with great music, relieve fatigue, and at the same time be free from interference from external noise-noise-canceling headphones may be the best choice.

1. Types and hazards of noise

The sound pressure level is a unit that expresses the magnitude of the sound amplitude and is 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 cars, trains, airplanes, ships, etc.; industrial noise mainly includes large-scale construction machinery noise, noise from equipment in factories and enterprises, and noise from construction sites; common domestic noise is some street-side e-commerce broadcasts. , Noisy speeches 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 the noise encountered in the civilian field. The spectrum is richer. These noises are usually above 90dB, and some noises can even be as high as 130dB.

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

(1) Affect life and work: Sleep is the most effective way to relieve fatigue. However, people cannot reach deep sleep in a strong noise environment, often causing dreams, awakenings and other conditions; in addition, noise will affect normal work and conversation, causing people to be irritable 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 harm of noise is the damage to human hearing. In a strong sound environment, people will feel uncomfortable in their ears. After a long time in a noisy environment, even in a quiet environment, they will still feel humming. If this happens, it will cause hearing loss and increase hearing threshold. If you stay in a high-intensity noise environment for a long time, your hearing will be irreversibly damaged and even deaf

(3) Nervous system damage: People who have been in a high-noise environment for a long time often suffer from dizziness, headache, palpitation, tinnitus and other neurasthenia;

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


2. Types of earphone noise reduction technology

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

2.1 Passive noise reduction technology

Passive noise reduction equipment occupies the majority share, and it is also seen everywhere in people’s daily life, such as noise reduction panels that are often seen in cities or on both sides of highways, anechoic room or KTV sound-absorbing wall design, high noise Contaminated jobs require wearing soundproof earplugs or earmuffs, etc.

Among the many noise protection equipment, noise reduction headphones are relatively effective and mainstream protection methods. The noise suppression method of general protective headphones is to use passive noise reduction, such as passive earplugs and earmuffs. The noise reduction mechanism is to rationally use the hard shell material or internal sound-absorbing material of the product, and use the method of isolation and absorption to prevent external sound from entering the ear canal. This noise reduction method has a particularly rich noise reduction effect on high-frequency components. It is especially obvious that this is also the most commonly used method of noise suppression. Passive noise reduction is easy to implement, but for noise in the frequency range below 800 Hz and lower, if you want to achieve the ideal noise reduction effect, you need to pay a high cost. In addition, the noise reduction material will be heavy, so Lost the value of actual use.

2.2 Active noise reduction technology

After long-term exploration, people clearly realize that the fundamental way of noise control is to effectively control the sound source. Therefore, starting from the analysis of the sound source of the sound source, the implementation of 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 the earmuffs can get a more uniform noise reduction effect in each frequency band.

Active noise reduction technology is also known as active noise control (active noise control, ANC for short) technology. It is an important research direction of modern noise control. Because of its small circuit size, light weight, easy control, and ability to handle smooth broadband noise. It has been widely used and developed vigorously in the field of acoustics and has gradually matured in recent years. At present, companies researching noise-cancelling headphones emerge in endlessly. Noise-cancelling headphones 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 cancellation of sound waves to artificially generate a secondary noise in the characteristic space that is opposite to the original noise in phase, with the same amplitude and frequency, and cancels the original noise to achieve the purpose of noise reduction. Unlike 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 the low-frequency wavelength is long, it is easy to achieve interference, so active noise reduction mainly realizes 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 foundation of electronic technology at the time, Paul Leug only demonstrated and stated its basic principles, without substantial experiments and products. In the following 20 years, active noise reduction technology has stagnated and has not achieved substantial development.

Until 1953, with the rapid development of electronic technology, Harry Olson and Everet May of the American RCA Company carried out a new theoretical demonstration based on Paul Leug's theory, and developed the "electronic sound absorber" for the first time. "Electronic sound absorber" is similar to the current active headrest system, which is to install the active noise reduction system on the seat headrest of an airplane or car occupant, and emit reverse noise through the principle of active noise reduction. Reduce noise in a small area of the human 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 is not called a product for promotion and application.

