【作者】 吴迪；

【导师】 崔志文；

【作者基本信息】 吉林大学 ， 声学， 2011， 硕士

【摘要】 材料的吸声系数是材料的各项声学性能参数中非常重要的一个,它对各种材料在生活和工业中的应用有着积极的指导意义。对材料吸声系数的测量通常采用标准的混响室方法,对应有相应的国际ISO标准和国家GBJ47-83标准。混响室方法要求材料被制成10到12平方米的标准试件。另外对应一些较小的材料还常采用驻波管方法测量其吸声系数。但在很多工业中,尤其是汽车行业,有些待测材料,如汽车内部的座椅,仪表盘,地毯等既无法做成标准混响室要求的10到12平方米试件,又无法将其切割成驻波管能够测量的小试件,必须将其作为一个整体测量吸声系数。这就要求我们必须开发一种能够满足这种测量实际的仪器,于是在上个世纪80年代人们开始逐渐开发出应用于汽车行业相关材料吸声系数测量的小混响室,也称其为Alpha舱。它是基于标准混响室测量方法的小型测量仪器。但是关于小混响室的相关技术参数并没有统一的ISO标准或国家标准。所以本文中如果我们研究的小混响室能够满足扩散声场的相关要求,让声场充分扩散,处处能量密度均匀,则这样的小混响室就能够作为测量工具来测定待测材料的吸声系数。在本文中我们首先通过对封闭空间声场的特性研究,给出的声场的声压表达式和简正振动频率表示式。进而得出了简正振动模式数目与频率和体积,总表面积和棱长总和的关系,并做了相关拟合分析。在分析中我们知道在频率较高时,声场中存在大量的简正波,如此多的简正波对一种驻波是波节的地方,对另一种驻波有可能刚好是波腹。这样大量简正波的叠加可以把声场的驻波效应“平均”掉,而使得封闭空间声场趋向均匀。此外如果置于声场中的声源发出的不是单频波而是具有一定带宽的声波,比如1/3倍频程的粉红噪声,在声源中心频率较高时就可能在声场中激起大量的简正波,而使得封闭空间声场近似与扩散声场,所以从这个意义上说,我们通常所说的扩散声场可以看成是封闭空间驻波声场的高频近似。但是我们通过研究发现,如果封闭空间的几何形状非常对称,甚至是简单的整数比时,其内部声场将存在严重的简并化,也就是很多简正波对应同一个频率,这样我们由公式得到的简正波就会与实际结果有较大的出入。特别在频率较低时,很可能在某一频率范围内没有简正频率,而在另一频率范围内存在较多的简正频率,使得简正频率的分布不均匀。所以我们在设计小混响室时要尽量使其各面互不平行,棱边长也不相等,其长宽高最好呈调和级数。另外要在封闭空间声场中加装扩散体以改进声场扩散,而对于高频纯音或窄带噪声的测试,扩散体也有一定的作用。扩散体在选择上有固定在壁面的扩散元件,悬吊式扩散板和旋转扩散体等,对于我们研究的小型混响室由于体积有限,并且声场较小,我们一般采用固定在壁面上的扩散元件。最后我们从理论上研究了声源在声场中心时能够激发的驻波方式仅仅为其在声场顶角位置上的八分之一。这就是说,声源置于顶角位置将比放在声场其它地方激起更多的驻波方式,而驻波越多则声场越趋向均匀。因此放在顶角上的声源将有利于产生扩散声场,特别在频率较低时更需如此。这样我们对小混响室的设计研究就有了一定的概念。接着我们引入统计能量方法对小混响室进行深入的研究,仿真出了系统的模型,并建立好系统的各个子系统。然后我们定义了模型需要的各种材料的相关物属性。并对系统模态密度,内损耗因子,耦合损耗因子做出了理论分析,并结合我们的模型进行了相关的计算给出了结果。在引入声源激励之后我们分析整个模型的相关各个参量,并着重分析了模型声腔的模态数和声腔内部的能量,声压级等,并采用将声腔细分为8个小声腔的办法,研究加入声源后,声源能量对声腔的影响。通过比对我们发现,单声源时加入声源的小声腔能量高于不加入声源的小声腔,并且距离越远的声腔能量,声压级都越小,这是不利于扩散声场产生和测量吸声系数的。当我们加入多声源时(一般为3个),小声腔中的能量和声压级得到了明显的改善,在低频时,非声源声腔的声压级平均提高5dB左右。这就使得小混响室能够产生较好的扩散声场,测量的结果也较为准确。最后我们分析了小混响室壁板的隔声能力,我们通过理论分析,得到了单层墙的质量作用定律,即当材料一定时,隔声量随着壁板厚度和频率的增加而增加,厚度或者频率每提高一倍,隔声能力提高6dB。但是对于声学环境要求较高的声学仪器,如我们的小混响室,通过提高壁板厚度来获得较高的隔声量显然是不经济也是很笨重的做法,所以我们引入双层壁板隔声问题,通过分析我们发现,当我们采用双层墙隔声时,可以大大的改善小混响室声场内外的声学环境,进而使得我们设计小混响室内部声场的不会收到外界的显著干扰。保证了测量的准确性。同时本文也指出,现行的双层墙隔声理论存在较大的理论误差,使得应用起来和实际出入非常大。本文通过分析发现,对于双层墙问题,我们应该考虑它的隔声量介于双层墙连接在一起时的隔声量和双层墙单墙独立隔声量的和之间。当空气层厚度较小时,应将其看作是连接双墙的一个小弹簧。由于它的存在,使得双墙的隔声量比单独的隔声量之和低很多,但仍大于完全连接在一起时的情况。这样本文就对小混响室的建立有了一个清晰完整的理论和方法,并且在实践中得到了良好的运用。更多还原

