The noise level is 30 ~ 40 decibels, which belongs to a relatively quiet normal environment; More than 50 decibels will affect sleep and rest. Due to lack of rest, fatigue cannot be eliminated, and normal physiological functions will be affected to some extent; Conversation interference above 70 decibels causes distraction, inattention, affecting work efficiency and even accidents; Working or living in a noise environment above 90 decibels for a long time will seriously affect hearing and lead to other diseases.
The basic principle of 1
The noise standard is based on Nyquist theorem, that is, in the thermal equilibrium state of temperature T, the thermal noise caused by the irregular thermal motion of electrons caused by resistance has a noise power of
P=kTB
Where k is Boltzmann constant,1.38×10j/k; T is the physical temperature of the resistor, and the frequency band width of the test system, Hz; P is the noise power output by the resistor W.
When b is constant, the temperature T has a definite relationship with the noise power P, so it can be considered that T is the measurement unit of noise value, which is called noise temperature. The noise standard is to select resistive elements or absorbing materials to make waveguide (or coaxial) terminal load and put it in a constant temperature heating furnace (or cooling room). When the load meets the absolute blackbody condition, according to Planck's blackbody radiation law, single-mode electromagnetic waves are radiated along the transmission line, and the radiation power per unit frequency band is
N=kT
Different load temperatures T can be divided into three noise sources: standard high temperature, standard low temperature and standard room temperature. Noise standards above room temperature are called standard high temperature noise sources; The noise standard below room temperature is called standard low temperature noise source; The noise standard with t equal to or close to room temperature is called the standard room temperature noise source. The standard high temperature noise source is troublesome to operate and maintain, and it is only suitable as the national measurement standard.
2 radiometer
Radiometer is a sensitive correlation receiver that can measure noise signals. It is used to transmit the noise standard (noise temperature standard) provided by standard noise sources, that is, as a measurement comparison index. Radiometers can be divided into three types: full power type, modulation type and correlation type. Figure 1 shows the composition block diagram of the improved Dicke radiometer, which is a modulated zero-point radiometer. The noise source to be calibrated is connected to the comparison channel for comparison with the standard noise source. In the first step, the standard noise source is added to the relevant receiver through waveguide switch 1, precision variable attenuator, modulator, magic T, waveguide switch 2 and isolator. The reference noise source connected to the reference channel is also applied to the relevant receiver through the balanced attenuator. Adjust the balanced attenuator of the reference channel so that the indication of the relevant receiver is zero. Step 2, turn the waveguide switch 1 to the noise source to be calibrated, keep the balance attenuator unchanged, and adjust the precision variable attenuator of the comparison channel to restore the receiver indication to zero. At this time, the noise temperature Tn of the calibrated noise source expressed by the noise temperature can be obtained by the following formula, as shown in Figure 2, where To is room temperature, 290 K; Ts is the noise temperature of the standard noise source, which is called the noise temperature of the noise source to be calibrated and evaluated; △A is the difference between the secondary readings of the precision attenuator.
Figure 1 block diagram of improved dirk radiometer
Three-level classification
Hearing impairment can be divided into acute and chronic. When exposed to strong noise, tinnitus and hearing loss will occur. As long as the time is not long, once you leave the noisy environment, you will soon return to normal, which is called auditory adaptation. If you are exposed to strong noise for a long time, your hearing will drop obviously, and it will take several hours, even ten to twenty hours, to return to normal after leaving the noisy environment. This is called hearing fatigue.
This temporary hearing loss is still within the physiological range, but it may develop into noise deafness. If you continue to be exposed to strong noise, hearing fatigue will not be recovered, and hearing will continue to decline, which will cause noise-induced hearing loss and become a pathological change. This symptom is manifested in high-frequency hearing loss in the early stage. But at this stage, patients have no abnormal feelings subjectively, and language listening has no influence, which is called hearing impairment.
If the course of the disease develops further, the hearing curve will continue to decline. When the average hearing drops more than 25 decibels, there will be abnormal language hearing, which makes it difficult to talk subjectively. This is called noise deafness. In addition, powerful acoustic storms, such as explosions and gunshots, can lead to acute explosive deafness, tympanic membrane rupture, middle ear ossicles dislocation, ligament tear, bleeding and partial or complete deafness.