Classic physical optics, acoustics, and changes in state of matter questions

What is sound?

Sound is caused by the vibration of objects, and energy is transmitted through the elastic medium (Elastic Medium) in the form of sound waves (Sound Wave). The medium can be liquid, gas, or solid matter. Sound cannot propagate in a vacuum due to the lack of medium. Generally speaking, there are two forms of waves, transverse waves and longitudinal waves. Transverse waves refer to the direction of vibration of an object being perpendicular to the direction of wave forward motion, also known as shear waves. Longitudinal waves refer to objects that vibrate in a direction parallel to the direction of wave forward, also called pressure waves or density waves.

How is sound produced?

The machine will make a sound when it is running. If you touch the casing of the machine with your hands, you will feel the casing vibrating. If the power is cut off, the sound will disappear when the shell stops vibrating, which means that the vibration of the object produces sound. Usually, an object that vibrates and produces 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 sound of wind is the result of gas vibration. Not all vibrations of objects can be heard by the human ear. Only the sounds produced by the vibration frequency in the range of 20-20000Hz can be heard by the human ear. Vibration in this frequency range is called acoustic vibration, which belongs to mechanical vibration. The energy emitted by the vibration of an object is transmitted to the receiver (such as a person) through the medium, and then the sound is displayed. Therefore, the formation of sound is composed of two links: the occurrence of vibration and the propagation of vibration. Without vibration there is no sound, and similarly without a medium to propagate vibration there is no sound. As an intermediate medium for transmitting sound, it must be a material with inertia and elasticity, because only the sound of the medium has inertia and elasticity so that it can continuously transmit the vibration of the sound source. Air is such a medium, and most of the sounds that the human ear hears are also transmitted through the air. The medium through which sound propagates can be gas, liquid or solid. The sound that propagates in the air is called air sound, the sound that propagates in water is called water sound, and the sound that propagates in solids is called solid sound (or structural sound). When sound propagates in a medium, the particle itself of the medium does not pass along with the sound. Instead, the particle vibrates back and forth near its equilibrium position. What is transmitted is the energy of material movement, not the material itself. The essence of sound is a form of motion of matter, and this form of motion is called wave. Therefore, sound is also called sound wave. Sound waves are alternating pressure waves and are mechanical waves.

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

(1) Wavelength: After a cycle of vibration, the distance the sound wave travels is called the wavelength, recorded as λ, and the unit is meters (m).

(2) Frequency: The number of times a medium particle vibrates in one second is called the frequency of the sound wave, recorded as f, and the unit is Hertz (Hz). The frequency of sound that can be heard by the human ear generally ranges from 20 Hz to 20,000 Hz. Sounds within this range are called audible sounds, sounds above 20,000 Hz are called ultrasound, and sounds below 20 Hz are called infrasound. Animals such as bats and dogs can hear ultrasound, and animals such as mice can hear infrasound. In the audio frequency range, the higher the frequency of the sound wave, the sharper the sound appears. On the contrary, it appears low. Sounds with a frequency lower than 300Hz are usually called low-frequency sounds; sounds between 300-1000Hz are called mid-frequency sounds, and sounds above 1000Hz are called high-frequency sounds.

(3) Speed ??of sound: The speed of sound waves propagating in the medium is called the speed of sound, recorded as v, and the unit is meters per second (m/s). Wavelength, frequency and sound speed are the three basic physical quantities that describe sound waves, and their mutual relationship is λ = v/f. The speed of sound is mainly related to the properties of the medium and the temperature. At the same temperature, the speed of sound in different media is different. At 20°C, the speed of sound in the air is about 340 m/s. Every time the temperature of the air increases by 1°C, the speed of sound increases by about 0.607 m/s.

(4) Sound field: The area in the medium where sound waves exist is called the sound field. In a uniform and isotropic medium, the sound field in which boundary effects are negligible is called a free sound field.

(5) Wave front (wave front): The curved surface formed by connecting the points reached by the sound wave fluctuation at a certain moment is called a wave front or wave front. Sound waves are generally divided into plane waves, cylindrical waves and spherical waves according to the shape of the wave front.

1. How to measure vibration?

Answer: The vibration of an object is the oscillation relative to a certain reference state of the object.

