It is found that the amplitude of the tuning fork is small, the amplitude of the waveform is also small, and the sound made by the tuning fork is also small. When the tuning fork is struck again, the amplitude of the tuning fork is large and the amplitude of the waveform diagram is also large. At this time, the tuning fork makes a loud sound.
Description: Loudness is related to the amplitude of tuning fork vibration. The greater the amplitude, the greater the loudness; The smaller the amplitude, the smaller the loudness. According to frequency classification, sound waves with frequencies below 20Hz are called infrasound waves; Sound waves with a frequency of 20Hz~20kHz are called audible waves; The sound wave with frequency of 20kHz~ 1GHz is called ultrasonic wave. Sound waves with a frequency greater than 1GHz are called ultrasound or microwave ultrasound.
Even without other sound sources, air particles are always oscillating irregularly, or always in turmoil, and they excite weak "white noise". There is no absolute silence in atmospheric space. The so-called background noise also includes many messy sounds in nature or human living environment, which have no information value for verbal communication. The walls or steep slopes of the room also have echo effect, and the noise is amplified and enhanced. Speech and its lagging echo are superimposed to form a complex echo. There is also white noise in electroacoustic instruments and equipment. People will feel uneasy when the noise without communication value is very strong. Interestingly, if you stay in an anechoic room with minimal noise for a long time, people will feel uneasy. Proper use of noise such as sand hammer in music brings artistic appreciation value.
There is an interesting story in ancient times about how people skillfully eliminate vibration. In the Tang Dynasty, a musical instrument hung in a monk's room in a temple in Luoyang often sounded automatically for no reason, so the monks became frightened into illness and sought treatment everywhere. He has a friend who is an official in charge of music in North Korea. He went to see him when he heard the news. At this moment, I just heard the bell ring in the temple and the musical instrument rang again. So the friend said: I can cure your illness, because I found the root of your illness. I saw a friend find an iron file and grind it on the instrument a few times, and the instrument will never sound automatically again. My friend explained that this musical instrument conforms to the vibration frequency of the bell in the temple, so when the bell rings, the musical instrument will ring accordingly. Now, if you file the instrument a little, it will change its natural vibration frequency, and it will no longer ring with the bell of the temple. The monk suddenly realized that the disease had been cured.
Pedestrians in the street, the noise of vehicles, the rumble of machines-these constant noises not only affect people's normal life, but also damage people's hearing. So people invented the muffler, which consists of an orifice plate with many small holes and a cavity. When the noise frequency is the same as the natural frequency of the muffler, it will vibrate violently with the air column in the small hole. In this way, a considerable part of noise can be "swallowed up" during vibration, and can also be converted into heat energy for utilization.
When the "sound source" vibrates in the air, it will compress the air for a while to make it "dense"; After a while, the air expands and becomes "sparse", forming a series of sparse and dense waves to transmit vibration energy. Waves whose vibration direction is consistent with the wave propagation direction are called "longitudinal waves".
If we are familiar with the theory of molecular motion, we will know that because the dielectric molecules we study are static and evenly distributed, for longitudinal waves, when the vibrator moves forward, it will occupy the space where the dielectric molecules are evenly distributed in front, compressing the original dielectric molecules into a small space and forming a dense part. The distance between molecules in the dense part becomes smaller, and the molecular force presented is repulsion. The repulsive force makes the molecules move centrifugally.
As a result of centrifugal movement, the small space that was originally a dense part became a sparse part, and the surrounding space became a new dense part. Then macroscopically, it is equivalent to that the original secret part has become sparse and the secret part has spread out. Then, the new sparse departments also dispersed. However, it should be noted that although sound waves are generally longitudinal waves, they can also have longitudinal waves and shear waves when propagating in solids, and the speed of shear waves is about 50%-60% of longitudinal waves.
The sound wave in the air is longitudinal wave, because gas and a considerable amount of liquid (collectively referred to as fluid) can't bear shear force, so the sound wave can't be shear wave when it propagates in the fluid; But solids can bear not only compressive (tensile) stress, but also shear stress, so there can be both longitudinal waves and shear waves in solids. Air particles vibrate in the same way as sound sources. When sound waves reach the eardrum, it will cause the eardrum to vibrate in the same way. The energy that drives the eardrum to vibrate comes from the sound source, which is ordinary mechanical energy.
