Sound is caused by the vibration of objects, and energy propagates in the form of sound waves through elastic media. The medium can be liquid, gas or solid matter. Because of the lack of medium, sound cannot spread in vacuum. Generally speaking, there are two forms of waves, namely, shear waves and longitudinal waves. Shear wave means that the vibration direction of an object is perpendicular to the forward direction of the wave, also known as shear wave. Longitudinal wave means that the vibration direction of an object is parallel to the advancing direction of the wave, which is also called pressure wave or density wave.

How is sound produced?

When the machine is running, it will make a sound. If you touch the shell of the machine with your hand, you will feel the shell vibrating. If the power supply is cut off and the shell stops vibrating, the sound will disappear, indicating that the vibration of the object produces sound. Usually, an object that vibrates and makes a sound is called a sound source. The sound source can be solid, such as various machines; It can also be liquid and gas. For example, the sound of running water is the result of liquid vibration and the wind is the result of gas vibration. Not all the vibration of objects can be heard by human ears, only the sound generated by vibration frequency in the range of 20-20000Hz can be heard by human ears. The vibration in this frequency range is called acoustic vibration and belongs to mechanical vibration. The energy transmitted by object vibration can only be transmitted to the receiver (such as people) through the medium, and the sound is displayed. Therefore, the formation of sound consists of two links: the occurrence of vibration and the propagation of vibration. No vibration, no sound. Similarly, there is no sound without the medium that transmits vibration. As an intermediate medium for transmitting sound, it must be a substance with inertia and elasticity, because only the medium itself has inertia and elasticity can it continuously transmit the vibration of the sound source. Air is such a medium, and most of the sounds people usually hear are also transmitted through air. The medium for transmitting sound can be gas, liquid and solid. Sound propagating in air is called air sound, sound propagating in water is called underwater sound, and sound propagating in solid is called solid sound (or structural sound). When sound propagates in a medium, the particles of the medium themselves do not pass with the sound, but vibrate back and forth near its equilibrium position, and the energy of the material movement is transmitted, not the material itself. The essence of sound is a form of material movement, which is called fluctuation. So sound is also called sound wave. Sound wave is an alternating pressure wave, which belongs to mechanical wave.

What are the basic physical quantities that describe sound waves? How are they defined?

(1) Wavelength: The distance traveled by sound waves after vibration for a period of time is called wavelength, denoted as λ, and the unit is meter (m).

(2) Frequency: The number of times the medium particles vibrate in one second is called the frequency of sound waves, which is denoted as F and the unit is Hertz (Hz). The frequency of sound that the human ear can hear is generally between 20 Hz and 20000 Hz. The sound in this range is called audible sound, the sound above 20000Hz is called ultrasonic wave, and the sound below 20 Hz is called infrasound. Animals such as bats and dogs can hear ultrasonic waves, while animals such as mice can hear infrasound. In the audio range, the higher the frequency of sound waves, the sharper the sound appears. On the contrary, it looks low. Usually the sound with frequency below 300Hz is called low-frequency sound; The sound of 300- 1000Hz is called intermediate frequency sound, and the sound above 1000Hz is called high frequency sound.

(3) Sound speed: The speed at which sound waves propagate in a medium is called sound speed, which is expressed by V and the unit is meters per second (m/s). Wavelength, frequency and sound speed are three basic physical quantities to describe sound waves, and their relationship is λ = v/f, and sound speed is mainly related to the properties and temperature of the medium. At the same temperature, the speed of sound in different media is different. At 20℃, the speed of sound in the air is about 340 m/s, and the speed of sound increases by about 0.607 m/s with the increase of air temperature 1℃.

(4) Sound field: The area where sound waves exist in the medium is called sound field. In homogeneous isotropic medium, the sound field with negligible boundary effect is called free sound field.

(5) Wavefront: The curved surface formed by the point where the sound wave waves arrive at a certain moment is called wavefront or wave front. Generally speaking, sound waves are divided into plane waves, cylindrical waves and spherical waves according to the shape of wave fronts.

