First, the production of sound

Sound is produced by the vibration of objects. For example, speech sounds are generated from the vibration of vocal cords in the larynx, loudspeaker (speaker) sound is generated from the vibration of paper basin, and mechanical noise is generated from the vibration of mechanical parts. We call a sound source an object that makes a sound. After the sound source sounds, it has to pass through a certain medium to propagate outward. For example, when a loudspeaker makes a paper bowl vibrate back and forth with an external signal, it also makes the air in its vicinity vibrate. When the paper basin vibrates in a certain direction, it compresses the adjacent air, making it denser. When the paper bowl vibrates in the opposite direction, this part of the air becomes thinner. The adjacent air vibrates along with the vibration of the paper basin, and at the same time makes the distant air do the same vibration. The wave of the air's vibration propagation is called the sound wave. The schematic diagram of sound wave propagation is shown in the figure. Sound wave travels in all directions at a certain speed. When the sound wave reaches the ear, it will cause corresponding vibration of the eardrum. This vibration passes through the auditory nerve and makes us feel sound.

So there are three basic requirements for hearing a sound.

One is the existence of a sound body or sound source.

The second is to have the propagation process of the elastic medium, such as air, or liquid, solid elastic medium; There is no elastic medium in a vacuum, so vacuum cannot transmit sound waves: not on the moon

The third is to produce the sense of sound through the in-ear hearing. The propagation of sound waves The propagation of sound waves can also be likened to water surface waves. When a stone is dropped into still water, circles of waves are seen on the surface of the water, rising and falling alternately from crest to trough. Because the surface of the water is fluctuating, the surface waves carry energy. If you float a very small piece of wood on the surface of the water, you can see that this little piece of wood moves up and down with the crest and trough of the water, and when the water is calm, the piece of wood stays in its original position.

Thus, the particles of water themselves do not follow the wave, but the wave energy of the water is transmitted from one part of the water surface to another nearby. This is similar to the way the air does not follow sound as it travels through the air, but vibrates near the equilibrium position. So the propagation of the sound wave is actually the propagation of the energy of the sound wave along with the sound wave. The space in which sound waves exist is called the sound field. However, sound wave is also different from water wave. The vibration direction of water wave is perpendicular to the propagation direction of the wave. Therefore, water wave is a shear wave. The propagation direction of sound wave is consistent with the vibration direction of density phase, so the expression form of sound wave in air is longitudinal wave.

It can be seen from the above that vibration and wave are motion forms closely related to each other, vibration is the source of wave, and wave is the propagation process of vibration. The nature of sound is a wave, so sound is also called a sound wave. For the sake of clarity, the physical process of sound is usually called sound wave, and the process relating to hearing is called sound.

Frequency, wavelength and sound velocity

The time it takes a sound source to complete a vibration is called a period and is recorded as seconds. The number of internal vibrations is called the frequency, which is called Hertz, and it is the reciprocal of the period, that is, the vibration of the sound source can produce sound waves, but not all of the sound waves produced by the vibration can be heard by people, because of the characteristics of the human ear. Only when the frequency of the ~ range of sound waves to the human ear, cause the eardrum vibration, to produce a sound sense. So sound waves with frequencies in the range are usually called audible. Sound waves below are called infrasound, and sound waves above are called ultrasound. Neither infrasound nor ultrasound can produce a sense of sound.

The distance that a sound wave travels through a medium per second is called the speed of sound propagation, or the speed of sound for short, and is written in meters per second. The velocity of sound is not the velocity of particle vibration but the propagation velocity of vibration state, and its magnitude has nothing to do with the characteristic of vibration, but with the elasticity, density and temperature of the medium.

The propagation velocity of sound wave is essentially the speed with which the medium molecules transmit momentum to the neighboring molecules. Obviously, the closer the molecular structure of the medium is, the smaller the internal loss characteristic is, and the higher the sound velocity value will be. For example, the dielectric properties of air, water, and steel determine that their sound velocity ratios are approximately. Since the temperature is closely related to the active degree of the molecular motion of the medium, the sound velocity increases correspondingly when the temperature of the medium rises. Take air as an example, the relationship between sound velocity and temperature can be expressed as (), where is air temperature ℃; The speed of sound in air at ℃ is equal to. For the usual ambient temperature, that is, when the ratio is much lower, the above equation can be simplified as (). Thus, it can be seen that the sound velocity increases with every increase in air temperature of ℃. Generally, the sound velocity of air towel at room temperature ℃ is. The distance that the sound source travels, the distance that the sound wave travels, or the distance that the sound wave travels between two adjacent particles that are in the same phase in their path, is called the wavelength of the sound wave, which is in meters. Therefore, there are the following relationships among the sound velocity, frequency and wavelength :() since the sound velocity of a certain medium is constant, the frequency is inversely proportional to the wavelength. For example, the wavelength of frequency in room temperature air is theta, the wavelength of theta is theta or theta.

Reflection and diffraction of sound waves

Geometric acoustic sound waves start from a sound source and travel in a certain direction in the same medium. The enveloping surface of each point reached by the fluctuation is called wave front. Waves whose wavefront is a plane are called nail waves, and those whose wavefront is a sphere are called spherical waves. An acoustic wave radiated by a point wave is a spherical wave but can be approximately considered a plane wave in a local area far enough away from the source.