Different sounds are different vibration modes and can distinguish different information. The human ear can distinguish between wind, rain and different people's voices, and can also distinguish various voices, which are different information waves from sound sources.
Some animals can emit ultrasonic waves from their throats through their mouths or noses, and use the folded sound to orient them. This method of spatial localization is called echo localization. For example, the "radar flying beast" bat can fly accurately at a very fast speed in complete darkness and will never collide with the object in front. If you cover its ears and block its mouth, you will lose the ability to avoid colliding with objects. The measurement by high-frequency pulse detector proves that when bats fly, they can produce ultrasonic pulses in their throats, and they can emit sounds that cannot be heard by human ears through their mouths.
Humans can hear the sound with the frequency of 20 kHz at most, while some bats can emit and hear the sound of 100 kHz. When encountering food or obstacles, the pulse wave will reflect back. Bats use their ears to receive the reflected waves of objects and determine the position of objects accordingly. They can distinguish the distance, shape and nature of objects by the differences between the echoes received by their ears. The size of an object is distinguished by the wavelength of the echo.
Most bats can pronounce with trembling tongues, some make shrill calls, and some can make sounds through their nostrils. They all help bats determine the direction of the echo and decide whether to go forward or turn. Bats can use ultrasonic waves to "navigate" in the air. They can catch flying insects quickly and accurately. In addition, some marine mammals can emit broadband sound waves underwater, even as high as 300,000 Hz. Such as toothed whales and dolphins, can use echolocation to determine the direction and know the location of objects or coasts through the reflection of nearby land sounds. Some seals and sea lions can also emit underwater ultrasonic waves.
The way to detect the direction and distance of an object by using the principle of wave reflection during propagation is called "echo location". The "echo location" of animals refers to the way that animals make spatial location by emitting sound waves and using the echoes reflected by objects. It has two functions: catching prey and avoiding objects.
According to the research, it is known that almost all species of Lepidoptera in the animal kingdom, such as toothed whales of Lepidoptera, cetaceans, seals and sea lions of Pinpoda, short-tailed shrews of Lepidoptera, oil birds in South America, Jin Siyan in Southeast Asia and some fish, have the ability of echolocation. They all have natural sonar systems in their bodies to complete echo localization. Sonar mainly consists of "acoustic transmitter", "echo receiver" and "distance indicator".
There are both physical and chemical changes in the transformation of acoustic energy, because this is the transformation of energy. The medium will produce a series of effects under the action of acoustic energy, such as mechanical effect, heating luminescence effect, chemical effect, discharge effect, biological effect and so on. Sound propagation must have three elements: sound source, media and receiver.
Sound source is an object that produces vibration; Media is the channel of energy flow; A receiver is a device that perceives sound. For example, when playing a musical instrument, the musical instrument is the sound source, the air is the medium of communication, and the ear is the receiving device for feeling the sound. The range of acoustic energy forms a sound field. The transmission of sound has energy loss, which is also called absorption. When the distance is far, we can't hear the sound, and the change of sound intensity is directly 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, there will be no sound. When the sound wave propagates to the surrounding interface, it will cause the vibration of other solids. Dolphin sonar has a high sensitivity. It can find a metal wire with a diameter of 0.2 mm and a nylon rope with a diameter of 1 mm several meters away, can distinguish two signals with a time difference of 200 burs, can find fish schools hundreds of meters away, and can walk through a pool full of bamboo poles flexibly and quickly blindfolded without touching. Dolphin sonar has a strong "target recognition" ability, which can not only identify different fish and distinguish different materials such as brass, aluminum, bakelite and plastic, but also distinguish the echo of its own voice from the sound waves played back by the person who recorded its voice. The anti-jamming ability of dolphin sonar is also amazing. If there is noise interference, it will increase the call intensity over the noise so that its judgment will not be affected. And dolphin sonar also has the ability to express feelings. It has been proved that dolphins are animals with "language", and their "dialogue" is carried out through their sonar system.
In particular, among the four remaining freshwater dolphins in the world, the most precious baiji in the middle and lower reaches of the Yangtze River in China has a clear "division of labor" in its sonar system, which is used for positioning, communication, reporting and police, and has a special function of phase modulation through frequency modulation. The unit of sound intensity is decibel. The larger the value, the greater the amplitude and the louder the sound, and it becomes noise to a certain extent. At a low level, we can no longer feel the sound, but it still exists.
