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1 The relationship between the sound pressure level and the logarithmic function of sound pressure. Sound intensity = power/area = energy (/area × time) = pressure × speed ie I = PV (1) where: P is the effective value of sound pressure: V The effective value of the sound particle velocity. From equation (1), the relationship between sound intensity and sound pressure can be expressed as I=PP/PV=P2/400 (2) where: PV is the acoustic impedance of air, approximately equal to 400J. Sound pressure level SPL = 10lgI/I0 = 10lgI + 120 (dB) (3) Where: I0 is the reference sound intensity, I0 = 10-12W/m2. SPL=10 lgP2/400+120=20 lgP/(2×10-5) (dB) (4) where: 2×10-5 is the reference sound pressure, N/m2.
Table 1 The change of the sound pressure level with the logarithmic function of sound pressure IP SPL IP SPL 10-12 2×10-5 0 10-5 6.33×10-2 70 10-11 6.33×10-5 10 10-4 2× 10-1 80 10-10 2×10-4 20 10-3 6.33×10-1 90 10-9 6.33×10-2 30 10-2 2 100 10-8 2×10-3 40 10-1 6.33 110 10-7 6.33×10-3 50 1 20 120 10-6 2×10-2 60 The sound pressure level may be defined as the base 10 logarithm multiplied by 20 for the ratio of sound pressure to reference sound pressure. From equations (2) and (4), it can be seen that different values ​​of sound intensity and sound pressure correspond to sound pressure levels of different values, and different SPLs are calculated one by one based on the values ​​of I0 and P0, as shown in Table 1. The following conclusions can be obtained from the analysis of Table 1: The sound pressure level changes with the logarithmic function of sound pressure: Based on the sound intensity of 10-12, when the sound intensity increases to 10 times the original sound pressure level, the sound pressure level increases by 10 dB, respectively. The sound pressure is 3 times the original. 2 The relationship between the synthetic sound pressure level and a single sound pressure level In practical production, when there are several noise sources, the synthetic sound pressure level is used to measure the noise of the whole machine. From equation (4), SSPL = 10lgP2 / 400 + 120 (5) Assume that the synthesized sound pressure level is represented by SSPL and ab, where a and b are two sound sources, and SPLa and SPLb are respectively a and b. The sound pressure levels of the two sound sources, Pa and Pb, respectively, are the sound pressures of the two sound sources. It can be derived from (3) and (4) that I=10 (SPLa/10-12) (6). SPLb = SPLa + t, t is a positive value, then by formula (5), formula (6) can be derived SSPLab = SPLa + 10lg (1 +10 t/10) (7)
Table 2 The relationship between the synthetic sound pressure level and the individual sound pressure level t D t D 0 3 11 11.3 1 3.5 12 12.3 2 4.1 13 13.2 3 4.8 14 14.2 4 5.5 15 15.1 5 6.2 16 16.1 6 7 17 17.1 7 7.8 18 18.1 8 8.6 19 19.1 9 9.5 20 20 10 10.4 When t=0, SSPLab=SPLa+3 (dB) When t=1, SSPLab=SPLa+3.5 (dB) When t=2, SSPLab=+4.1 ( dB) and so on, according to different t values, calculate different SSPLab, as shown in Table 2. In Table 2, D = SSPLab-SPLa. The following two conclusions can be drawn from the analysis of Table 2: When the decibel values ​​of two sounders are equal, the sound pressure level of the synthesized sound pressure level is 3 times higher than the sound pressure level of a single sounding body: When the sound pressure difference between two sounding bodies gradually increases The synthetic sound pressure level decibel value tends to be the decibel value of the highest sounding body among the two sounding bodies, namely SSPLab≈SPLb, so reducing the decibel value of the maximum noise source is the main way to reduce machine noise. In the decibel value of a plurality of sounding bodies, each of the two is synthesized by the above method, and their synthesized sound pressure levels are calculated.
Table 3 Sound pressure level experimental results of machine tool noise Measurement point position noise (dB) Reverse speed
1000 rpm positive speed
800 rpm 1 83.2 80 4 70 70 6 60 60 7 58 58 9 70 70 12 67 67 2 83.2 83 4 63 65 8 54 60 10 57 58 11 67 65 3 65 67 3 Comparison between application and experimental results CW6163A type common The lathe has a dimension of 3540mm×1770mm×1571mm and a net weight of 4480kg. The sound pressure levels at work are SPLa=65dB, SPLb=60dB, SPLc=58dB, SPLd=65dB, SPLe=67dB, and SPLf=83dB. According to the method described in 2 can be found SPLab = 66.2dB, SPLabc = 66.6dB, SPLabcdef = 83.4dB, precision machine tools and general machine noise should not exceed 85dB, high-precision machine tools must not exceed 75dB, so you can determine the machine noise is qualified Within range. Using four different rotation speeds (spindle idling), with the background noise kept at 5dB, use the precision sound level meter A to perform two-stage position measurement on the whole machine. Position 1, 4, 6, 7, 9, 12 For a set of measuring points, positions 2, 5, 8, 10, 11, and 3 are a set of measuring points. Each set of measuring points is measured with positive and negative two rotation speeds. The results of the experiments (schematic sketches) are shown in Table 3. As can be seen from the above table, the maximum values ​​of the four groups of measurements are 83.2dB, 80dB, 83.2dB, and 83dB, respectively. The total noise of the whole machine is 83.2dB, which is -0.2dB different from the above calculation result.
4 Conclusion According to the decibel value calculation of the noise source of the machine tool, it can be judged whether the noise of the whole machine is within the qualified range. The decibel value of the synthetic boost stage always approaches the decibel value of the highest occurrence in the body. Therefore, reducing the decibel value of the machine's maximum main noise source is the key to reducing machine noise.
Noise is an important quality index of machine tool products. It directly affects the physical and mental health of the operating workers and the competitiveness of machine tool products in the international market. Machine noise is emitted by multiple sounding bodies. How to determine the qualification of the whole machine noise in the case of multiple noise sources and no advanced analytical instruments is a concern for designers and craftsmen. Based on the above, an addition and subtraction method for the decibel value of the sound pressure level is deduced, and a remarkable effect is obtained in the practical application of production. And concluded that the key to improving machine tool noise is to improve the maximum noise source of the machine tool.