1. How to measure vibration?

A: The vibration of an object is an oscillation relative to the object in a certain reference state. There are three physical quantities in vibration: displacement s, velocity υ and acceleration α. In modern vibration measurement, except for some special cases, optical measurement is generally used, and the device that converts vibration motion into electrical signals (or other physical quantities) is called vibration sensor. According to whether the measured vibration motion is displacement, velocity or acceleration, vibration sensors can be divided into displacement sensors, velocity sensors and acceleration sensors. Because displacement and velocity can be obtained by integrating velocity and acceleration respectively, velocity sensor can also be used to measure displacement, and acceleration sensor can also be used to measure velocity and displacement. Accelerometers are usually used in laboratories to measure the vibration of objects. An accelerometer is a piezoelectric transducer, which can convert the acceleration of vibration or shock into a voltage (or charge) proportional to it. Its simple structure is shown in the figure.

2. In practice, particle vibration system has complex structure and various forms. What are the rules in the research of various vibration systems?

A: The so-called particle vibration system is inseparable from mass and elastic objects. If there is damping, it should be taken into account. The actual particle vibration system can be equivalent to the situation shown on the right:

(1) No external force:

(2) When subjected to external force:

3. How to list the vibration equation?

Answer: ① Select appropriate coordinates to analyze the force of the analysis object; ② According to Newton's second law, the vibration equation is listed; (3) If there are multiple analysis objects, select their coordinates respectively and then analyze them.

Example 1: vibration pickup analysis

(1) Take the static state of mass block M as the coordinate origin, and select the downward positive direction X;

(2) Take the downward vibration direction of the foundation as the positive direction y;

(3) Assuming that M has a small downward displacement, the force is analyzed;

(4) Column vibration equation:

Example 2: Dynamic Vibration Absorber

(1) Select the coordinates x and y as shown in the figure;

(2) Assuming that M and M have a small downward displacement, the force is analyzed;

(3) Column vibration equation:

4. What are * * * vibration frequency, natural frequency and fundamental frequency?

A: The frequency determined by the mass and stiffness of the structure itself is called the natural frequency, and the lowest natural frequency is called the fundamental frequency. The vibration frequency of the structure is called * * * vibration frequency.

5. How to control vibration?

A: The vibration system can be regarded as consisting of a spring and a damping mass. The simplest vibration system consists of a damping mass and a spring. Assuming that the vibration is generated by the engine inside the machine, the frequency of the engine (that is, the frequency of the force) must be greater than the natural frequency of the machine, which can be achieved by changing the mass or elasticity of the machine. There are several ways to control vibration: 1) Vibration isolation. It installs devices with certain elasticity between the vibration source and the foundation, and between the foundation and the mechanical equipment that needs vibration prevention, so that the nearly rigid connection between the vibration source and the foundation or between the equipment and the foundation becomes elastic connection, thereby isolating or reducing the transmission of vibration energy and achieving the purpose of vibration reduction and noise reduction. 2) Damping and vibration reduction. It converts the energy of mechanical vibration into heat energy or other energy that can be lost, so as to achieve the purpose of vibration reduction.

6. What is the specific intuitive meaning of wave number?

Answer: Take a string with fixed ends as an example: As shown in Figure 2- 1-3, there are n waves in n vibrations, that is, n is related to the wave number, and k is related to n, so from an intuitive point of view, k is expressed as the wave number.

7. In the longitudinal vibration equation of the bar, when the bar is subjected to longitudinal force, why is the relative expansion and contraction at X and x+dx different?

Answer: In the longitudinal vibration equation of a bar, when the bar is subjected to longitudinal force, why is the relative expansion and contraction at X and x+dx different?

8. why is the circular section with radius a?

Solution: Then

manufacture

get

1. What is an ideal fluid medium?

A: The ideal fluid medium has the following assumptions:

(1) The medium has no viscosity, and there is no energy loss when sound waves propagate.

(2) When there is no acoustic disturbance, the medium is macroscopically static, that is, the initial velocity is zero. The medium is homogeneous, and the static pressure and density are constant.

(3) When sound waves propagate, the dense and sparse processes in the medium are adiabatic;

(4) small amplitude sound waves.