Sound lines are often used to indicate the direction of sound wave propagation. The direction of sound lines is perpendicular to the wave front. The study of the propagation of sound waves from the point of view of sound lines is called geometric acoustics. In contrast, the study of acoustic problems from a wave point of view is called physical acoustics. Reflection of sound waves When an acoustic wave travels through a wall or obstacle much larger in size than its wavelength, the sound wave is reflected. If the sound wave is a spherical wave, after reflection is still a football wave. As shown in the figure, the reflection wave is represented by a dashed line, which is emitted as if from an image of the sound source. The and points are symmetric points on the reflecting plane. The reflected wave has the same radius as the incident wave at the same time. If the direction of advance is represented by a sound line, the reflected sound line may be regarded as originating from a virtual source. Therefore, it is easy to determine the direction of the reflected wave by using the symmetric relation between sound source and virtual source. As with the law of geometric-optical reflection, the Angle of reflection of an acoustic wave is equal to the Angle of incidence. When the reflecting surface is a curved surface, as shown in the figure, the law of acoustic reflection can still be used to find the reflected sound line of the acoustic wave on the curved surface. For example, if you want to find the reflection ray at a point on the surface, you can determine the reflected sound line by taking the section of the surface passing through the point as the mirror and making its incidence Angle equal to the reflection Angle. It can be seen from the figure that the convex surface has an obvious scattering effect on the incident acoustic wave, which contributes to the sound field diffusion uniformity. The figure uses the feature of concave surface reflection to make the sound converge to a certain area or appear sound focus, thus causing uneven distribution of sound field, which should be avoided in the design of indoor sound quality. Diffraction of sound Waves The principle of geometrical acoustics described above is based on a similarity to geometrical optics, that is, sound travels in straight lines, but this assumption is valid only if the reflecting surface or obstacle and the size of the hole is much larger than the sound wave. When the ulnar acoustic wave wavelength of an obstacle or a hole is small, the sound wave will produce diffraction (also known as diffraction) or bending, that is, the sound wave will bypass the obstacle or change its direction of advance through the hole, as shown in the figure. If the diameter of the hole size is much lower than the acoustic wavelength (the), sound waves through the hole is not as straight as the light propagation, but can be spread around to the back of the mask change original direction. The holes in the particle can be seen as a new source, generate new spherical wave, and have nothing to do with the original waveform. Normally we can hear the other side of the voice in the side of the wall, is also the result of the diffraction. The lower the frequency of the sound source, the diffraction phenomenon is obvious: On the contrary, the higher the frequency, the more difficult to produce diffraction, and transmission also has strong directivity.. Sound waves in the spread of the refraction of sound waves on the way to meet different medium interface, besides reflects, bent. Sound wave propagation direction will change after the refraction, as shown in the figure, compared with the normal incidence Angle and the relationship between refracting Angle is as follows: (), and for the two kinds of medium velocity. By type,; when, at the time. The sound waves from the sound of small medium, medium refraction into the sound waves propagation fold from the boundary surface normal direction; on the other hand, the sound waves from the speed of sound small big medium, medium refraction into the sound waves propagation direction from the normals. Therefore, the refraction of sound waves is determined by the velocity of sound, even if there are in the same medium velocity gradient also produces the refraction change (sound). For example, outdoor square, the atmosphere during the day higher ground temperature (warm), and thus the speed of sound bigger type, Sound velocity decreased with increasing of height from the ground, and sound propagation direction bending upwards, as shown, so are less likely to be in a square behind. On the contrary, low ground temperature (cold) in the evening, and sound velocity is small, the speed of sound increases with the increase of the height, the sound propagation direction is bending down, as shown. This phenomenon can be used to explain why the voice spread further apart during the night than during the day. In addition, the wind speed will also affect the direction of propagation of, when the actual wind velocity is the average sound velocity vectors with wind speed. Therefore, when the sound wave travels with the wind is from the direction of the velocity to the speed of fast, so the sound propagation direction of bending down, When the wind is upwind, the direction of sound propagation bends upward and produces a negative area (static area), as shown on the left, which explains why sounds propagated upwind from the source are often hard to hear.

Transmission and absorption of sound waves

When an acoustic wave enters an object such as a wall, as shown in the figure, the sound energy is partly reflected, partly transmitted through the object, and partly lost due to the vibration of the object or the friction or heat conduction of the medium as the sound travels within the object, which is often referred to as material absorption. According to the law of conservation of energy, it is assumed that the total sound energy incident on the object in unit time is, the reflected sound energy is, the absorbed sound energy of the object is, and the ratio of transmitted sound energy to incident sound energy is, namely, the transmission coefficient. The ratio of reflected and incident sound energy is called the reflection coefficient. Generally, the material with low value is called sound insulation material, and the material with low value is called sound absorption material. In fact, what an object absorbs is only the sound absorption coefficient of a material. Considering the space where the incident and reflected waves are located, the following formula is commonly used to define the sound absorption coefficient of a material :() when the incident sound energy is all reflected; , the incident sound energy is all absorbed. Therefore, the value is between ~. If the sound absorption coefficient of a material is said to be the incident sound energy is absorbed. The higher the value, the better the sound absorption performance. The sound absorption coefficient is not only related to the property of the material itself, but also related to the frequency and incidence direction of the sound wave. Generally speaking, solid and smooth floors and walls have a very small sound absorption coefficient, while porous (aerated) materials are commonly used as sound absorption materials with high efficiency. Generally, the sound absorption capacity of porous materials is related to the thickness of the materials. The thickness increases and the low-frequency sound absorption increases. But the material thickness has little influence on the high frequency. Theoretically, when the thickness of the material is equal to the wavelength, it has the maximum sound absorption effect at this frequency. But for low frequency, at this time the material thickness is often above, so it is not economical. If a thin porous material is used at a distance from the hard wall of the back, the sound-absorbing performance is almost the same as if all the cavities were filled with similar materials.