During the same period, in 1956, still at RCA, W.B.Conover applied active noise reduction technology to the noise control of a large 15000kVA transformer. Through demonstration and experimentation, there is a significant noise reduction effect directly in front of the speaker. This effect is inversely proportional to the distance, that is, the longer the distance, the smaller the noise reduction effect. In addition, the noise reduction effect is related to the forward deviation angle. Generally speaking, the noise reduction effect is smaller. Through testing, the noise reduction can reach about 20dB at 30cm directly in front of the speaker.

Until the 1970s, no substantive results of active noise reduction technology were applied. In the late 1970s, France’s Jessel, Mangiante and Canevet based on the Huygens principle proposed the JMC active noise reduction control algorithm which can be applied to free-field space. Through the demonstration test, it is finally applied to the noise treatment of large transformers. 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 a continuous surface.

The adaptive active noise control theory was produced in the early days of active noise reduction theory. Conover explained the principle of adaptive active noise reduction: according to the human ear to monitor the noise reduction effect, while manually adjusting the gain and phase control of the circuit to maximize the amount of noise reduction. This is the original idea of adaptive active noise reduction. Due to the limitations of electronic technology development 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 adaptive filtering methods to active suppression of pipeline noise. In the early 1980s, Chaplin, J.C.Burgess and C.F.Ross achieved certain results in the field of adaptive active noise reduction, especially in pipe 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 achieved innovative breakthroughs. It explains the theory of intrinsic coherence, demonstrates the arrangement of microphones and secondary sound sources in the noise reduction system, and further studies the design method of adaptive active noise reduction in the cabin.

In the 1990s, PA.Nelson and SJElliott and others set out to study active noise reduction in a sealed cabin, and finally completed a 6-channel active noise reduction system in the cabin of the BAE748 twin-propeller aircraft. Developed and achieved certain results. Among them, the environmental noise was reduced by 13dB, 9dB, and 6dB at 88Hz and the second and third harmonics respectively. The results are consistent with the theoretical analysis. This development is the highest level of development and testing since the principle of active noise reduction technology was proposed, which shows that the technology can already be applied in engineering practice.

After 2000, with the continuous development of electronic technology, active noise reduction technology has also been developed by leaps and bounds. For example, SongY and Gong Y have applied feedback active noise reduction technology in headphones; Gonzalez A and Ferrer M passed Active noise reduction applications eliminate car engine noise; Castane-Selga R and Sanchez Pena apply active noise reduction technology to motorcycle helmets, which has a significant effect on eliminating wind noise generated by motorcycles at high speeds; Kochan K and Sachau D applied space active noise reduction technology on military transport aircraft, which effectively suppressed cabin noise and improved the comfort of the aircraft.

Fourth, the working principle of active noise reduction

The principle of active noise reduction technology is to use the principle of superimposed interference cancellation of sound waves in space. The noise reduction system picks up environmental noise and actively generates another noise through circuit processing, which is equivalent in frequency and amplitude to the original noise, but the phase is opposite. This actively generated sound wave and the original noise sound wave cancel each other out in space to achieve the purpose of reducing noise. The noise reduction principle is shown in Figure 2-1. Since 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 a compensation for passive noise reduction in low frequency bands. Therefore, this technology is mainly used for the protection of broadband noise or low frequency noise.

图 2-1 降噪原理图

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 feed-forward noise reduction headphones

Among them, the feedforward active noise reduction earphone is to keep the microphone position away from the secondary sound source. It is composed of an external microphone, a secondary sound source, earphone structural parts, and a noise reduction control circuit. The external microphone generally faces the outside and passes through the earmuffs. The sound-transmitting hole picks up the environmental noise outside the earmuffs, the structure is 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 adjust gain, phase control and other parameters adaptively according to time-varying environmental noise. Therefore, its noise reduction performance is not stable. It has a better noise reduction effect for some steady-state noises. It is generally only used in some lower-end earplug products. The main reason is that the earplugs are small in size and the internal layout of the feedforward noise reduction is easy to design and implement.

