Sound energy is a form of energy. Its essence is the transfer and transformation of mechanical energy in the form of waves through the propagation medium after the object vibrates. In turn, the transfer and transformation of other energies also occur. Can be reduced to mechanical energy to produce sound. Change can be reversed. Strike the tuning fork to collect the sound waveform.
The experiment found that: when the tuning fork is lightly tapped, the amplitude of the tuning fork is small, and the amplitude of the waveform is small, and the sound emitted by the tuning fork is also small; when the tuning fork is struck hard, the amplitude of the tuning fork is large, and the amplitude of the waveform is large. The tuning fork also makes a loud sound.
Explanation: Loudness is related to the amplitude of the tuning fork vibration. The larger the amplitude, the louder the loudness; the smaller the amplitude, the smaller the loudness. Classified by frequency, sound waves with a frequency lower than 20Hz are called infrasound waves; sound waves with a frequency of 20Hz~20kHz are called audible waves; sound waves with a frequency of 20kHz~1GHz are called ultrasonic waves; sound waves with a frequency greater than 1GHz are called special ultrasound or microwave ultrasound.
Even if there is no other sound source, air particles are always oscillating irregularly, or they are always turbulent, and they stimulate weak "white noise". There is no such thing as absolutely silent atmospheric space. The so-called background noise also includes the messy sounds of many sound sources in nature or human living environment, which have no information value for speech communication. The walls of the room or steep hills also have an echo effect, and the noise is amplified and enhanced. The sound of speech and its delayed echoes add up to form a complex echo. White noise is also found in electroacoustic instruments and equipment. People get upset when the noise, which has no communication value, is strong. Interestingly, people will feel uneasy if they stay in an anechoic room with minimal noise for a long time. The appropriate use of noises such as maracas in music brings artistic appreciation value.
There is an interesting story in ancient times about how people cleverly eliminated vibration. During the Tang Dynasty, a musical instrument hanging in a monk's room in a temple in Luoyang often sounded automatically for no apparent reason. The monk became frightened and became ill, and sought treatment everywhere to no avail. He had a friend who was an official in charge of music in the imperial court, and he went to visit him after hearing the news. At this time, I heard the ringing of the bell in the temple, and the musical instrument sounded again. So my friend said: I can cure your disease because I have found the root cause of your disease. I just saw a friend find an iron file and grind it on the instrument a few times, and the instrument would no longer make any noise automatically. My friend explained that the vibration frequency of this instrument is consistent with that of the bells in the temple, so when the bell is struck, the instrument will ring accordingly. Now, if you file the instrument a little bit, it will change its natural vibration frequency. , it can no longer ring with the temple bell. The monk suddenly understood and the disease was cured.
Pedestrians on the street, the noise of vehicles, the rumble of machines - these continuous noises not only affect people's normal life, but also damage people's hearing. So people invented a muffler, which is composed of an orifice plate with many small holes and a cavity. When the frequency of the transmitted noise is the same as the natural frequency of the muffler, it will produce a violent explosion with the air column in the small holes. **Vibrate. In this way, a considerable part of the noise can be "swallowed" during vibration, and can also be converted into heat energy for use.
When the "sound source" vibrates in the air, it compresses the air for a while, making it "dense"; for a while, the air expands and becomes "sparse", forming a series of waves that vary in density and density. Vibrational energy is transmitted. This kind of wave whose vibration direction of the medium particle is consistent with the propagation direction of the wave is called a "longitudinal wave".
If we are familiar with molecular kinetic theory, we will know that since the medium molecules we study are stationary and evenly distributed, then for longitudinal waves, when the oscillator moves forward, it will It occupies the space in front where the medium molecules were evenly distributed, and compresses the original medium molecules into a small space to form a dense part. The distance between molecules in the dense part becomes smaller, and the molecular force present is repulsion. The repulsive force causes the molecules to move centrifugally around them.
As a result of the centrifugal movement, the small space that was originally the dense part becomes the sparse part, and the surrounding space becomes the new dense part. So, from a macro perspective, it is equivalent to the original dense part becoming sparse, and the dense part spreading out. Then, the new sparse part also spreads. However, it should be noted that although sound waves are generally longitudinal waves, when propagating in solids, they can also have both longitudinal and transverse waves. The speed of transverse waves is about 50%-60% of the speed of longitudinal waves.