2. What is decibel (dB)?

A: Decibel is a logarithmic unit commonly used in natural science. It is defined as comparing some data with a reference value. In acoustics, there are sound pressure level, sound intensity level and sound power level. Is to compare the sound pressure, sound intensity and sound power with their respective reference values, and then take the logarithm. Sound pressure level is defined as: sound intensity level is defined as: sound power level is defined as. From the definitions of sound pressure level, sound intensity level and sound power level, it can be seen that the decibel number of sound level should be calculated according to the law of logarithmic operation, not arithmetic.

3. What is plane wave?

Answer: Sound waves only propagate in the X direction, but the amplitude and phase of all particles on the yz plane are the same. Because the wave front of this sound wave is plane, it is called plane wave.

4. What are the definitions and relationships of sound power, sound intensity, sound pressure level, sound power level and sound intensity level?

Answer: The average sound energy passing through the area S perpendicular to the sound propagation direction per unit time is called the average sound power, that is; The average sound energy flow through the unit area perpendicular to the sound propagation direction is called sound intensity, that is; ; The sound pressure level is expressed by the symbol SPL, which is defined as: pe is the effective value of the measured sound pressure, pref is the reference value, and PREF = 2 *10-5pa; The sound intensity level is represented by the symbol SIL, which is defined as: I is the sound intensity to be measured, Iref is the reference value, and IREF =10-12w/m2; The sound power level is represented by the symbol SWL, which is defined as, W is the sound intensity to be measured, Wref is the reference value, and Wref= 10- 12W.

5. How does the human ear hear sound?

Answer: The vibration energy of an object propagates in air and other media to form sound waves. Sound waves in a specific frequency range (20-20000Hz) can be heard by people, which is what we usually call sound. Sound waves below 20 Hz form infrasound waves, and sound waves above 20000Hz form ultrasonic waves, which are generally not felt by human ears. The stimulus suitable for the ear is the dense wave of air vibration, but the frequency of vibration must be within a certain range and reach a certain intensity before it can be felt by the cochlea and cause hearing. Generally, the vibration frequency that human ears can feel is between 16-20000Hz, and for each of these frequencies, there is a minimum vibration intensity that can just cause hearing, which is called auditory threshold. When the vibration intensity continues to increase above the hearing threshold, the sense of hearing will be enhanced accordingly, but when the vibration intensity increases to a certain limit, it will not only cause hearing, but also cause pain in the tympanic membrane, which is called the maximum hearing threshold. Because each vibration frequency has its own hearing threshold and maximum value or hearing threshold, a coordinate diagram can be drawn to show the human ear's perception range of vibration frequency and intensity, as shown in the figure. Among them, the lower curve represents the auditory threshold of vibration with different frequencies, and the upper curve represents their maximum auditory threshold, and the area contained by them is called auditory field. The coordinates of the frequency and intensity of all sounds that people can feel should be within the listening range. It can be seen from the audiogram that the most sensitive frequency of human ear is between 1000-3000Hz; However, the frequency of daily language is slightly lower, and the speaking intensity is between the hearing threshold and the maximum hearing threshold.

6. What is pitch?

A: Pitch is one of the three qualities of sound, which refers to a signal with a specific and usually stable pitch. Generally speaking, it is the degree to which the sound sounds high or low. Mainly depends on the frequency, but also on the sound intensity. The human ear responds to high-frequency sounds of high notes and low-frequency sounds of low notes. The change of pitch with frequency (Hz) is basically logarithmic.

7. What is timbre?

A: It refers to a signal with a specific and usually stable pitch. Generally speaking, it is the degree to which the sound sounds high or low. Mainly depends on the frequency, but also on the sound intensity. The human ear responds to high-frequency sounds of high notes and low-frequency sounds of low notes. The change of pitch with frequency (Hz) is basically logarithmic.

8. What are the acoustic boundary conditions?

Answer: The sound pressure in the two media is continuous at the interface, that is, p1= p2; In addition, if the normal velocities of the media on both sides of the interface are v 1 and v2, respectively, the normal velocities of the two media at the interface are equal, that is, v 1=v2, because the two media keep constant contact. These are two conditions of acoustic boundary.

9. What are the conditions of sound interference?

Answer: Interference sound waves must have the same frequency and constant phase difference, both of which are indispensable. After interference, the energy of the sound field is not simply equal to the sum of the average energy density of each sound wave, but related to the phase difference. The energy of the sound field is equal to the sum of the average energy density of each sound wave without interference, so there is.

What is directivity?

