Environmental noise measuring instrument based on 89C51 microcontroller

　　1 Introduction　
　
　　Environmental noise monitoring is an important link for humans to improve their quality of life and strengthen environmental protection. Large-scale environmental noise monitors have been erected on the streets in prosperous neighborhoods and residential areas of major cities. However, most of the current domestic portable noise testers are expensive imported special equipment. Except for those owned by professional environmental protection departments such as health and measurement, they cannot be popularized as civilian products. This article introduces a low-cost, portable digital display environmental noise measuring instrument with 89C51 microcontroller as the core and using V/F conversion technology. The instrument works stably and has good performance. After calibration and calibration, it can meet the needs of general civilians. It can be widely used in industrial and mining enterprises, government agencies, schools and other occasions where environmental noise needs to be measured and controlled.

       2. Measurement mechanism of sound pressure level.

　　The hearing threshold of the human ear is generally 20 μPa, and the pain threshold is generally 200 Pa. The difference is 107 times. Such a wide sound pressure range is difficult to measure, and the human ear has a very special ability to distinguish the relative changes in sound pressure. linear characteristics. Therefore, sound pressure level Lp is commonly used in acoustics to reflect changes in sound pressure, and the sound pressure level of sound pressure p is expressed as

　　Lp=20 lg (p/p0) (dB) (attached)

       Among them: the reference quantity p0 is 20μPa. When p=p0, Lp=0 dB, and when p=200 Pa, Lp=140 dB.

　　The sound pressure level can be measured with a sound level meter, and the true sound pressure level or average sound pressure level can be obtained by inputting 1 kHz pure tone for 0.2 s to 0.25 s or more than 0.5 s. Taking into account the human ear's perception of loudness at different frequencies, in noise measurement, the inverse curve of the 40phon equal loudness curve is often used to weight the sound pressure level. That is, the A sound level is measured using the A weighting network and written as dB (A ). The attached table gives the relationship between the center frequency of the octave band and the correction amount of the A sound level:

  

       3 Hardware circuit composition　　

       The hardware structure principle of the environmental noise measuring instrument is shown in Figure 1:　
　

 

　　                                           Figure 1 Hardware schematic diagram

　　Environmental noise is converted into electrical signals by high-sensitivity, non-directional electret microphones. The frequency characteristics of the microphone used have non-uniformity of less than 1.5 dB in the range of 50 to 14 000 Hz. It can be used for outdoor measurements after adding a windproof cover and a rainproof cover. The three-stage amplification circuit is composed of an operational amplifier LM324. Carefully adjusting the relevant peripheral component parameters can make its amplitude-frequency characteristics similar to the A-weighted curve. D1, C1, and R1 form a peak detection network, and its output DC level reflects the noise sound pressure.

　　The voltage/frequency conversion circuit is composed of LM331, and the output frequency signal becomes a TTL level and is sent to the P3.4 pin of the microcontroller as a counting pulse of T0. The conversion circuit has good linearity and strong anti-interference ability. The output frequency range is above 10 ~ 10 000 Hz. Its change ratio reaches 103, which is better than the ordinary 8-bit parallel A/D converter and helps to improve the measurement range of the system. In Figure 1, Rs can be used to adjust the gain deviation and change the output frequency.

　　The core part of the system is the AT89C51 microcontroller, whose command system is fully compatible with MCS-51 and has a 4kB E2PROM on the chip, which can easily form a minimum measurement system. Its P3.5 pin is connected to the 100 kHz square wave output by the timer composed of NE555, and the T0 timing count is controlled through the T1 interrupt. The counting pulse frequency input from the T0 terminal also reflects the measured sound pressure. After software processing, the noise sound pressure level display value is output from the P1 port, decoded by 74LS248 and then driven to display the two-digit 1.8 in high-brightness LED digital tube. The decoder's light-off end is appropriately controlled to make the two digital tubes glow in turn. Realize dynamic display and reduce power consumption.

       4 Software design

　　The software of the environmental noise measurement system adopts a modular design and consists of a main program, an interrupt service program, a table lookup subprogram and a display subprogram. The flow chart of each program module is shown in Figure 2:

　　The main program is in a cyclic working state, mainly completing the initialization of the timer/counter and interrupt system, and cyclically calling table lookup and display subroutines. Whenever T1 counts the external 100 kHz clock for 0.5 s, it applies for an interrupt. After responding to the interrupt, the CPU reads the count values ​​​​in the TH0 and TL0 registers and re-initializes T0 and T1 in order to detect the next data.