Wave interference When two or more waves with the same frequency, the same vibration direction and constant phase difference are superimposed in space, the vibration is strengthened or weakened in different places in the overlapping area. This phenomenon is called "wave interference". A wave source that meets the above conditions is called a coherent wave source, and the wave it emits is called a coherent wave. This is the simplest case in the superposition of waves.
After the superposition of two coherent waves, there is a certain amplitude at each position in the superposition area. In some positions, the amplitude is equal to the sum of the amplitudes of the vibrations caused by the two waves respectively, and the combined vibration of these positions is the strongest. Called "constructive interference"; However, the amplitudes of some positions are equal to the difference of the amplitudes of the vibrations caused by the two waves respectively, and the combined vibration of these positions is the weakest, which is called "destructive interference". This is an important feature of waves.
The phenomenon that reflected waves return to the original medium when they reach the interface with another medium. For example, when a sound wave encounters an obstacle, it obeys the law of reflection. In the same medium, due to the inhomogeneity of the medium, the wave will return to the original density medium, that is, reflection will occur.
In the process of propagation, when the refracted wave of a wave enters another medium from one medium, the propagation direction is deflected, which is called wave refraction. In the same medium, due to the inhomogeneity of the medium itself, the propagation direction of waves will also change. This phenomenon is also called wave refraction. It also obeys the law of refraction of waves.
A branch of acoustic physics that studies the generation, propagation, reception and function of sound waves. According to different research methods, objects and frequency ranges, it intersects with many other disciplines, forming many unique marginal disciplines, such as atmospheric acoustics, hydroacoustics, electroacoustic, bio-acoustics, psychological acoustics, language acoustics, architectural acoustics, environmental acoustics, geometric acoustics, physical acoustics, physiological acoustics, molecular acoustics, acoustic energy, ultrasound, infraacoustics, micro-acoustics, musical acoustics and so on. With the development of modern industry, acoustics is the product of the combination of classical acoustics, electronic technology and various industrial applications, and it continues to develop with the development of industry.
This kind of sound is called "rhythm sound". Sound with a single fundamental frequency. Pure tone (or pure tone) has a nearly single resonance waveform. This rhythm can be generated by a tuning fork. Musical instruments produce complex tones, which can be decomposed into a fundamental frequency and some overtones with higher frequencies. See "Speech".
A vibration system in which a sound source radiates sound waves to the surrounding medium is called a "sound source". For example, stringed instruments such as erhu and violin sound by the vibration of strings; Flute and other wind instruments vibrate by air column to produce sound; Membrane instruments such as gongs and drums sound through the vibration of plates or membranes; Singing or talking depends on the vibration of the vocal cords in the throat. Anything that makes a sound vibrates, so all kinds of vibrating objects are called sound sources. Solid, liquid and gas can vibrate to produce sound, and they can all be regarded as sound sources.
In the acoustoelastic medium, the propagation process of each particle vibration is called "sound wave". This is a mechanical wave. The sound wave emitted by the sound generator, the vibration frequency is between 20 Hz and 20000 Hz, which can cause people's hearing, so it is also called audible sound wave. Mechanical waves with frequency of 10-4 ~ 20 Hz are called infrasound waves, and mechanical waves with frequency of 2× 104 ~ 2× 108 Hz are called ultrasonic waves. Infrasound waves and ultrasonic waves generally do not cause human hearing. From the physical point of view, there is no essential difference between the acoustic vibration at the frequency of 20 ~ 20000 Hz and the acoustic vibration outside this frequency. So sound waves in a broad sense include infrasound waves and ultrasonic waves. Whether it can arouse people's hearing does not depend entirely on the frequency of mechanical waves, but also on the sound intensity. Sound waves propagate in the form of longitudinal and transverse waves in solids, but only in the form of longitudinal waves in liquids and gases.
Sound speed is also called sound speed. Refers to the speed at which sound propagates in a medium. It is related to density, elastic coefficient and the state of medium. In solids, sound waves can propagate in two forms: longitudinal waves and shear waves. What is the propagation speed of longitudinal waves? V= E is better than ρ (can't type it out, o(∩_∩)o? Sorry, just understand)
E is the elastic modulus of a solid, and ρ is its density.
In gases and liquids, sound waves are longitudinal waves, and their propagation speed (like longitudinal waves, they are not typed).
K is the bulk elastic modulus of the medium.
The propagation speed of sound in air increases with the increase of temperature, which is similar to the propagation speed of absolute temperature T.