Answer: Define the sound pressure amplitude in any θ direction, θ=0? The ratio of sound pressure amplitude on the axis is the radiation direction characteristic of the sound source, that is. For dipole sound source, its directional characteristic is zigzag on polar coordinate diagram; The sound source pointing characteristics of the same sphere are as follows.

2. What is the principle of mirroring?

A: The radiated sound field of a small ball source in front of a rigid wall can be regarded as a synthetic sound field generated by a small ball source and a "virtual source" (that is, a mirror image) in a symmetrical position. That is to say, the influence of a rigid wall on the sound source is equivalent to that of a virtual sound source, which is the mirror image principle. When the sound source is close to the absolute soft boundary, the boundary surface will also affect the radiation of the sound source, and the phase of the virtual sound source is opposite to that of the real sound source.

3. What is sound attenuation?

Answer: The attenuation of sound is related to the size, shape and environmental conditions of the sound source. If the sound source is small and in an open environment, the calculation of sound attenuation is relatively simple; However, if the sound source is located indoors (reverberation sound field), the calculation is more complicated. If the noise is relatively small relative to the measuring point in an open environment (ideally it can be regarded as a point sound source), the sound energy in the sound field is inversely proportional to the square of the distance, and the sound pressure level will be reduced by 6dB every time the distance is doubled. A linear sound source, such as the traffic on a highway, produces a cylindrical sound field, and the sound energy in the sound field is opposite to the distance. Every time the distance is doubled, the sound pressure level will decrease by 3dB. However, the near-field sound pressure does not satisfy the above law, and the sound energy does not change much with distance. At this time, the sound attenuation calculation should consider the absorption of air, especially the absorption of high-frequency components.

1. How to measure the sound absorption coefficient of materials by standing wave tube method?

Answer: The standing wave tube method can only measure the vertical sound absorption coefficient of sound-absorbing materials, and the sound absorption coefficient can be calculated under the conditions of uniform and random incidence according to the measurement results. If the frequency of sound wave propagation in the pipeline is lower than the cutoff frequency of the pipeline, only plane waves propagate in the pipeline. Based on the standing wave distribution characteristics of plane waves in finite pipes, the sound absorption coefficient of acoustic materials at the end of pipes can be obtained. The plane wave reflects back from the surface of the material, and as a result, a standing wave sound field is established in the tube. From the surface of the material, using the movable probe microphone, there is an alternating distribution of maximum and minimum sound pressure in the tube, and the maximum and minimum sound level difference (or maximum and minimum ratio) of sound pressure can be measured, and the vertical incidence sound absorption coefficient can be determined.

2. What is the law of sound wave propagation in pipes with abrupt cross sections and pipes with lateral branches?

A: The propagation of sound waves in the above two kinds of pipes follows the following two boundary conditions: continuous sound pressure and continuous volume velocity. It should be noted here that, unlike acoustic wave projection, the propagation of acoustic wave in pipeline is continuous in volume velocity, not in normal velocity. This is because the singing near the interface is non-uniform, and according to the law of mass conservation, the volume velocity should be continuous. As long as these two conditions are followed, all problems will be solved.

3. What points should be paid attention to in acoustic waveguide theory?

Answer: The conditions for generating sound waves propagating in the Z direction in the tube are as follows. Therefore, the (0,0) order wave, that is, the plane wave, can certainly propagate in the pipeline. If the pipeline only propagates plane waves, the working frequency of the sound source should be less than the cutoff frequency of the pipeline.

4. What is group speed? What is phase velocity?

A: As shown in the figure, the propagation of plane sound wave can be expressed by the motion of a beam, and its motion speed is c0. Higher-order wave is a plane wave propagating obliquely at a certain angle to the axis of the pipeline. Its propagation direction is represented by AB, and its wavefront can be aa? With bb? Equal parallel lines represent. Assuming that the wavefront is equivalent to amplitude and phase, then when the wavefront is aa? Position, amplitude and phase at the pipe wall are at point E, and after time T, the wave moves the distance AB along the beam direction, and the wavefront has reached bb? The velocity of moving from point A to point C is called the phase velocity of higher-order waves. However, the propagation distance of energy along the tube axis is only AD, which is called energy propagation velocity or group velocity.