Sound waves in the air are longitudinal waves. The reason is that gases and a considerable amount of liquids (collectively called fluids) cannot withstand shear force. Therefore, sound waves cannot be transverse waves when propagating in fluids; but solids can not only withstand Compressive (tensile) stress can also withstand shear stress, so there can be both longitudinal and transverse waves in solids. The air particles vibrate in the same way as the sound source body vibrates. When the sound waves reach the human eardrum, they cause the eardrum to vibrate in the same way. The energy that drives the ear drum to vibrate comes from the sound source body, which is ordinary mechanical energy.
Different sounds are different modes of vibration, which can distinguish different information. The human ear can distinguish the sound of wind, rain and different people's voices, as well as various speech sounds. They are all different information waves coming from the sound source.
Some animals can emit ultrasonic waves generated from the throat through the oral cavity or nasal cavity, and use the reverberated sound to orient. This method of spatial orientation is called echolocation. For example, the "Radar Flying Beast" bat can fly accurately at extremely fast speeds in complete darkness, never colliding with objects in front of it. If its ears are covered and its mouth is blocked, it will lose the ability to avoid collision with objects. After measurement by a high-frequency pulse detection device, it was confirmed that when bats fly, ultrasonic pulses that are inaudible to human ears are generated in the throat and can be emitted through the mouth.
Humans can hear sounds with a frequency of up to 20 kilohertz, while some bats can emit and hear sounds as high as 100 kilohertz. When encountering food or obstacles, the pulse wave will be reflected back. The bat uses its two ears to receive the reflected wave of the object and determine the position of the object accordingly. It can also identify the difference between the echoes received from its two ears. The distance, shape and nature of the object; the size of the object is distinguished by the wavelength in the echo.
Most bats can vibrate their tongues to make sounds, some make high-pitched chirps, and some can make sounds through their nostrils. They all help the bat determine the direction of the echo and decide whether to move forward or turn. Bats can use ultrasonic waves to "navigate" in the air and capture flying insects quickly and accurately. In addition, some marine mammals can emit sound waves with a wide frequency band underwater, even as high as 300,000 Hz. For example, toothed whales and dolphins can rely on the reflection of sound from nearby land to use echolocation to determine the direction and know the location of objects or coasts. Some seals and sea lions can also emit underwater ultrasonic waves.
The method of detecting the orientation and distance of objects by using the principle of reflection during wave propagation is called "echolocation". Animal "echolocation" refers to the way animals orient themselves in space by emitting sound waves and using the echoes reflected from objects. It has two functions: catching prey and avoiding objects.
According to research, it is known that almost all species of the suborder Microbats in the animal kingdom, the genus Fruit Bats of the suborder Macrobats, toothed whales of the order Cetaceans, seals and sea lions of the order Pinnipeds, and the insectivores of the order Mashimaidae, The short-tailed shrews of the shrew family, the oilbirds of South America, the swiftlets of Southeast Asia, and some fish all have the ability to echolocate. They all have natural sonar systems in their bodies that complete echolocation. Sonar is mainly composed of "acoustic transmitter", "echo receiver" and "distance indicator".
The conversion of sound energy involves both physical changes and chemical changes, because this is the conversion of energy. The medium will produce a series of effects under the action of sound energy, such as mechanical effects, heating and luminescence effects, chemical effects, discharge effects and biological effects. The propagation of sound must have three elements: sound source, transmission medium and receiver.
The sound source is the object that generates vibration; the propagation medium is the channel through which energy flows; the receiver is the device that senses the sound. For example, when playing a musical instrument, the musical instrument is the sound source, the air is the transmission medium, and the ears are the receiving devices for sensing sound. The range of sound energy forms a sound field. There is energy loss in the transmission of sound, also called absorption. When the distance is relatively far, we cannot hear the sound, and the change in sound intensity is proportional to the square of the propagation distance (inverse square law).
When sound waves propagate in a medium, if there is no medium to propagate, no sound will be produced. When sound waves propagate to surrounding interfaces, they will cause vibrations in other solids, etc.
The sensitivity of dolphin sonar is very high. It can detect metal wires with a diameter of 0.2mm and nylon ropes with a diameter of 1mm several meters away. It can distinguish two signals with a time difference of only 200 seconds. It can detect fish schools hundreds of meters away. Cover your eyes and move flexibly and quickly through a pool full of bamboo poles without touching the bamboo poles; dolphin sonar has a strong "target recognition" ability and can not only identify different fish, but also distinguish brass, aluminum, and bakelite , plastics and other different materials, it can also distinguish between the echo of its own sound and the sound wave that people record and play back; the anti-interference ability of dolphin sonar is also amazing. If there is noise interference, it will raise its cry The intensity of the sound can overwhelm the noise so that one's own judgment is not affected; moreover, dolphin sonar also has the ability to express emotions. It has been confirmed that dolphins are animals with "language", and their "conversation" is precisely through their sonar system.