Where r is the ratio of constant pressure specific heat Cp to constant volume specific heat Cv, and r is the gas constant. Usually expressed by the following formula
Vt = 33 1.45+0.6 liter
Vt is the speed of sound in air at t℃, in meters per second, and 33 1.45 meters per second is the speed of sound in air at 0℃. When the temperature rises by 65438 0℃, the speed of sound increases by about 0.6 m/s.
Reflection of sound waves The sound waves emitted by the sound source are reflected back when encountering obstacles in the propagation process, which is called "sound wave reflection". The echo wall of Beijing Tiantan Park, the echo in the valley, the thunder and the reverberation in the building are all different situations caused by sound wave reflection.
Echo When sound is projected on a large area at a certain distance from the sound source, part of the sound energy is absorbed and the other part is reflected back. If the listener hears it, it is directly from the sound source.
Sound, this kind of reflected sound is called "echo". If the speed of sound is known, the distance from the reflecting surface to the sound source can be calculated by measuring the time interval from sound emission to sound reflection. Based on this principle, an underwater acoustic locator for measuring the depth of seawater is designed. Echo is a common phenomenon in valleys or halls. In summer, thunder rumbles, which is also the echo of thunder reflected many times through the dense clouds in the sky. Broadly speaking, all other signals of this nature are echoes. Such as reflected ultrasonic signals. Use echo detector, underwater acoustic direction finder, ultrasonic flaw detector, etc. It is made of echoes, fish schools are detected by sound waves, or underground oil mines are detected by the reflection of explosive sound waves on the ground.
After the reverberation sound source stops working, the sound continuation phenomenon is called "reverberation". In various buildings, sound waves are reflected and absorbed many times through walls and ceilings. The duration of reverberation is related to the absorption of sound waves by materials that reflect sound waves. If only a small part of acoustic energy is absorbed at a time, the reverberation time will be prolonged, and the sound will overlap and be unclear. If the reverberation time is too short, although the sound is clearly distinguished, it makes people feel dull and monotonous, which is not suitable for the effect of music performance and affects the appreciation of performance. Generally, the appropriate time is between 1 ~ 2 seconds, and the reverberation time is reduced by 60 decibels. The sound absorption performance of various substances is expressed by sound absorption coefficient, which is the percentage of absorbed sound energy to incident wave energy. Soft and porous materials have large sound absorption coefficient, while hard and smooth materials have small sound absorption coefficient. Therefore, reverberation time is an important acoustic characteristic of buildings. For example, in capital theatre, the reverberation time is 1.36 seconds when the seat is full and 3.3 seconds when the seat is empty. The reverberation time of the Great Hall of the People in Beijing is 1.6 seconds when it is full and 3 seconds when it is empty.
A tuning fork can demonstrate the interference of sound waves. The two forks of a tuning fork are two identical wave sources. When the tuning fork sounds, the two radiated waves interfere, which makes the tuning fork rotate around the longitudinal axis of the fork handle, or the tuning fork does not move. When we walk around the tuning fork that makes sound, we will hear the sound of the tuning fork become stronger and weaker. When the two waves generated by the tuning fork interfere, there will be alternating strengthening areas and weakening areas. In the reinforced area, the vibration of the air is strengthened, and the sound we hear is also stronger. In the weakened area, the vibration of the air is weakened and the sound we hear is also weakened. The interference of sound waves refers to the phenomenon that two series of continuous sound waves with the same frequency meet and overlap in the same area, which only happens under certain conditions and is not a common phenomenon.
Diffraction of sound waves In general, some sound waves propagate in a straight line, while others are obviously diffracted. Because the wavelength of sound wave is about 1.7 cm to 17 m, which is close to the size of ordinary indoor obstacles, doors and windows. When the sound wave encounters an obstacle with the same wavelength as the sound wave in the propagation process, the sound wave can bypass the obstacle and pass behind it, that is, diffraction occurs.
* * * If the frequency of the sounding device is the same as that of the external sound, it will sound due to the vibration of * * *. This phenomenon of * * * vibration is acoustically called "* * * sound". Many musical instruments use vibrators and air columns to enhance the sound produced by musical instruments. Place two * * * sound boxes opposite each other (box mouths are opposite), and then tap a tuning fork on one * * * sound box. After a period of time, hold the tapped tuning fork with your hand to stop it from vibrating. At this time, you can hear the sound that another * * * sound box has not been struck, thus confirming the generation of * * * sound. Because one of the two * * * sound boxes vibrates, the nearby air diffuses around, and the other * * * sound box and tuning fork ring, so it can also make sound. * * occurs when the natural frequencies of two objects are the same, or one of them is an integer multiple of the natural frequency of the other as a sound source. The speaker attached to the tuning fork plays a role in strengthening the sound. The body or barrel of violin, pipa, erhu and other stringed instruments also has the function of a speaker. Once upon a time, the built stage often placed several vats under the stage, which also used the sound of * * * to make the sounds made by actors and musical instruments on the stage have a loud and round effect.