1. What are anechoic chamber and reverberation chamber?

Answer: Principle of anechoic room: The sound absorption coefficient of the room wall is close to complete sound absorption, that is, the average sound absorption coefficient is close to 1, and the indoor sound field is close to free sound field. Sound absorption wedge is often used to achieve sound absorption effect.

Principle of reverberation room: the sound absorption coefficient of the room wall is close to total reflection, that is, the average sound absorption coefficient is close to 0, and the indoor reverberation is strong.

2. What is a diffuse sound field?

Answer: A sound field that meets the following conditions is called a diffuse sound field:

1) Sound travels in a straight line at the speed of sound c0 in the form of sound rays, and the propagation probability of sound energy carried by sound rays in all directions is the same;

2) The sound lines are independent, and the phase changes are random when superimposed;

3) The average indoor acoustic energy density is equal everywhere.

3. What is the sound absorption coefficient?

Answer: Sound absorption is the phenomenon of energy loss after sound waves hit the surface of materials. Sound absorption can reduce the indoor sound pressure level. The index describing sound absorption is sound absorption coefficient A, which indicates the ratio of sound energy absorbed by the material to incident sound energy. Theoretically, if a material completely reflects sound, then its A = 0；; If a material absorbs all the incident acoustic energy, its a= 1. In fact, the A of all materials is between 0 and 1, which means that it is impossible to completely reflect and absorb. Different frequencies will have different sound absorption coefficients. People use the frequency characteristic curve of sound absorption coefficient to describe the sound absorption performance of materials at different frequencies. According to ISO standard and national standard, the frequency range of sound absorption coefficient in sound absorption test report is 100-5KHz. The average sound absorption coefficient obtained by averaging the sound absorption coefficient of 100-5KHz reflects the overall sound absorption performance of the material. There are two methods to measure the sound absorption coefficient of materials, one is reverberation chamber method, the other is standing wave tube method. The reverberation chamber method measures the sound absorption coefficient when the sound is randomly incident, that is, the proportion of energy loss when the sound enters the material from all directions, while the standing wave tube method measures the sound absorption coefficient when the sound is vertically incident, and the incident angle of the sound is only 90 degrees. The sound absorption coefficients measured by the two methods are different, and the reverberation chamber method is the most commonly used method in engineering, because in the practical application of buildings, the sound incidence is random. In some measurement reports, the sound absorption coefficient will be greater than 1, which is caused by the measured laboratory conditions. Theoretically, the sound energy absorbed by any material cannot be greater than the incident sound energy, and the sound absorption coefficient is always less than 1. Any measured sound absorption coefficient value greater than 1 cannot be regarded as greater than 1 in actual acoustic engineering calculation, but is calculated as 1 at most. In a room, sound will quickly fill every corner, so placing sound-absorbing materials on any surface of the room has sound-absorbing effect. The greater the sound absorption coefficient of sound-absorbing materials, the more sound-absorbing areas and the more obvious sound-absorbing effect.

1. Development and principle of architectural acoustics.

A: Architectural acoustics is a science to study the acoustic environment in buildings. It mainly studies the indoor sound quality and noise control of the building environment. The earliest record of architectural acoustics can be found in Ten Books of Architecture written by Roman architect Vitruvi in the first century BC. The book describes the sound adjustment methods in ancient Greek theaters, such as using * * * sounding cylinders and reflecting surfaces to increase the volume of performances. In the Middle Ages, European churches used walls with large internal space and low sound absorption coefficient to produce long reverberation and create a mysterious religious atmosphere. At that time, vibrators absorbing low-frequency sound were used to improve the acoustics of the theater. 15 ~ 17 th century, some theaters built in Europe were mostly annular boxes with stepped seats near the ceiling. At the same time, due to the absorption of sound energy by audience and clothes, and the scattering of sound by complex concave-convex decoration inside the building, the reverberation time is moderate and the sound field distribution is relatively uniform. This design of a theater or other building may be just to solve the problem of sight at first, but it has achieved a good listening effect unintentionally. /kloc-in the 6th century, China built the famous Royal Dome of the Temple of Heaven in Beijing, which has an echo wall with a diameter of 65 meters, so that weak sounds can spread along the wall for one or two hundred meters. In front of the steps of the imperial vault, there is also a three-tone stone that can be heard several times. From 18 to 19 century, the development of natural science promoted the development of theoretical acoustics. By the end of 19, classical theoretical acoustics reached its peak. At the beginning of the 20th century, Sabin put forward the famous reverberation theory, which brought architectural acoustics into the academic field. Since the 1920s, due to the appearance of electron tubes and the application of amplifiers, the measurement of extremely low sound volume has been realized, which has paved the way for the further development of modern architectural acoustics. The basic task of architectural acoustics is to study the physical conditions of indoor sound wave propagation and acoustic processing methods to ensure good indoor listening conditions; Study and control the noise interference and harm in a certain space inside and outside the building.