Different sounds can be algebraically superimposed. Ultrasonic wave is a sound wave with a frequency higher than 20000 Hz. It has good directivity, strong penetration ability, easy to obtain concentrated acoustic energy and long propagation distance in water. It can be used for ranging, measuring speed, cleaning, welding, crushing, sterilization and disinfection. It has many applications in medicine, science, military, military, industry and agriculture.
Ultrasound is named because its lower frequency limit is about equal to the upper hearing limit of human beings. Under the same amplitude, the vibration energy of an object is directly proportional to the vibration frequency. When ultrasonic wave propagates in medium, the vibration frequency of medium particles is very high, so the energy is very high.
Ultrasonic wave and audible sound are essentially the same, and their similarity is a mechanical vibration mode, which usually propagates in elastic medium in the form of longitudinal wave and is a form of energy transfer. The difference is that the ultrasonic wave has high frequency and short wave length, and it has good beam and directivity when traveling in a straight line within a certain distance. At present, the frequency range for abdominal ultrasound imaging is between 2 and 5 MHz. Commonly used as 3∽3.5 MHz (vibration per second 1 Hz, 1 MHz = 10 6 Hz, that is, vibration per second10 million, and the frequency of audible waves is16-2000.
There is no essential difference between the propagation law of reflection, refraction, diffraction and scattering of ultrasonic waves in media and audible sound waves. But the wavelength of ultrasonic wave is very short, only a few centimeters, even a few 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, so the diffraction ability of ultrasonic waves is very poor, and it can travel in a straight line in a uniform medium. The shorter the wavelength of ultrasonic wave, the more remarkable this characteristic is. Power characteristics-when sound propagates in the air, it pushes the particles in the air to vibrate back and forth and do work on them. Sound wave power is a physical quantity, indicating the speed at which sound waves do work.
At the same intensity, the higher the frequency of sound wave, the greater its power. Because the frequency of ultrasonic wave is very high, its power is very large compared with ordinary sound wave. Cavitation-when ultrasonic waves propagate in a medium, there is a period of alternating positive and negative pressures. In the positive pressure stage, ultrasonic waves squeeze the medium molecules to change the original density of the medium and increase it; In the negative pressure stage, the medium molecules are sparse and further dispersed, and the medium density decreases. When only ultrasonic waves with large amplitude act on the liquid medium, the average distance between the molecules of the medium will exceed the critical molecular distance to keep the liquid medium unchanged, and the liquid medium will rupture to form microbubbles. The rapid expansion and closure of these small cavities will cause violent collisions between liquid particles, thus generating pressures of thousands to tens of thousands of atmospheres. This strong interaction between particles will make the temperature of the liquid suddenly rise and play a good role in stirring, thus emulsifying two immiscible liquids (such as water and oil), accelerating the dissolution of solute and accelerating the chemical reaction. This effect produced by the action of ultrasound in liquid is called ultrasonic cavitation.
Sound wave with frequency higher than 2× 10 kHz. A branch of acoustics that studies the generation, propagation and reception of ultrasonic waves, as well as various ultrasonic effects and applications, is called ultrasound. The devices for generating ultrasonic waves include mechanical ultrasonic generators (such as air whistle, whistle and liquid whistle), electro-ultrasonic generators based on electromagnetic induction and electromagnetic action, electro-acoustic transducers based on electrostrictive effect of piezoelectric crystals and magnetostrictive effect of ferromagnetic substances.
The mechanical action of ultrasonic wave can promote the emulsification of liquid, the liquefaction of gel and the dispersion of solid. When standing waves are formed in the ultrasonic fluid medium, tiny particles suspended in the fluid condense at nodes due to mechanical force, and form periodic accumulation in space. The action of ultrasound can promote or accelerate some chemical reactions. For example, pure distilled water generates hydrogen peroxide after ultrasonic treatment; Nitrite is produced by ultrasonic treatment of water dissolved with nitrogen; Dye aqueous solution will change color or fade after ultrasonic treatment. These phenomena are always accompanied by cavitation. Ultrasound can also accelerate the hydrolysis, decomposition and polymerization of many chemicals.