In particular, the most precious of the only four freshwater dolphins left in the world - the white-tip dolphin in the middle and lower reaches of the Yangtze River in my country, has a clear "division of labor" in its sonar system, which is used for positioning and communication. Some are used for alarm and police purposes, and have the special function of modulating the phase through frequency modulation. The unit of sound intensity is "decibel". The larger the value, the greater the amplitude and the louder the sound. When it reaches a certain level, it becomes noise. When it reaches the top level, we can no longer feel the sound, but it is still there.
Different sounds can be algebraically superimposed. Ultrasound is a sound wave with a frequency higher than 20,000 Hz. It has good directionality, strong penetrating ability, and is easy to obtain concentrated sound energy. It propagates over a long distance in water and can be used for distance measurement, speed measurement, cleaning, welding, gravel, sterilization and disinfection. wait. It has many applications in medicine, science, military, industry, and agriculture.
Ultrasound is named because its lower frequency limit is approximately equal to the upper limit of human hearing. Under the condition of the same amplitude, the energy of an object's vibration is proportional to the vibration frequency. When ultrasonic waves propagate in the medium, the frequency of the medium particle vibration is very high, so the energy is very large.
Ultrasound and audible sound are essentially the same. Their main similarity is a mechanical vibration mode, which usually propagates in elastic media in the form of longitudinal waves, which is a kind of energy propagation. Form, its difference is that the ultrasonic frequency is high, the wavelength is short, and it has good beam emission and directionality when propagating along a straight line within a certain distance. The frequency range used in abdominal ultrasonic imaging is currently between 2∽5 MHz, and is commonly used as 3∽3.5 MHz (one vibration per second is 1Hz, 1 MHz = 10^6Hz, that is, 1 million vibrations per second, the frequency of audible waves is between 16-20,000HZ).
The propagation rules of ultrasonic waves such as reflection, refraction, diffraction, and scattering in the medium are not essentially different from the rules of audible sound waves. But the wavelength of ultrasonic waves is very short, only a few centimeters or even thousandths of a millimeter. Compared with audible sound waves, ultrasonic waves have many strange characteristics: Propagation characteristics - the wavelength of ultrasonic waves is very short, and the size of ordinary obstacles is many times larger than the wavelength of ultrasonic waves. Therefore, the diffraction ability of ultrasonic waves is very poor. It can propagate in a directional straight line. The shorter the wavelength of the ultrasonic wave, the more significant this characteristic is. Power characteristics: When sound propagates in the air, it pushes the particles in the air to vibrate back and forth and does work on the particles. Sound wave power is a physical quantity that indicates how fast sound waves do work.
Under the same intensity, the higher the frequency of the sound wave, the greater the power it has. Because the frequency of ultrasonic waves is very high, its power is very large compared with ordinary sound waves. Cavitation ─ When ultrasonic waves propagate in the medium, there is an alternating cycle of positive and negative pressure. In the positive pressure phase, the ultrasonic waves squeeze the medium molecules, changing the original density of the medium and making it increase; in the negative pressure phase, the When the phase is compressed, the medium molecules are sparse and further dispersed, and the density of the medium is reduced. When ultrasonic waves with only large amplitude are used to act on the liquid medium, the average distance between the medium molecules will exceed the critical molecular distance that keeps the liquid medium unchanged. , the liquid medium will break and form microbubbles. These small cavities expand and close rapidly, causing violent collisions between liquid particles, thereby generating pressures of several thousand to tens of thousands of atmospheres. This violent interaction between particles will cause the temperature of the liquid to rise suddenly, playing a good stirring role, thereby emulsifying two immiscible liquids (such as water and oil), and accelerating the dissolution of solutes. chemical reaction. The various effects caused by the action of ultrasonic waves in liquids are called ultrasonic cavitation.
Sound waves with frequencies higher than 2×10 kilohertz.
The branch of acoustics that studies the generation, propagation, reception of ultrasonic waves, as well as various ultrasonic effects and applications is called ultrasonics. Devices that generate ultrasonic waves include mechanical ultrasonic generators (such as air whistles, steam whistles, liquid whistles, etc.), electric ultrasonic generators made using the principles of electromagnetic induction and electromagnetic action, and the electrostrictive effect of piezoelectric crystals and ferromagnetism. Electroacoustic transducers made of the magnetostrictive effect of matter, etc.