2. Development and principle of environmental acoustics.

A: Environmental acoustics is a branch of environmental physics, which mainly studies the acoustic environment and its interaction with human activities. There are all kinds of sound waves in the environment of human life, some of which are used to transmit information and carry out social activities, which are needed by people; Some will affect people's work and rest, and even endanger people's health. This is what people don't need, called noise. In order to improve the sound environment of human beings and ensure that the language is clear and easy to understand and the music is beautiful. Since the beginning of the twentieth century, people have studied the sound quality in architecture, which has promoted the formation and development of architectural acoustics. Since 1950s, with the rapid development of industrial production and transportation, the urban population has increased dramatically, and there are more and more noise sources, resulting in increasingly intense noise pollution in human living environment. Therefore, it is necessary not only to improve the sound quality in the building, but also to control the noise in a certain spatial range inside and outside the building to prevent the harm of noise. The research of these problems involves physics, physiology, psychology, biology, medicine, architecture, music, communication, law, management and many other disciplines. After long-term research, the results gradually converge, forming a comprehensive science-environmental acoustics. The term environmental acoustics was formally used in the 8th International Acoustics Conference held in 1974. The content of environmental acoustics is mainly to study the generation, transmission and reception of sound and its physiological and psychological effects on human body; Study the technology and management measures to improve and control the quality of acoustic environment.

3. Development and principle of hydroacoustics.

A: Hydroacoustics is a branch of acoustics. It mainly studies the generation, propagation and reception of sound waves under water to solve acoustic problems related to underwater target detection and information transmission. With the development and utilization of the ocean, hydroacoustics has been developed and widely used. Around 827, Swiss and French scientists first measured the speed of sound in water quite accurately. 19 12 "Giant" collided with an iceberg and sank, which prompted some scientists to study the echolocation of icebergs, which marked the birth of hydroacoustics. An electric underwater acoustic transducer was designed and manufactured in Fessenden, USA. 19 14 can detect icebergs two nautical miles away. 19 18, Langevin made a piezoelectric transducer to generate ultrasonic waves, and used the vacuum tube amplification technology that just appeared at that time to detect long-distance targets in water, and received the echo of submarines for the first time, which created modern hydroacoustics and invented sonar. Subsequently, the innovation of underwater acoustic transducer, underwater acoustic research on the mechanism of temperature gradient affecting sound propagation path and the change of sound absorption coefficient with frequency made sonar improve continuously and played an important role in the Atlantic campaign against German submarines during World War II. After World War II, in order to improve the ability to detect long-range targets (such as submarines), the focus of underwater acoustic research turned to low frequency, high power, deep sea and signal processing. At the same time, the application field of underwater acoustics is more and more extensive, and many new devices have appeared, such as underwater acoustic guided torpedo, active and passive scanning sonar of acoustic mine, underwater acoustic communicator, underwater acoustic buoy, underwater acoustic speedometer, echo detector, fish detector, underwater acoustic navigation beacon, geomorphologist, depth profiler, underwater acoustic releaser, underwater acoustic telemetry and controller, etc. The research topics of modern hydroacoustics cover a wide range, including: new underwater acoustic transducers; Nonlinear acoustics in water: temporal and spatial structure of underwater acoustic field; Underwater acoustic signal processing technology; Noise and reverberation in the ocean, scattering and fluctuation, target reflection and ship radiation noise; Acoustic characteristics of marine media, etc. In particular, hydroacoustics is infiltrating with marine, geology, aquatic biology and other disciplines, forming research fields such as marine acoustics.