There are three physical quantities in vibration: displacement s, velocity υ and acceleration α. In modern vibration measurement, except for optical measurement in some specific cases, electrical measurement is generally used. The device that converts vibration motion into electrical (or other physical quantity) signals is called a vibration sensor. Depending on whether the measured vibration motion is displacement, velocity or acceleration, vibration sensors can be divided into displacement sensors, speed sensors and acceleration sensors. Since displacement and velocity can be obtained by integrating velocity and acceleration respectively, the velocity sensor can also be used to measure displacement, and the acceleration sensor can also be used to measure velocity and displacement. Accelerometers are commonly used in laboratories to measure the vibration of objects. An accelerometer is a piezoelectric transducer that can convert the acceleration of vibration or impact into a voltage (or charge) proportional to it. Its simple structure is shown in the figure.

2. The actual particle vibration system has a complex structure and diverse forms. What are the rules for studying various vibration systems?

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

(1) When no external force is applied:

(2) When subjected to external force:

p>

3. How to formulate the vibration equation?

Answer: ① Select the appropriate coordinates to perform force analysis on the analysis object; ② List the vibration equation according to Newton’s second law; ③ If there is more than one analysis object, select the coordinates for them respectively, and then Perform analysis.

Example 1: Vibration pickup analysis

(1) Take the stationary state of the mass M as the origin of the coordinates, and select the downward positive direction x;

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

(3) Assume that M has a small downward displacement, and conduct force analysis;

(4) Column vibration equation: < /p>

Example 2: Dynamic vibration absorber

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

(2) Assume that M and m have small downward displacements , perform force analysis;

(3) Column vibration equation:

4. What are the maximum vibration frequency, natural frequency and fundamental frequency?

Answer: 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 frequency when the structure vibrates is called the maximum vibration frequency. .

5. How to control vibration?

Answer: The vibration system can be regarded as consisting of a spring and a mass block with damping. The simplest vibration system consists of a damped 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 forcing force) must be made greater than the natural frequency of the machine, which is achieved by changing the mass or elasticity of the machine. There are mainly the following methods to control vibration: 1) Vibration isolation. It is to install a device with a certain degree of elasticity between the foundation where the vibration originates, the foundation and the machinery and equipment that need to be anti-vibration, so that the nearly rigid connection between the vibration source and the foundation or between the equipment and the foundation becomes an elastic connection to isolate or reduce vibration. The transmission of energy achieves the purpose of vibration reduction and noise reduction. 2) Damping and vibration reduction. It is to convert 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 both ends fixed as an example: From Figure 2-1-3, we know that n vibrations have n wave numbers, that is, n is related to the number of waves, and k is related to n. Therefore, intuitively, k is expressed as the number of waves.

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

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

8. Why is there a circular cross-section with radius a?

Solution: Then

Let

Obtain

1. What is an ideal fluid medium?

Answer: The ideal fluid medium has the following assumptions:

(1) There is no viscosity in the medium, and there is no energy loss when sound waves propagate;

(2) ) When there is no sound disturbance, the medium is macroscopically stationary, that is, the initial velocity is zero.

The medium is uniform, and the static pressure and static density are constants;

(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)?

Answer: Decibel is a logarithmic unit commonly used in natural sciences. Its definition is to compare certain data with a reference value. In acoustics, there are sound pressure level, sound intensity level, sound power level, etc., respectively. It 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 . It can be seen from the definitions of sound pressure level, sound intensity level and sound power level that the calculation of decibel level cannot be carried out according to the rules of arithmetic, but should be carried out according to the rules of logarithmic calculation.

3. What is a plane wave?

Answer: The sound wave only propagates along the x direction, and the amplitude and phase of all particles on the yz plane are the same. Since the wave front of this sound wave is a plane, it is called a plane wave.

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

Answer: The average sound energy passing through the area S perpendicular to the direction of sound propagation per unit time is called the average sound power, that is, the average sound energy flow passing through the unit area perpendicular to the direction of sound propagation is It is called sound intensity, that is; the sound pressure level is represented by the symbol SPL, which is defined as, pe is the effective value of the sound pressure to be measured, pref is the reference value, 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, 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, Wref =10-12W.