The air column blows at one end of the thin glass tube, or a vibrating tuning fork is placed at the nozzle, so that the sound of air in the tube can be heard. That is to say, the air column in the tube becomes a sounding body due to its own vibration. If the lower end of the glass tube is communicated with the rubber tube of the water funnel. Put a vibrating tuning fork with frequency V near the nozzle, and move the funnel up and down to change the length of the air column in the pipe. The maximum sound can be heard at a certain length, because the air column in the tube is forced to vibrate under the vibration of the continuous sinusoidal sound pressure generated by the tuning fork. When the vibration frequency of the tuning fork coincides with the natural vibration frequency of the air column, * * vibration occurs. Adjust it slowly, so that the water level in the pipe continues to drop, and the sound intensity can reach the maximum under several other suitable gas column lengths. This phenomenon, which is caused by sound waves, is called * * * sound. The length l of the air column has the following relationship with the wavelength λ of the sound wave:
Because the frequency v of the sound source tuning fork is fixed and the sound velocity v in the air column has a certain value, the ringing frequency can be obtained from the specific wavelength λ=u/v where the vibration occurs. It is convenient to measure the wavelength of sound wave and calculate the speed of sound by using air column sound.
A tuning fork is an acoustic instrument. It is made of steel, shaped like the letter U, but thin and long. The handle at the lower end is inserted into an empty wooden box with an opening at one end to increase the pronunciation intensity. When the vibration frequency of the tuning fork is constant, its pitch is constant, its vibration is the vibration of the rod, and its sound is extremely clear. Therefore, it is often used as a standard to measure tones. Tap the upper end of the tuning fork with a rubber hammer, and its two tuning forks vibrate left and right to make a sound. The pitch of a tuning fork depends on its length and thickness. The slender thigh arm vibrates slowly and the sound is low; Those short and thick thigh arms vibrate quickly and have a high pitch.
Generally, the vibration of tuning fork is not easy to produce overtone vibration, occasionally overtone vibration occurs, and it is immediately reduced because of the large impedance of the rod, so the tuning fork is easy to produce fundamental wave vibration. There is also interference when the tuning fork vibrates, and the vibration of its two tuning forks is inward or outward at the same time. If two forks vibrate inward at the same time, A is dense in the middle, as shown in figure 1-33, while B and B' on the outer side of the fork are sparse. When two bifurcations vibrate outward at the same time, the middle becomes sparse, while B and B' become dense. Because the amplitudes, wavelengths or frequencies of the vibrations of the two tuning forks are equal, the position shown by the dotted line in figure 1-33 is silent when the densities meet and interfere. In this way, you can put the vibrating tuning fork on your ear and turn it slowly to find the position of such a silent zone.
The tuning fork can keep the frequency of its sound unchanged for a long time, and the amplitude and temperature of vibration are not easy to change the frequency. The frequency of tuning fork is as high as 90 thousand cycles per second. Usually the tuning fork is made of steel or elastic steel. Because the tuning fork is small in size, the sound output rarely reaches the air, and because of its vibration mode, only a very small amount of longitudinal wave components vibrate along the fork handle, so the vibration system is rarely blocked. Connecting the * * * buzzer with the wooden handle of a tuning fork can increase the sound output, and because the * * * buzzer and the tuning fork have different overtones, only the pitch produces the * * * buzzer. Tuning forks have many uses and are used as tuning standards when tuning musical instruments.
The pitch of sound is called "pitch". Pitch mainly depends on the frequency of sound waves. When the intensity of sound waves increases, it will also make sound waves with the same frequency have higher pitch. Usually, the sound waves emitted by musical instruments are not single tones or pure tones, but have the complexity of their waveforms, so the pitch is actually determined by many factors. The difference between music and noise is that music has a certain frequency, while the frequency of noise is constantly changing, which makes people feel the rapid change of tone. There is a difference between pitch and overtone in tone, that is, the vibration frequency of overtone is an integer multiple of pitch, including the first overtone, the second overtone and so on. In fluctuation, the period is inversely proportional to the frequency, so the overtone period is shorter than the main period, and their ratio is also an integer multiple.