1. What is speech recognition technology?

A: Speech recognition technology was one of the top ten important technologies in the information technology field from 2000 to 20 10. Speech recognition is an interdisciplinary subject, which is gradually becoming the key technology of man-machine interface in information technology. The combination of speech recognition technology and speech synthesis technology enables people to get rid of the keyboard and operate through voice commands. The application of voice technology has become a competitive emerging high-tech industry. Speech recognition technology is a kind of high technology, which allows machines to convert speech signals into corresponding texts or commands through the process of recognition and understanding. At present, the mainstream speech recognition technology is based on the basic theory of statistical pattern recognition. A complete speech recognition system can be roughly divided into three parts: (1) speech feature extraction: its purpose is to extract speech feature sequences that change with time from speech waveforms. (2) Acoustic model and pattern matching (recognition algorithm): Acoustic model usually generates the obtained speech features through learning algorithm. In recognition, the input speech features are matched and compared with acoustic models (patterns) to obtain the best recognition results. (3) Language model and language processing: The language model includes a grammar network composed of speech recognition commands or a language model composed of statistical methods, and language processing can carry out grammar and semantic analysis. For speech recognition systems with small vocabulary, language processing is often unnecessary.

2. What is the principle of active noise control?

A: A typical single-channel adaptive active noise control system is shown in the figure. Active noise control system includes two parts: controller and electroacoustic part. The controller is divided into analog and digital; The electroacoustic part mainly includes secondary sound source, reference sensor and error sensor.

3. What is active control?

A: Active active control is a kind of active control. For example, if an active controller is used to monitor the growth of plants, the controller usually uses some mathematical models of plant dynamics to monitor, but plants will adjust their growth at any time because of changes in temperature or other environmental conditions. In this way, the controller will be constantly adjusted to meet the needs with the changes of plants. If the plants change too fast, the controller will collapse. Active control is to continuously or periodically monitor equipment and update its internal equipment dynamic model.

4. What are the applications of active control in acoustics?

Answer: An active muffler can be developed by applying the principle of active noise control. Generally speaking, random noise within two octaves can be eliminated, and the noise reduction amount is between 15dB-20dB. For single-frequency noise, the noise reduction can reach Zi) dB-30 dB, and the typical noise elimination frequency band is between 40 Hz and 400 Hz. Active control of interior noise: In addition to the traditional vibration and noise reduction measures, active control technology can be used to reduce the low-frequency noise of any type of vehicles. Military vehicles such as luxury cars, armored vehicles and tanks are in urgent need of active control technology, and the interior noise mainly comes from two aspects; ① The noise of the vehicle itself, such as engine noise and exhaust noise, is coupled or transmitted into the interior of the carriage through structural vibration; ② Friction noise between wheels and the ground and external noise (mainly traffic noise) enter the carriage. For the active control of vehicle self-noise, feedforward control system can be adopted, because the vibration signal of vehicle body or engine speed signal can be extracted as reference signal. For the active control of noise outside the vehicle, it is difficult to obtain the reference signal, so feedback control is often used. There are two schemes to arrange the secondary sound source: one is to take the whole carriage as an acoustic cavity, place the secondary sound source inside the carriage, and control the acoustic energy in the cavity to minimize it; The second scheme is to introduce secondary sound sources near the human ears on both sides of the seat return to reduce the acoustic anchor in the local space where the human ears are located. This kind of active support device is called source support, which can greatly reduce the noise, but it limits the range of people's activities and affects the ride comfort of passengers. In addition, there are active anti-noise earmuffs and active sound absorbers.

5. What are white noise and pink noise?

A: Random noise with the same energy at all frequencies is called white noise. Judging from the frequency response of our ears, it sounds like a very bright "hissing" sound (every octave higher, the frequency will double. Therefore, the energy in the high frequency region is also significantly enhanced). Pink noise refers to random noise with the same energy in each octave. Our ears will receive these sounds with "flat" frequency response (because pink noise is based on octaves rather than a single frequency, the energy will not increase when the frequency becomes higher). Because of this characteristic and RTA's attention to an octave range or 1/3 octave range, pink noise is very useful for measuring the frequency response of audio equipment and determining the room's sound reinforcement application.

I don't know if these are useful to you, but if they are, there are others.