5. How do human ears hear sounds?

Answer: The energy of the vibration of an object propagates in media such as air, forming sound waves. Sound waves within a specific frequency range (20-20,000 Hz) can cause the human ear to hear, which is what we usually think of as sound. Sound waves below 20 Hz form infrasound, and sound waves above 20,000 Hz form ultrasound, which are generally not felt by the human ear. The suitable stimulation for the ear is density waves 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 the human ear can feel is between 16 and 20,000 Hz, and for each frequency, there is a minimum vibration intensity that can just cause hearing, which is called the hearing threshold. When the vibration intensity continues to increase above the hearing threshold, the auditory experience will also increase accordingly. However, when the vibration intensity increases to a certain limit, it will not only cause hearing, but also cause pain in the eardrum. This limit is called the maximum audible threshold. Since each vibration frequency has its own hearing threshold and maximum or hearing threshold, a coordinate diagram representing the range of vibration frequency and intensity experienced by the human ear can be drawn, as shown in the figure. The lower curve represents the hearing thresholds of vibrations at different frequencies, and the upper curve represents their maximum hearing thresholds. The area covered by the two is called the hearing domain. For all sounds that people can feel, their frequency and intensity coordinates should be within the listening range. It can be seen from the auditory domain diagram that the most sensitive frequency of the human ear is between 1000-3000Hz; while the frequency of daily speech is slightly lower than this, and the intensity of speech is at a medium intensity between the hearing threshold and the maximum audible threshold.

6. What is pitch?

Answer: Pitch is one of the three major qualities of sound. It refers to a signal with a specific and usually stable pitch. In layman's terms, it refers to the pitch of a sound. It mainly depends on the frequency, but also on the intensity of the sound. Sounds with high frequencies are perceived as high-pitched by the human ear, while sounds with low frequencies are perceived by the human ear as low-pitched. Pitch changes with frequency (Hz) in an essentially logarithmic relationship.

7. What is timbre?

Answer: It refers to a signal with a specific and usually stable pitch. Generally speaking, it refers to the pitch of a sound. It mainly depends on the frequency, but also on the intensity of the sound. Sounds with high frequencies are perceived as high-pitched by the human ear, while sounds with low frequencies are perceived by the human ear as low-pitched. Pitch changes with frequency (Hz) in an essentially logarithmic relationship.

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 due to acoustic disturbance are v1 and v2 respectively , because the two media maintain constant contact, the normal velocities of the two media at the interface are equal, that is, v1 = v2. These are the two conditions for acoustic boundaries.

9. What are the conditions for sound wave interference?

Answer: The sound waves that interfere must have equal frequencies and constant phase differences. Both are indispensable. After interference occurs, the energy of the sound field cannot simply be equal to the sum of the average energy densities of each sound wave, but is related to the phase difference. For sound waves that do not interfere, the energy of the sound field is equal to the sum of the average energy densities of each sound wave, so there is .

1. What is directivity?

Answer: Define the ratio of the sound pressure amplitude in any θ direction to the sound pressure amplitude on the θ=0? axis as the radiation directivity characteristic of the sound source, that is. For a dipole sound source, its directional characteristics are , which is shaped like on the polar coordinate diagram; the directional characteristics of the same-direction small ball sound source are .

2. What is the principle of mirroring?

Answer: The radiation sound field of a small spherical source in front of a rigid wall can be regarded as the synthetic sound field generated by the small spherical source and a "virtual source" (i.e. mirror image) at a symmetrical position, which is also That is to say, the influence of the rigid wall on the sound source is equivalent to the effect of a virtual sound source. This is the mirror principle. When the sound source is close to the absolute soft boundary, the boundary surface will also affect the radiation of the sound source. At this time, the phase of the virtual sound source is opposite to the phase 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; but if the sound source is located indoors (reverberant sound field), the calculation is more complicated. If the noise is in an open environment and its size is small relative to the location of the measuring point (ideally it can be regarded as a point sound source), then the sound energy in the sound field is inversely proportional to the square of the distance. Doubling the sound pressure level will reduce 6dB. Linear sound sources such as traffic on the road produce a cylindrical sound field. The sound energy in the sound field is inversely proportional to the distance. Every time the distance doubles, the sound pressure level will decrease by 3dB. However, the sound pressure in the near field will not meet the above rules, and the sound energy changes slightly with distance. At this time, the calculation of sound attenuation must consider the absorption of sound by the air, especially the absorption of high-frequency components is obvious.

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

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

2. What are the rules for the propagation of sound waves in tubes with sudden cross-sections and tubes with side branches?