Male pronunciation, its frequency is about 90 ~ 140 Hz, and the voice is low. The frequency of female pronunciation is about 270 ~ 550 Hz, and its voice is high. The frequency range of sound waves that human ear organs can feel varies from person to person. The average person's hearing range is 50 ~ 50~ 15000Hz, and the sensitive hearing range is 20 ~ 20~20000Hz. The vocalization of human mouth is about 100 ~ 8000 Hz. The frequency range of various musical instruments is 40 ~ 40~ 14000Hz, and the frequency of speakers is 40 ~ 8000 Hz. If the sound is emitted by a taut string, the thinner, shorter and tighter the string, the higher the tone will be; On the contrary, the tone is low.
Loudness is also called volume. The sound intensity perceived by human ears is the subjective quantity of sound. Loudness depends on the amplitude at the sound reception. For the same sound source, the farther the amplitude propagates, the smaller the loudness. When the propagation distance is constant, the greater the amplitude of the sound source, the greater the loudness. Loudness is closely related to sound intensity, but the relationship between loudness and sound intensity is not simple linear, but close to logarithmic. When the frequency of sound and the waveform of sound wave change, people's feeling of loudness will also change.
The quality of sound is called timbre, also called timbre, which is one of the attributes of sound. It is determined by the number, frequency and amplitude of overtones. Different musical instruments can still distinguish their own characteristics under the same basic vibration frequency because of their different timbres. For example, the ensemble of Erhu, Qin Yue and Pipa, due to different timbres, people's hearing can distinguish the names of various musical instruments.
The three elements of sound timbre, tone and loudness are the three main attributes of sound, so they are called the three elements of sound.
Pitch In polyphony, the lowest frequency sound is called "pitch". The tone of music is determined by the frequency of pitch. For example, the piano sound of 100Hz not only emits the sound with the frequency of 100Hz, but also emits many weak sounds with different frequencies. The pure tone of 100Hz is called piano pitch.
The remaining pure tones whose overtone frequency is integer multiple or non-integer multiple of pitch 1 are called overtones. A sound equal to an integer multiple of the pitch frequency is also called homophonic. A vocal body that emits a simple harmonic vibration emits a very simple pure tone. The sound produced by musical instruments is generally polyphony composed of several pure tones with different frequencies and amplitudes, among which the pure tone with the lowest frequency is called pitch, and there are overtones with an integer multiple of pitch. The tone of music is determined by the number, frequency and amplitude of overtones.
The pressure produced by sound waves is called sound pressure. In the process of sound wave propagation, the particles near any point in the air are sometimes loose and sometimes tight due to the sound wave, so the pressure changes accordingly. The difference between the pressure at this point when sound waves propagate in the air and the pressure when no sound arrives is called the sound pressure at this point. The unit of sound pressure is Pascal, abbreviated as Pa. The magnitude of sound pressure is related to the vibration speed of particles in the sound transmission medium, the density of the medium and the propagation speed of sound waves. If ρ represents the air density, μ represents the sound speed and V represents the vibration speed of air particles, then the sound pressure P is
P=pμv
The sound pressure of the leaves blown by the breeze is about 0.0 1 Pa. The sound pressure of talking loudly in the room is about 0. 1 Pa.
Energy flow density of sound intensity sound wave propagation. That is, the sound energy passing through the unit area perpendicular to the propagation direction in unit time. Because the strength of sound is related to the amplitude of sound source. If the amplitude of the sound source is large, the energy transmitted per unit time is large, so the sound wave is stronger. The sound wave emitted by the sound source at a certain point propagates outward, and the sound intensity at the distance from the wave source R is
Where e is the energy emitted by the sound source per second, and the unit of sound intensity I is watt/square meter. Sound intensity is related to the distance and loudness of sound propagation, but loudness is not linear with the increase of sound intensity, and there is a difference between them. Sound intensity is an objective fact and a physical quantity of sound intensity, which is not affected by human ear function. However, loudness is related to people's feelings. When the eardrum vibrates due to sound waves, the response to the same sound wave varies from person to person. Ear sensitive loudness, ear bad loudness. For sound waves with different frequencies, the ear sensation is also different. Every sound wave that can cause normal hearing needs a certain range of sound intensity. For each given frequency, to cause hearing, its sound intensity also has two extreme values. According to the experimental results of normal hearing, if the frequency is taken as the abscissa and the sound intensity as the ordinate, the upper and lower limits of sound intensity of various frequencies are connected, and the sound intensity below the lower limit cannot cause hearing. Any sound intensity exceeding the upper limit will make people feel pain. So the upper limit curve is called pain threshold, the lower limit curve is called hearing threshold, and the area between the two curves is the hearing range. Therefore, all sound waves that can cause human hearing should have a frequency of 20 ~ 20000 Hz and a sound intensity of10-12w/m2 ~1w/m2. This shows that the sound intensity changes greatly.
