Design And Simulation Of Alpha Cabin And Assistant Software Developing Of Pass-by Noise Standard Measurement

Alpha cabin consists of a main chamber, generally have a number of sound source and microphone receiver, signal acquisition and processing system. Alpha cabin has its obvious advantages: Alpha cabin cost is significantly lower compared to reverberation chamber, and its portability also good; and wears standing wave tube ratio, Alpha cabin can measure samples of irregular shape and size.It is precisely because of these advantages of Alpha cabin, automotive engineering industry commonly used Alpha cabin to test the car sound absorption properties of various components, such as material absorption coefficient of engine sound insulation cover, acoustic impedance. Currently, there are no uniform ISO standards or national standards about the structure on the Alpha cabin. So as long as it meets the cabin sound field as diffuse sound field, and sound energy density distribution is more uniform, the Alpha cabin can be carried out as a small reverberation chamber which can measure sound absorption characteristics of the sample.In this paper, when the Alpha cabin space simulation models for different problems, we take different simulation modes: stationary mode, time dependent mode and eigenfrequency analysis. Based on Alpha cabin setting boundary conditions, this paper also describes the finite element method and the basic steps, including sub-domain of discrete division of the choice of interpolation function, the establishment of equations and linear equations, combined with simulation to solve the equations.In this paper, we use acoustic wave formulation to get the interior sound field, and based on the simple rectangular model to discuss the geometric parameters of Alpha cabin. Alpha cabin is equivalent to a filter, and adjust the ratio of scale length and breadth of this cabin can improve the frequency response characteristics of the acoustic wave filter, if the improved frequency response curve to be more gentle than before, then you can regard it as feasible. In general, the length and breadth of large reverberation chamber has a similar harmonic scale value, and the Alpha cabin can refer to the scale relationship of reverberation chamber. Here are the five principles of the Alpha cabin design:1. The Alpha internal compartment of any two sides must not be parallel to each other, which can reduce the number of standing waves.2. Any two edges of Cabin should not be the same, which can increase the distribution inhomogeneity of eigenfrequency of the cabin.3. The greatest scale of reverberation chamber must meet the following relationship: 13Lm ax≤1.9? V. V is the volume of small reverberation chamber.4. The Alpha cabin should be relatively small in size, so we can get high total sound pressure level more easily; and The Alpha cabin is easy to assemble and to facilitate transport.5. In order to ensure the Alpha cabin to meet the low frequency limit up to 200Hz (the cut-off frequency), its internal shortest edge length should be at least half the wavelength of 200Hz sound wave, which is 0.85m.Without no diffuser, when the center frequency of the sound source move from 500Hz to 2000Hz, inΔf = 30Hz frequency band, the number of eigenfrequency is from 8 up to 139, that is the number of wave modes is greatly increased. So in the high frequency band, the cabin's sound field is similar to a diffuse sound field. Although there is no prominent means to eliminate the standing wave phenomenon to a certain extent in the low frequency band, we can add the certain diffuser into the cabin to eliminate some overlapped mode. In this paper, we use finite element simulation to compare the diffusers of different body shapes, such as cylinder and cone-type diffusers. In the low frequency part, due to the small scale of space, resulting in very uneven distribution of sound field, the sound pressure curves are not even, but in the high frequency part, the curves have the opposite effect and the effect of the cone and cylinder shape is not very obvious. Due to the eigenfrequency mode number calculation of various frequency, we find out that when the rate is a certain constant value, the diffuser of two different structures almost indicate the same diffusion effect. For the conical arrangement and rotation of diffusers, the paper did a similar simulation, which find out that the arrangement of diffusers for the Alpha model cabin does not increase the number of eigenfrequency mode, and sound pressure level of the distribution is no more uniform, whereas the volume of the model make a huge influence for the number of eigenfrequency. By changing the inward embedded depth of cone-diffuser, this paper simulate the eigenfrequency increasing. Simulation results show that for different center frequency, the number of cabin mode waves are different, which is actually very similar to mode analysis, and when the rate of the volume of cone diffuse to the total cabin volume is up to 11%, the number of waves mode of Alpha cabin the may get the peak value. Comparing the three microphone receive, the three received signals illustrate that diffusers make the sound field more uniform, while the fluctuations of the third microphone signal curve in the low frequency area may be due to small size of the reverberation room, which is too small to bring even distribution of sound field.This latter part of the thesis is about Assistant Software Developing of Pass-by Noise Standard Measurement. Based on the vehicle type, PMR, single shift and multiple shifts, the power factor, Test method B will carry out the different accelerate test and constant speed test. By the supporting software it will save much time on the classification test, and dealing with the acquired data. It can also be used conveniently, clearly to carry out the standard B, and it can be regarded as a guideline for experiments to obtain data, process data and generate reports. The functions of the supporting software are as following 7 points: (1) may choose testing methods that are compatible with test method A; (2) tests of the environment, including meteorological environment and background noise; (3) input parameters, including vehicle load, engine power, engine speed, seats and gear parameters for future use links; (4) car classification, based on previous input parameters to classify the test object, and thus take a different test procedures; (5) shifts test function, which is based on the acceleration of different shifts and the regulated speed of inlet the outlet line ; (6) test function, save the data of the measure test; (7) generate the test report.