Ultrasound also has obvious influence on photochemical and electrochemical processes. After ultrasonic treatment, the characteristic absorption bands of various amino acids and other organic aqueous solutions disappeared, showing uniform general absorption, indicating that cavitation changed the molecular structure. The sound wave emitted by the vibration of an object spreads around, and the sound wave energy gradually spreads. The diffusion of energy reduces the energy per unit area and weakens the sound heard. The sound energy per unit area decreases with the square of the sound source distance.
When the sound wave propagates in the solid medium, the internal friction between particles is caused by the viscosity of the medium, and part of the sound energy is converted into heat energy; At the same time, due to the heat conduction of the medium, the dense part and the sparse part of the medium exchange heat, which leads to the loss of acoustic energy, which is the absorption phenomenon of the medium. This attenuation of medium is called absorption attenuation. It is generally believed that absorption attenuation is proportional to the first power of sound wave frequency and the square of frequency.
When there are particle structures in the medium (such as suspended particles and bubbles in liquid, particle structures, defects and impurities in solid, etc.). ), the attenuation of sound waves 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 frequency; When the particle size is close to the wavelength, the scattering attenuation is proportional to the square of frequency. According to the mechanical characteristics of 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:: 1000Hz high frequency noise.
According to the time-varying characteristics, noise can be divided into steady noise, unsteady noise, fluctuation noise, intermittent noise and impulse noise. The energy of sound in propagation decreases with the increase of distance, so the purpose of noise reduction can be achieved by keeping the noise source away from the place where it needs to be quiet. The radiation of sound is generally directional, and the received sound intensity is different at the same distance from the sound source but in different directions.
But when most sound sources radiate noise at low frequency, the directivity is very poor; With the increase of frequency, the directivity increases. Using sound-absorbing materials and sound-absorbing structures, the acoustic energy of noise in transmission is converted into heat energy. When the incident acoustic energy is completely reflected, α=0, indicating that there is no sound absorption; When the incident sound wave is not reflected at all, α= 1, which means it is completely absorbed.
The sound absorption coefficient of general materials or structures is α=0~ 1. The greater the α value, the better the sound absorption energy, which is the most commonly used parameter to characterize the sound absorption performance at present. Sound absorption is the phenomenon of energy loss after sound waves hit the surface of materials, and sound absorption can reduce the indoor sound pressure level. The index describing sound absorption is sound absorption coefficient A, which indicates the ratio of sound energy absorbed by the material to incident sound energy.
Theoretically, if a material completely reflects sound, then its A = 0；; If a material absorbs all the incident acoustic energy, its a= 1. In fact, the A of all materials is between 0 and 1, which means that it is impossible to completely reflect and absorb. Different frequencies will have different sound absorption coefficients. People use the frequency characteristic curve of sound absorption coefficient to describe the sound absorption performance of materials at different frequencies. According to ISO standard and national standard, the frequency range of sound absorption coefficient in sound absorption test report is 100-5KHz. The average sound absorption coefficient obtained by averaging the sound absorption coefficient of 100-5KHz reflects the overall sound absorption performance of the material.
The noise reduction coefficient NRC is often used in engineering to roughly evaluate the sound absorption performance in the language frequency range. This value is the arithmetic average of the sound absorption coefficient of the material at four frequencies of 250,500, 1K and 2K, rounded to the nearest 0.05. Generally speaking, materials with NRC less than 0.2 are considered as reflective materials, and materials with NRC greater than or equal to 0.2 are considered as sound-absorbing materials. When it is necessary to absorb a lot of acoustic energy to reduce indoor reverberation and noise, it is often necessary to use materials with high sound absorption coefficient. For example, centrifugal glass wool and rock wool are sound-absorbing materials with high NRC, and the NRC of centrifugal glass wool with a thickness of 5cm and 24kg/m3 can reach 0.95.
Sound level meter generally consists of condenser microphone, preamplifier, noise meter picture attenuator, amplifier, frequency meter network and effective value indicator. The working principle of the sound level meter is that the microphone converts the sound into an electrical signal, and then the preamplifier performs impedance transformation to match the microphone with the attenuator. The amplifier adds the output signal to the network, weights the signal in frequency (or connects the filter), and then amplifies the signal to a certain amplitude through the attenuator and amplifier and sends it to the root mean square detector.