A measure of sound intensity. The logarithm of the ratio of sound intensity I to standard sound intensity I0 is called the "sound intensity level" of sound intensity I, which is expressed by L, namely
The unit is bell, expressed in bell. This unit is too large for practical use, so Bell's110, that is, decibels (expressed in dB), is often used as the unit, so the expression of sound intensity level is
I0 is the reference standard of sound intensity. If I0= 10- 12W/m2 is selected internationally, the sound intensity value of the audible threshold can be obtained.
The softest sound is 0. Usually, the sound intensity level during a call is 60 ~ 70dB.
Musical instruments used to play music, opera accompaniment, movie sound effects, etc. Generally classified by pronunciation, such as wind instruments: its sound is realized by the vibration of the air column. This kind of musical instruments include sheng, pipe, flute, Xiao, etc. String instruments, such as Qin Yue, Pipa, Violin, Erhu, Ma Touqin, etc., produce sounds through the vibration of strings; Percussion instruments, such as drums, gongs, bells, cymbals, hairpin, etc.
Music is the sound that the sound source vibrates regularly according to its periodicity, which is pleasing to the ear. Its waveform diagram is a periodic curve. Musical instruments vibrate according to the law, which is music; Otherwise, it is noise and harshness. The three elements of musical sound are tone, loudness and tone, which respectively reflect the characteristics of musical sound.
Noise is produced by irregular and aperiodic vibration of sound source, or by irregular combination of sounds with different intensities and frequencies. For example, the engine of the vehicle, the sound of the horn, the noise of various machines in the construction site or factory, the crying and noise of the baby, all harsh sounds are noise, also called noise. It has a great influence on people's life and work. Noise makes people fidgety, tired, nervous and distracted. It affects people's study, work, rest and sleep, and even causes diseases (such as deafness and heart disease) and accidents. 90dB is the highest limit of hearing protection in the world. Noise is one of the three public hazards (sewage, waste gas and noise) that pollute the environment at present. Noise has no pollutants and will not accumulate. It pollutes a large area, and its energy is finally completely converted into heat energy and transferred. Therefore, noise control is an extremely important aspect of environmental protection.
The frequency of ultrasonic waves is higher than 20000Hz, which exceeds the upper limit of the frequency that normal people can receive and cannot cause auditory sound waves. Its frequency is usually in the range of 2× 104 ~ 5× 108 Hz. It travels at the same speed as sound waves. Ultrasonic wave has many characteristics because of its high frequency and short wave length: because its attenuation in liquid and solid is smaller than that in air, its penetration is large; The directivity of ultrasonic wave is very strong, and the wavelength of sound wave is large, so it is easy to diffract in the propagation process, while the wavelength of ultrasonic wave is short, so it is not easy to diffract, and it will propagate along a straight line like light wave. Ultrasonic waves will be reflected when they meet impurities and refracted when they meet interfaces. Ultrasonic wave has high power, easy concentration of energy and strong effect on materials, and can be used for welding, cutting, drilling and cleaning parts. It is used for nondestructive testing in industry, such as flaw detection, thickness measurement and elastic modulus measurement, to study the microstructure of substances. In medicine, it can be used for clinical detection, such as "B-ultrasound" to measure liver, gallbladder, spleen, kidney and other diseases, or for sterilization, treatment, diagnosis and so on; In navigation and fishery, it can be used for navigation, fish detection and ocean depth measurement. Ultrasound is widely used in various fields.
Infrasound waves are also called infrasound waves. It is an acoustic wave below 20Hz, which cannot cause human hearing. It travels at the same speed as sound waves. In many natural changes, such as earthquakes, typhoons, tsunamis and volcanic eruptions, infrasound waves are generated. Man-made secondary sound sources will also appear in the process of nuclear explosion, jet flight, moving cars, ships and compressors. All carsickness and seasickness are also affected by infrasound waves when cars and ships are driving. Infrasound waves can also be used to monitor and detect atmospheric changes.