The mechanical action of ultrasonic waves can promote the emulsification of liquids, liquefaction of gels and dispersion of solids. When a standing wave is formed in an ultrasonic fluid medium, the tiny particles suspended in the fluid condense at the wave nodes due to the action of mechanical force, forming periodic accumulations in space. The action of ultrasound can promote or accelerate certain chemical reactions. For example, pure distilled water will produce hydrogen peroxide after ultrasonic treatment; water with dissolved nitrogen will produce nitrous acid after ultrasonic treatment; aqueous dye solutions will change color or fade after ultrasonic treatment. The occurrence of these phenomena is always accompanied by cavitation. Ultrasound can also accelerate the hydrolysis, decomposition and polymerization processes of many chemical substances.
Ultrasound also has a significant impact on photochemical and electrochemical processes. After ultrasonic treatment of aqueous solutions of various amino acids and other organic substances, the characteristic absorption spectral bands disappear and become uniform general absorption, which indicates that cavitation has changed the molecular structure. The sound waves emitted by the vibration of the object spread around, and the sound wave energy gradually spreads. The diffusion of energy reduces the energy present per unit area, and the sound heard becomes weaker. The energy of sound waves per unit area decreases with the square of the distance from the sound source.
When sound waves propagate in a solid medium, the viscosity of the medium causes internal friction between particles, thereby converting part of the sound energy into heat energy; at the same time, due to the heat conduction of the medium, the density and density of the medium Heat exchange occurs between sparse parts, resulting in the loss of sound energy. This is the absorption phenomenon of the medium. This attenuation of the medium is called absorptive attenuation. It is generally believed that absorption attenuation is proportional to the first power of the sound wave frequency and the square of the frequency.
The attenuation of sound waves caused by the presence of granular structures in the medium (such as suspended particles and bubbles in liquids, granular structures, defects, inclusions in solids, etc.) is called scattering attenuation. It is generally believed that when the particle size is much smaller than the wavelength, the scattering attenuation is proportional to the fourth power of the frequency; when the particle size is similar to the wavelength, the scattering attenuation is proportional to the square of the frequency. According to the mechanical characteristics of the sound source, noise pollution can be divided into: noise caused by gas disturbance, noise caused by solid vibration, noise caused by liquid impact, and electromagnetic noise caused by electromagnetic action. Noise can be divided into high-frequency noise according to the frequency of sound: 1000Hz.
The time-varying properties of noise can be divided into: steady-state noise, unsteady-state noise, fluctuation noise, intermittent noise and pulse noise, etc. The energy of sound in propagation attenuates as the distance increases, so keeping the noise source away from the place where quiet is required can achieve the purpose of noise reduction. The radiation of sound is generally directional. At places with the same distance from the sound source but different directions, the received sound intensity will be different.
However, when most sound sources radiate noise at low frequencies, the directivity is very poor; as the frequency increases, the directivity increases. Apply sound-absorbing materials and sound-absorbing structures to convert the propagating noise sound energy into heat energy, etc. When the incident sound energy is completely reflected, α=0 means no sound absorption; when the incident sound wave is not reflected at all, α=1 means it is completely absorbed.
The sound absorption coefficient of general materials or structures is α=0~1. The larger the α value, the better the sound absorption energy. It is currently the most commonly used parameter to characterize sound absorption performance. 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, the a of all materials is between 0 and 1, which means that 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.
In engineering, the noise reduction coefficient NRC is often used to roughly evaluate the sound absorption performance in the speech frequency range. This value is the arithmetic of the sound absorption coefficient of the material at the four frequencies of 250, 500, 1K, and 2K. Average, rounded to 0.05. It is generally considered that materials with NRC less than 0.2 are reflective materials, and materials with NRC greater than or equal to 0.2 are considered sound-absorbing materials. When it is necessary to absorb a large amount of sound energy to reduce indoor reverberation and noise, it is often necessary to use materials with high sound absorption coefficients. For example, centrifugal glass wool and rock wool are high NRC sound-absorbing materials. The NRC of 5cm thick 24kg/m3 centrifugal glass wool can reach 0.95.
A sound level meter generally consists of a condenser microphone, a preamplifier, a noise meter picture attenuator, an amplifier, a frequency meter network and an effective value indicator meter. The working principle of the sound level meter is: the microphone converts the sound into an electrical signal, and then the preamplifier converts the impedance to match the microphone and the attenuator. The amplifier adds the output signal to the network, performs frequency weighting on the signal (or connects an external filter), and then amplifies the signal to a certain amplitude through the attenuator and amplifier, and sends it to the effective value detector.