Acoustic study of sound wave is a disturbance of the propagation, disturbance has speed.
1. Frequency range of acoustic research
Acoustic study of sound wave is a disturbance of the propagation, disturbance has speed. In a broad sense, the scope of acoustic research starts from 10-4 Hz, that is, 10-4 vibrations per second. Conversely, it is a period of 104 seconds, very slow. From 10-4 to 20 Hz, we call it infrasound. It's a sound that can't be heard by people's ears. It's a sound that changes very slowly. In fact, the response range of human ears to sound waves is very narrow, from 20 Hz to 20000 Hz. Beyond this range, human ears can't hear. Therefore, sound waves within this range are called audible sound, and those above 20000 Hz are called ultrasonic.
The infrasound frequency band has many uses in Atmospheric Physics and geophysics. Earthquakes, typhoons, such as the "UTA" in the South China Sea a few days ago, durian and a series of other typhoons, all produce infrasound. From thousands of kilometers away, the signal can be received by using a very sensitive acoustic sensor, and then it can be processed and located. The typhoon is located in the Where, the intensity of the size, can be known. In fact, seismic wave monitoring is infrasound monitoring. There are many monitoring stations throughout the country to monitor the infrasound generated by the nuclear explosion and its propagation along the earth's surface. Now the international nuclear arms control, acoustic method is also a means of monitoring.
When we reach the audible frequency band, we will be familiar with it. We know that walking on the side of the road, we feel the noise is very noisy; microphones and speakers used everywhere are the research scope of Electroacoustics; whether we can hear a speech clearly in a room is the research scope of architectural acoustics. Of course, language and music are the research scope of audible sound. At over 20000 Hz, people's ears can't hear. What can we do with sound waves in the ultrasonic frequency band?
What you are most familiar with is that every year when we do physical examination, we need to do B-ultrasound, which is to use ultrasound to detect whether there is any disease or defect in our body; in addition, ultrasound can also check materials and check some things in industry; there is a very familiar animal that uses ultrasound as its eyes, that is, bats, which send out ultrasound and then detect There are no obstacles in front of it, so the bats use the sonar in the air.
2. Intensity range of acoustic research
The intensity of sound waves is generally expressed in terms of sound pressure. Sound pressure refers to the part of pressure that changes with time on the basis of average pressure level (atmospheric pressure in air). The range of sound pressure changes from 10 ⁻ Pascal to 10 ⁶ Pascal. For such a range of intensity, it is inconvenient to express and use, so generally speaking, we do not talk about how much Pascal or how much kilogram of atmospheric pressure is the pressure, so what is the intensity of the sound pressure? It is decibel.
The concept of decibel has been widely used now. A sign is erected on the main road to say how many decibels is the noise of this place today. What is the concept of decibel? In fact, decibel represents the subjective reflection of people on sound. In vision, because of the different backgrounds, the objects of the same height often give people different feelings; the universality of the relationship between the strength of people's feelings and the original background is summed up by a man called Weber, so it is called Weber's theorem.
The same is true for acoustics. What does it mean that a person's ear's sense of the loudness of a sound is directly proportional to the logarithm of the intensity, rather than simply to the intensity stimulus itself? For example, the sense of our hand can be easily felt by putting a chalk head on when there is nothing in our hand; but if we first put a brick on him and then put a chalk head gently In the above, it may be difficult to feel it. What's the reason? It's because there is a brick in the background. Such a relationship is written as a formula that the feeling is directly proportional to the amount of change, but also inversely proportional to the original basic quantity. The integral on both sides of this relationship is the logarithm.
Therefore, the intensity of sound is not described by linear quantity, but by such a ratio of change quantity and basic quantity, which is based on a 10-based numerical description. The unit is bell, in memory of inventor bell. Multiply by 10, it's called decibel, which is 1 / 10 bell. This is for the intensity stimulation of energy, and for pressure, because energy is directly proportional to the square of pressure, the sound pressure level is multiplied by the logarithm of sound pressure by 20.
When we use the concept of decibel to express the sound intensity, the range of intensity we study is about 0 dB to 180 dB. Here, between 0 dB and 20 dB, it's actually the hearing threshold of a person. Everyone is different. A relatively healthy person with good hearing may be able to hear a sound of 0 decibel. A person with a slightly poor hearing may only feel it at 20 decibel, so it is a very quiet situation below 20 decibel.
For example, if you go to a place where there are no insects in the countryside, you may be able to find such a "quiet" place. But in general, it's noisy all over the place, so it's hard to find such a quiet place. Maybe only in the laboratory can you find such an area. Above this, 20-30 decibels is an ideal place to rest. For example, in a very quiet bedroom or ward, it's very quiet. At this time, you can hear the clattering of the watch beside your ear, and feel annoyed by the mosquito.
At 40 decibels, in the study or library of a quiet residential area not close to the road, you can read, study and think at ease. The general face-to-face conversation is relatively close, and the sound level is about 50-60 decibels.
In a commercial office, if there are typewriters, computers and so on, they are generally around 60 decibels, not more than 70 decibels, which we think can be tolerated.
More than 70 decibels, to 80 decibels, is the average noise on the road. When large vehicles pass, it's 80 or 90 decibels, and it's very noisy.
It's 100 decibels to the construction site. Now everyone hears the sound of pouring mud every day. It's almost more than 100 decibels nearby. In addition, when you see workers using pneumatic drills on the road, the noise nearby is more than 100 decibels. When you walk by, you will find it very hard to hear and noisy.
Higher sounds are not very common. For example, when a jet takes off, it is almost 120 decibels when it is 100 meters away. Even higher, for example, when it comes to 25m from a jet engine, it can reach 140dB. At this time, people's feeling is no longer noisy, and their ears have hurt.
In the upward direction, the chest will vibrate, which is dangerous to people. So the range of acoustic research is from about 0 dB to 200 dB, which can be said to be the limit. It is difficult to achieve the general level of 180 dB, which will produce serious nonlinear effects. Now under laboratory conditions, it can achieve the sound intensity of 180 dB.
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This paper is excerpted from the article "basic acoustic handout" of Baidu Library
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Reference documents for the calibration：
GB 6882-2008 Acoustics-Determination o sound power levels of noise sources-precision methods for anechoic and semi-anechoic rooms