Answer: The propagation of sound waves in the above two types of tubes follows the following two boundary conditions: continuous sound pressure and continuous volume velocity. What should be noted here is that unlike sound wave projection, the propagation of sound waves in pipes has volume velocity continuity rather than normal velocity continuity. This is because the sound near the interface is non-uniform and according to the law of conservation of mass, the volume velocity should be continuous. As long as you follow these two conditions, all problems will be solved.

3. What key points should be paid attention to in the theory of acoustic wave tube?

Answer: The conditions for generating sound waves propagating along the z direction in a tube are , . It can be seen that (0,0) sub-waves, that is, plane waves, must be propagated in the pipeline. If the pipeline only propagates plane waves, the operating frequency of the sound source should be less than the cut-off frequency of the pipeline.

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

Answer: As shown in the figure, the propagation of plane sound waves can be represented by the movement of a wave beam, and its movement speed is c0. A high-order wave is a beam that is at an angle to the tube axis and propagates diagonally. For plane waves, the propagation direction is represented by AB, and the wave front can be represented by parallel lines such as aa? and bb?. Assuming that the wave front is equivalent to the amplitude phase, then when the wave front is at the position aa?, the amplitude phase reaching the tube wall is point E. After t time, the wave has moved a distance AB along the direction of the beam, and the wave front has reached bb ?, then the speed of movement from point A to point C is called the phase velocity of the higher-order wave; and the propagation distance of energy along the tube axis is only AD, which is called the energy propagation speed or group velocity.

1. What is an anechoic chamber and what is a reverberation chamber?

Answer: Principle of anechoic chamber: 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 the free sound field. Sound-absorbing spikes are commonly used to achieve sound-absorbing effects.

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

2. What is diffuse sound field?

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

1) Sound propagates in a straight line at the speed of sound c0 in the form of sound rays. The sound energy carried by the sound rays travels in all directions. The transmission probability is the same;

2) The sound rays are independent of each other. When the sound rays are superimposed, their phase changes are random;

3) The average sound energy in the room Density is equal everywhere.

3. What is sound absorption coefficient?

Answer: Sound absorption is the phenomenon of energy loss after sound waves hit the surface of materials. Sound absorption can reduce indoor sound pressure levels. The index describing sound absorption is the sound absorption coefficient a, which represents the ratio of sound energy absorbed by the material to the incident sound energy. Theoretically, if a material completely reflects sound, then its a=0; if a material absorbs all incident sound energy, then its a=1. In fact, a of all materials is between 0 and 1, which means it is impossible to reflect all materials and absorb them all. There will be different sound absorption coefficients at different frequencies. The sound absorption coefficient frequency characteristic curve is used to describe the sound absorption performance of materials at different frequencies. According to ISO standards and national standards, the frequency range of the sound absorption coefficient in the sound absorption test report is 100-5KHz. The value obtained by averaging the sound absorption coefficients of 100-5KHz is the average sound absorption coefficient, which reflects the overall sound absorption performance of the material. There are two methods for measuring the sound absorption coefficient of materials, one is the reverberation chamber method and the other is the standing wave tube method. The reverberation chamber method measures the sound absorption coefficient when sound is randomly incident, that is, the proportion of energy loss when sound is injected into the material from all directions, while the standing wave tube method measures the sound absorption coefficient when sound is normally incident, and the sound incident angle is only 90 degrees. . The sound absorption coefficients measured by the two methods are different. In engineering, the sound absorption coefficient measured by the reverberation chamber method is most commonly used, because in practical construction applications, sound incidence is random. In some measurement reports, the sound absorption coefficient may appear to be greater than 1. This is due to the laboratory conditions of the measurement. 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 used as greater than 1 in actual acoustic engineering calculations, and is calculated as 1 at most. In a room, sound will quickly fill every corner, so placing sound-absorbing materials on any surface in the room will have a sound-absorbing effect. The greater the sound absorption coefficient of the sound-absorbing material, the larger the sound-absorbing area, and the more obvious the sound-absorbing effect.

1. The development and principles of architectural acoustics.

Answer: Architectural acoustics is the science that studies acoustic environmental issues in buildings. It focuses on indoor sound quality and noise control in the built environment. The earliest records about architectural acoustics can be found in the "Ten Books on Architecture" written by the Roman architect Vitruvius in the first century BC. The book describes the sound adjustment methods in ancient Greek theaters, such as the use of sound cylinders and reflective surfaces to increase the volume of performances. In the Middle Ages, European churches used large internal spaces and walls with low sound absorption coefficients to produce long reverberation sounds and create a mysterious religious atmosphere. At that time, vibrators that absorbed low-frequency sounds were also used to improve the sound effects of theaters.

