Research on Noise Sources of Machine Tools

Noise is a crucial quality indicator for machine tool products, directly impacting the physical and mental health of operators and the global competitiveness of the machines. Machine noise originates from multiple sound sources, making it challenging for engineers and craftsmen to determine whether the overall noise level meets standards, especially when advanced analytical tools are not available. Based on this challenge, an addition and subtraction method for decibel values of sound pressure levels was developed and has shown significant practical results. The study concludes that improving the maximum noise source is the key to reducing machine tool noise. **1. The Relationship Between Sound Pressure Level and Logarithmic Function of Sound Pressure** Sound intensity (I) is defined as power per unit area, or energy per unit area per second, which can also be expressed as I = P × V, where P is the effective sound pressure and V is the effective particle velocity. From this, the relationship between sound intensity and sound pressure becomes: $$ I = \frac{P^2}{400} $$ where 400 J represents the acoustic impedance of air. The sound pressure level (SPL) is calculated using the logarithmic formula: $$ SPL = 10 \log_{10} \left( \frac{I}{I_0} \right) = 10 \log_{10} \left( \frac{P^2}{400} \right) + 120 $$ with $ I_0 = 10^{-12} \, \text{W/m}^2 $ being the reference sound intensity, and $ 2 \times 10^{-5} \, \text{N/m}^2 $ as the reference sound pressure. This leads to: $$ SPL = 20 \log_{10} \left( \frac{P}{2 \times 10^{-5}} \right) $$ Table 1 illustrates how the sound pressure level changes with different sound pressures. For example, when the sound intensity increases by 10 times, the SPL increases by 10 dB. Similarly, when the sound pressure triples, the SPL increases accordingly. These relationships help in understanding the logarithmic nature of sound perception. **2. The Relationship Between Synthetic Sound Pressure Level and Individual Sound Pressure Levels** In real-world scenarios, when multiple noise sources are present, the synthetic sound pressure level (SSPL) is used to evaluate the total noise of the system. Assuming two sound sources with SPLs of $ SPL_a $ and $ SPL_b $, where $ SPL_b = SPL_a + t $, the combined SSPL can be calculated as: $$ SSPL_{ab} = SPL_a + 10 \log_{10} \left(1 + 10^{t/10}\right) $$ Table 2 shows how the difference in decibel levels between two sources affects the combined result. When the decibel values are equal, the combined level is 3 dB higher than either individual source. As the difference increases, the SSPL approaches the level of the louder source. This implies that to reduce overall noise, the focus should be on minimizing the loudest noise source. **3. Comparison Between Application and Experimental Results** The CW6163A lathe, with dimensions of 3540 mm × 1770 mm × 1571 mm and a weight of 4480 kg, was tested under various conditions. Measured sound pressure levels included SPLa=65 dB, SPLb=60 dB, SPLc=58 dB, SPLd=65 dB, SPLe=67 dB, and SPLf=83 dB. Using the method described, the combined SPLs were calculated as 66.2 dB, 66.6 dB, and 83.4 dB, respectively. According to industry standards, precision machine tools should not exceed 85 dB, while high-precision ones must stay below 75 dB. Therefore, the machine’s noise level was within acceptable limits. Experiments were conducted at four different spindle speeds with background noise kept at 5 dB. Measurements were taken at six positions, both at positive and negative rotation speeds. The maximum noise levels recorded were 83.2 dB, 80 dB, 83.2 dB, and 83 dB, closely matching the calculated value of 83.2 dB. **4. Conclusion** By calculating the decibel levels of individual noise sources, it is possible to assess whether the overall noise of a machine falls within acceptable limits. The synthetic sound pressure level always tends to approach the highest single source, confirming that reducing the loudest noise source is the most effective way to lower machine noise. This approach not only improves working conditions but also enhances the product's marketability.

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