Identification of main noise sources of loaders by

Identify the main noise sources of loaders by sound intensity method Abstract: This paper briefly expounds the basic principle of sound intensity measurement, and takes loaders as an example to explore the application of sound intensity measurement method in distinguishing the noise sources of construction machinery

key words: sound intensity measurement method to identify noise

sound intensity measurement method is a new technology developed by Lu in the 1980s in acoustic measurement and signal processing. The Vectoriality of sound intensity makes the measurement of sound intensity less limited by the environment. It is easy to carry out the near-field measurement, and the location of the main noise source can be easily determined. Therefore, sound intensity and oil quantity have become one of the effective means for noise identification and sound power evaluation in recent years

1 basic principle of sound intensity measurement

sound intensity refers to the sound energy per unit area passing through the vertical sound wave propagation direction in unit time. It is an acoustic quantity with size and direction that describes the flow of sound energy. In the sound field, the sound intensity of point a is defined as:

IR = paur (1)

where IR - the sound intensity of point a in the R direction, pa - the sound pressure of point a, and ur - the vibration velocity of air particles in the R direction

the commonly used sound intensity measurement method is the double microphone method. The basic principle of the two microphone method is as follows: let the distance between point a and point R in the sound field be Δ The sound pressure of two points A1 and A2 of R is set as PA and Pb; For the inviscid theoretical medium, the Euler equation of point a is:

where ρ—— Air density, using the difference of sound pressure between A1 and A2, approximate the sound pressure gradient of point a in formula (1), and get

ur = - (1)/ρΔ r) ∫ (PA Pb) DT

the sound pressure at point a can also be approximated by the average value of the sound pressure at points A1 and A2:

substituting formula (3) and formula (4) into formula (1) to obtain the sound intensity at point a by vector multiplication

2 identify the noise source of the loader by sound intensity method

in the measurement, the 3360 sound intensity analysis system of b&k company is used, which includes 2134 sound intensity analyzer, 4715 display, 3519 sound intensity probe and zh025 remote meter. The basic working principle of the test system is described above, and its workflow is shown in Figure 1

when using the double microphone sound intensity measurement method to measure the sound intensity, these problems do not exist in PCL implants. The acoustic distance between the two microphones Δ R is an important parameter that affects the measurement accuracy. Later Δ If R is too large, it will increase the finite difference (using the sound pressure difference to approximate the error caused by the sound in the gradient), and if R is too small, it will increase the phase loss error (there is an error caused by the phase difference of the sound waves in the two microphone channels), and only when Δ When R is far less than the distance between cloud point and sound source, the near-field error in sound intensity measurement can be ignored. For this purpose, b&k company provides microphone groups with different diameters in Δ R can reach the effective frequency measurement range (the measurement error is within ± 1dB (a)). Based on previous experience and the need for noise control measures to be taken in the future, according to Δ R cannot exceed the rule of 1/5 of the shortest wavelength. We select a group of microphones with a spacing of 12mm and a diameter of 1/2 to form a sound intensity probe, and its effective frequency measurement range is 125hz-5khz

we take the ZL50 Wheel Loader produced by China YITUO Group Company as the prototype, and use the sound intensity measurement method to identify the noise source of the whole machine

in order to reduce the impact of background noise on test 1, the test was conducted in the semi anechoic room of the technical center of YITUO Group. Three sides of the loader hood are selected. The exhaust pipe takes the bottom plate, side wall and hydraulic valve in the cab as the measuring surface, and more than 800 measuring points are arranged. All measuring points are arranged in a plane parallel to the measured surface and 0.1M away from the measured surface. Figure 2 shows the layout of several main survey points. During the measurement, the machine is installed without understanding, the engine is in neutral gear, and the engine operates at the maximum throttle. The working device performs shovel, unloading and lifting actions similar to the working condition. Using the above measuring system, the sound intensity signals of each measuring point are measured and recorded in turn, and the three-dimensional sound intensity line diagram of each measuring surface is obtained. Figure 3 shows the three-dimensional sound intensity distribution and isointensity line diagram of some measurement surfaces. Table 1 shows the measurement data obtained by arranging measurement points with the exhaust pipe as a linear sound source

Figure 2 Schematic diagram of measuring point layout of part of the measuring surface

(a) upper part of the hood (b) bottom plate in the cab (c) left side of the whole machine

(a)

Figure 3 three-dimensional intensity distribution and isointensity line diagram of part of the measuring surface

(a) left side of the hood (b) upper part of the hood

the sound field distribution of the measuring surface can be clearly seen from the measured three-dimensional sound intensity distribution diagram. On the three-dimensional sound intensity distribution diagram of the upper part of the hood given in Figure 2, there is an obvious convex belt at the joint between the hood and the cab (the numbers 1, 2, 3 and 4 in Figure 2 correspond to 1, 2 and 34 in Figure 3 respectively), indicating that the noise radiation value here is high; The lower one of the other two bulges is at the inlet of the engine filter, and the higher one is at the interface between the engine exhaust pipe and the muffler. There is an estimated plane on the three-dimensional sound intensity distribution map on the left side of the hood. Here is the ventilation on the left side of the hood. The noise of the engine radiates directly from here, resulting in high sound intensity values here. It can be seen from the isointensity line diagram and measurement data that the exhaust pipe is the first radiation noise source of the prototype, up to 112.3db (a); The second is the locust producing crack between the engine hood and the cab, up to 105dB (a); The next industry is the vent hole on the left side of the engine hood and the engine air inlet, up to 103dB (a); It shows that the radiated noise from the engine is the main noise source of the machine. Although the overall noise of the engine meets the national standard, the test results show that there is still much work to be done in reducing the noise of the engine; At the same time, it also shows that the structure of the engine hood of the prototype ignores the noise reduction design, which is unreasonable in terms of noise reduction. Further structural improvements are needed to reduce the radiated noise of the whole engine hood

through the analysis of sound intensity, we can not only obtain the distribution of noise radiation field and identify the main noise sources in the complex noise sources, but also further analyze the main noise sources that affect the driver's ear noise in the cab through the sound intensity spectrum obtained from the sound intensity analysis. Figure 4 shows the 1/3 octave band intensity spectrum of the driver's ear noise and the 1/3 octave band intensity spectrum of some sound sources

Table 1 regards the exhaust pipe as a linear sound source and arranges the measurement data obtained from the measurement points

compares the 1/3 octave band sound intensity spectrum of the exhaust pipe and the rear wall of the cab with the 1/3 octave band sound intensity spectrum of the driver's ear, which feeds back these information to the technical director PEBA. It can be seen that both sound intensity spectra have peaks at 125Hz, It shows that the exhaust pipe noise in this frequency band is the main sound source affecting the driver's ear noise; The change of the sound intensity of the ear side noise near 1kHz is similar, indicating that the energy of the ear side noise in this frequency band mainly comes from the rear wall of the cab and the noise radiation. By analogy, the sound source that has the greatest impact on the driver's ear noise in each measurement frequency band can be analyzed

3 conclusion

from the above analysis, it can be seen that the main noise sources of the loader can be easily identified by using the sound intensity measurement technology, and through the analysis of the sound intensity spectrum, the noise sources near the driver's ear on each measurement frequency band can be found. At the same time, the frequency band that is most likely to cause driver fatigue and reduce work efficiency in the loading noise sources is also determined. From Wanli Yaoba, targeted measures can be taken to better control the noise of the loader, Improve the acoustic comfort of the whole machine and reduce noise pollution