Some theaters built in Europe from the 15th to the 17th century mostly had circular boxes and stepped seats arranged close to the ceiling. At the same time, due to the absorption of sound energy by the audience and clothing, and the scattering effect of the complex concave and convex decoration inside the building, Make the reverberation time moderate and the sound field distribution relatively uniform. This kind of design of theaters or other buildings may have been originally designed to solve the sight problem, but it has inadvertently achieved better hearing effects. In the 16th century, China built the famous Temple of Heaven in Beijing, which has a 65-meter-diameter echo wall that allows weak sounds to propagate along the wall for one to two hundred meters. In front of the steps of the Imperial Vault, there is a three-tone stone that can be heard echoing several times. From the 18th to the 19th centuries, the development of natural science promoted the development of theoretical acoustics. By the end of the 19th century, classical theoretical acoustics had reached its peak. At the beginning of the 20th century, the American Sabine proposed the famous reverberation theory, which brought architectural acoustics into the field of science. Starting in the 1920s, due to the emergence of vacuum tubes and the application of amplifiers, the measurement of very small acoustic quantities was possible, which opened the way for the further development of modern architectural acoustics. The basic tasks of architectural acoustics are to study the physical conditions and acoustic treatment methods of indoor sound wave transmission to ensure good indoor listening conditions; to study and control noise interference and hazards in certain spaces inside and outside buildings.

2. The development and principles of environmental acoustics.

Answer: Environmental acoustics is a branch of environmental physics, which mainly studies the acoustic environment and its interaction with human activities. There are various sound waves in the environment where humans live. Some of them are used to transmit information and carry out social activities, which are needed by people; some can affect people's work and rest, or even harm human health, and are not needed by people. They are called for noise. In order to improve the human acoustic environment, ensure that language is clear and understandable, and music is beautiful and pleasant. Since the beginning of the 20th century, people have conducted research on sound quality issues in buildings, which has promoted the formation and development of architectural acoustics. Since the 1950s, with the rapid development of industrial production and transportation, the urban population has grown rapidly, the number of noise sources has increased, and the noise generated has become stronger and stronger, causing increasingly serious noise pollution in the human living environment. Therefore, it is not only necessary to improve the sound quality within the building, but also to control the noise within a certain space within the building and outside the building to prevent the harm of noise. The research on these issues involves physics, physiology, psychology, biology, medicine, architecture, music, communications, law, management science and many other disciplines. After long-term research, the results have gradually converged to form a comprehensive science. --Environmental acoustics. At the Eighth International Conference on Acoustics held in 1974, the term environmental acoustics was officially used. The content of environmental acoustics is mainly to study the generation, transmission and reception of sound, and its physiological and psychological effects on the human body; to study the technology and management measures to improve and control the quality of the acoustic environment.

3. The development and principles of hydroacoustics.

Answer: Hydroacoustics is a branch of acoustics. It mainly studies the generation, propagation and reception of sound waves underwater to solve acoustic problems related to underwater target detection and information transmission processes. Hydroacoustics has developed with the development and utilization of the ocean and has been widely used. Around 827, Swiss and French scientists measured the speed of sound in water quite accurately for the first time. In 1912, the passenger ship "Giant" collided with an iceberg and sank, prompting some scientists to study the echolocation of icebergs, which marked the birth of hydroacoustics. Fessenden in the United States designed and manufactured an electric hydroacoustic transducer that could detect icebergs two nautical miles away in 1914. In 1918, Langevin made a piezoelectric transducer to generate ultrasonic waves, and applied the vacuum tube amplification technology that had just emerged at the time to detect long-range targets in the water. He received the echo from a submarine for the first time, ushering in the modern era. Hydroacoustics also led to the invention of sonar. Subsequently, the innovation of hydroacoustic transducers, achievements in hydroacoustic research on the mechanism of temperature gradient affecting the sound propagation path, the change of sound absorption coefficient with frequency, etc., enabled sonar to be continuously improved and used against German submarines during World War II. played an important role in the Battle of the Atlantic. After World War II, in order to improve the ability to detect long-distance targets (such as submarines), the focus of hydroacoustic research shifted to low frequency, high power, deep sea and signal processing.