图 2-2 前馈式有源降噪系统图

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

图 2-3 前馈式有源降噪耳机

5.2 Feedback noise reduction headphones

The feedback type active noise reduction headset is composed of an inner microphone, a secondary sound source, earphone structural parts, and a noise reduction control circuit. The inner microphone faces the earmuff and is generally placed at the entrance of the ear canal. The inner microphone picks up and enters the ear After the noise of the cover, it is sent to the ANC noise reduction processing control circuit for processing, and then the secondary sound signal with the opposite phase, the same amplitude and the same frequency is sent to the secondary sound source, and the reverse noise is emitted through the secondary sound source to achieve the reduction The effect of noise is shown in Figure 2-4. The internal microphone is generally placed near the secondary sound source and will pick up the noise near the secondary sound source. Its noise reduction system will form a feedback loop for adaptive adjustment of noise reduction parameters. Therefore, the microphone is usually placed close to the secondary sound source. The location of the source can more truly 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 more complicated. In addition, because of the existence of the feedback loop, instability will occur when the feedback control system is not designed properly, such as self-excitation of howling, which is the weakness of this type of active noise reduction earmuffs.

图 2-4 反馈式有源降噪系统图

As shown in Figure 2-5, it is the K495NC feedback type active noise reduction headset of AKG. The arrow in the figure refers to the internal microphone, which is placed at the entrance of the ear canal to pick up the noise signal in the earmuff.

图 2-5 反馈式有源降噪耳机

In addition, in recent years, with the continuous development of electronic technology, a composite active noise reduction system using a combination of feedforward and feedback has gradually become the focus and direction of everyone's research. The composite active noise reduction headset is composed of an external microphone (reference microphone), an internal microphone (error microphone), a secondary sound source, headphone structural parts, and a noise reduction control circuit. The system composition diagram is shown in Figure 2-6.

图 2-6 复合式有源降噪系统图

Figure 2-7 shows Sennheiser’s PXC450 composite active noise reduction earphones. In Figure 2-7, the small hole in the left picture is a feed-forward noise reduction external microphone. The lower left is the internal structure, and the right picture is The arrow points to the feedback-type noise reduction internal microphone, which is placed at the entrance of the ear canal to pick up the noise signal in the earmuff.

图 2-7 森海赛尔复合式有源降噪耳机

With the continuous development of electronic technology, due to the structural size limitation of earplugs, at present, except for some low-end noise reduction earplugs that cannot use feedback or compound noise reduction, they still use feedforward structure to achieve active noise reduction. Most active noise reduction headphones adopt feedback type or composite active noise reduction system structure to achieve better product noise reduction performance. Table 2-1 is the performance comparison of the three noise reduction structures.

图 2-1 降噪原理图

Six, the design points of noise reduction headphones

Active noise reduction earphones are mainly composed of power supply, control circuit, pickup, speaker and ear cavity, among which the control circuit is the core component of earphone. 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 the three main processes of measurement, reverse control, and sound superposition. It measures the phase and amplitude of the noise signal picked up by the pickup, and the sound produced by the speaker after reverse processing (called It is the noise suppression signal) for superposition. If the amplitude of the noise suppression signal and the original signal are not exactly the same or the phase is not exactly 180° apart, the noise will only be reduced, and the effect of complete cancellation will not be achieved.

(2) The frequency response of electro-acoustic devices: electro-acoustic devices include pickups and speakers. The sound transmission path in the space between the electro-acoustic devices inside the earmuffs passes through complex processes such as electro-acoustic conversion and acoustic-electric conversion, and the diaphragm This mechanical vibration system composed of coils, etc. has its own vibration mode. It may have multiple natural frequencies. The frequency response may show peaks at each natural frequency of the system, and the phase response is in each natural frequency of the system. The frequency drops by 90 degrees. At this time, the waveform distortion and time delay appear in the time domain, and the amplitude-frequency function is not flat in the frequency domain. Therefore, speakers and microphones with relatively flat amplitude and frequency and slow phase frequency changes are needed.

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

to sum up

In summary, the active noise reduction headphone system generally consists 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 earphone. Attention should be paid when designing.