At the same time, the fields of hydroacoustic applications are becoming more and more extensive, and many new devices have appeared, such as: hydroacoustic guided torpedoes, acoustic mine main and passive scanning sonar, hydroacoustic communicators, acoustic buoys, acoustic speed meters, echo sounders, fish Group detectors, acoustic navigation beacons, geomorphological instruments, deep and shallow bottom stratigraphic profilers, hydroacoustic releasers and hydroacoustic telemetry, controllers, etc. The research topics of modern hydroacoustics cover a wide range of topics, mainly including: new hydroacoustic transducers; nonlinear acoustics in water; the spatiotemporal structure of hydroacoustic fields; hydroacoustic signal processing technology; noise and reverberation, scattering and fluctuation in the ocean, Target reflection and ship radiation noise; acoustic characteristics of marine media, etc. In particular, hydroacoustics is interpenetrating with disciplines such as oceanography, geology, and aquatic biology, forming research fields such as ocean acoustics.

1. What is speech recognition technology?

Answer: Speech recognition technology is one of the ten important technological development technologies in the field of information technology from 2000 to 2010. Speech recognition is an interdisciplinary subject. Speech recognition is gradually becoming a key technology for human-computer 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 high technology that allows machines to convert speech signals into corresponding texts or commands through the process of recognition and understanding. The current 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 a sequence of speech features that changes over time from the speech waveform. (2) Acoustic model and pattern matching (recognition algorithm): The acoustic model usually generates the acquired speech features through a learning algorithm. During recognition, the input speech features are matched and compared with the acoustic model (pattern) to obtain the best recognition result. (3) Language model and language processing: The language model includes a grammatical network composed of recognizing voice commands or a language model composed of statistical methods. Language processing can perform grammatical and semantic analysis. For small vocabulary speech recognition systems, the language processing part is often not required.

2. What is the principle of active noise control?

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

3. What is active active control?

Answer: Active active control is a type 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 for monitoring, but because plants will adjust their growth at any time due to changes in temperature or other environmental conditions . In this way, the controller will continuously adjust to suit needs as the plants change. If the plants change too quickly, the controller will crash. Active active control monitors the plant and continuously or periodically updates its internal model of plant dynamics.

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

Answer: An active muffler can be developed by applying the principle of active noise control. Generally speaking, it can eliminate twice For random noise in the frequency range, the noise reduction amount is between 15dB-20dB; for single-frequency noise, the noise reduction amount can reach ZI)dB-30dB. The typical silencing frequency band is between 40Hz~400Hz. Active control of noise inside the cabin: In addition to traditional vibration and noise reduction measures, active control technology can be used to reduce low-frequency noise inside the cabin of any type of vehicle, such as luxury cars, armored vehicles, tanks, etc. Military vehicles have an extremely urgent need for active control technology, and noise in the cabin mainly comes from two aspects; ① vehicle self-noise such as engine noise and exhaust noise enters the cabin through structural vibration coupling or transmission; ② friction noise between wheels and the ground and External noise (mainly traffic noise) enters the cabin. For active control of vehicle self-noise, a feedforward control system can be used because the vehicle body vibration signal or engine speed signal can be picked up as a reference signal. For active control of vehicle external noise, it is difficult to obtain a reference signal, so feedback control is often used.

There are two options for the placement of secondary sound sources: one is to use the entire car as an acoustic cavity, place secondary sound sources inside the car, and control the sound energy in the cavity to minimize it; the second option is to use the two sides of the seat return , introducing a secondary sound source near the human ear, so that the amount of sound anchor in the local space where the human ear is located is reduced. This kind of active control device is called an active support. It can achieve a large amount of noise reduction, but it limits the range of people's activities, affecting the riding comfort of passengers. There are also active anti-noise earmuffs, active sound absorbers, etc.

5. What are white noise and pink noise?

Answer: Random noise with the same energy at all frequencies is called white noise. From the frequency response of our ears it sounds like a very bright "hiss" sound (the frequency doubles for every octave higher. Therefore the energy in the high frequency area is also significantly enhanced). Pink noise refers to random noise with the same energy in each octave. Our ears will receive these sounds with a "flat" frequency response (because pink noise is based on octaves rather than individual frequencies, the energy does not increase as the frequency becomes higher). Because this feature and the Real-Time Analyzer (RTA) focus on an octave or 1/3 of an octave of sound, pink noise is useful for measuring the frequency response of audio equipment and determining room sound reinforcement applications.

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