【Abstract】 The sound absorption coefficient is an important acoustical property of a kind of material, and this attribution indicates significance in engineering industry. The measurement of absorption coefficient usually adopts the standard reverberation room methods to the corresponding ISO standards and national GBJ47-83 standards. Reverberation room methodologies requires that the material should be made from 10 to 12 square meters test pieces. Additionally some small materials can also measured by standing wave tube for its absorption coefficient.However in many fields, especially in the automotive industry, some tested materials, such as automotive interiors, seats, dashboard, carpet, cannot be made for the requirements of the standard chamber test piece 10-12 square meters, or the test standard of standing wave tube, cut into small specimens, so the test piece must be measured for absorption coefficient as a whole. This needs us to develop a way to meet this measurement, so in the 80s of last century a new way has been developed to make this kind of measurement, and the small reverberation chamber is called Alpha Cabin. It is based on the standard measurement method for small chamber measuring instruments. However, a small reverberation chamber on related technical parameters and there is no uniform ISO or national standards. So, if we study this small chamber to meet the relevant requirements of diffuse sound field, the full spreadness of the sound field, and the uniform of energy density, this small reverberation chamber can be made as a measurement tool to determine the absorption coefficient of the material.In this paper due to the study for enclosed space of the sound field characteristics, I deviate the sound field pressure expression and the normal vibration frequency formula. Then I study a number of the normal vibration modes and frequency and volume, total surface area and total edge length relationship, and do the relevant fitting analysis. In the analysis we know that the higher the frequency, the sound field consists of the more normal modes, so much the normal mode of a standing wave where the wave section, to another may happen to be standing wave antinodes. This superposition of a large number of the normal mode of the standing wave sound field can effect the "average", and tend to make uniform the sound field enclosed space. In addition, if placed in the sound source sound field is not given but with a certain single-frequency wave acoustic bandwidth, such as 1/3 octave pink noise, the higher the center frequency of the sound source may be stirred in a large number of sound field The normal wave, which makes the enclosed space sound field is approximately diffuse sound field, so in this sense, we usually refer to the diffuse sound field can be regarded as enclosed space standing wave high-frequency approximation. But We found that if the enclosed space of the geometric shape is very symmetrical, and even simple integer ratio, the internal sound field will be serious degeneracy, which also is a lot of the normal mode corresponding to the same frequency, so that we by the formula obtained by normal mode actual results will differ materially. Especially in the low frequency, probably not in a frequency range of the normal frequency, while in another frequency range of more memory in the normal frequency, making the uneven distribution of the normal frequency. So we designed a small chamber to the surface as possible when not parallel to each other, not equal edge length, the length and breadth was the best harmonic series. In addition to the acoustic field in a closed space installed diffusers to improve the sound field diffusion, and for high-frequency pure tone or narrowband noise tests, the proliferation of the body also has a role. Diffusers in the choice of a fixed component in the spread of the wall, suspended and rotating diffuser diffuser, etc., for our small study size limited the reverberation chamber, and the smaller sound field, we generally use a fixed spread on the wall components. Finally, we studied theoretically sound source in the sound field center only way to stimulate the standing wave sound field angle for position in the one-eighth. This means that the angle position of the sound source placed elsewhere than on the sound field of standing waves stirred up more, rather more standing wave field tend to make a sound more evenly. So on the angle on the sound source will help to produce diffuse sound field, especially in the low frequency is more to be the case. So by the previous way we study for the small chamber, we will make a certain design concept of the small reverberation chamber.Then we introduce the statistical energy method for small chamber study further. I made a simulation model of the system and establish a good system in various subsystems. Then I defined the model requires a variety of materials related to physical properties. And the modal density of the system, the loss factor, coupling loss factor has made a theoretical analysis, combined with our model calculation shows the relevant results. After the introduction of sound source excitation model we analyze the various parameters related to, and analyzes the model number of tunes and tune the modal internal energy, sound pressure level, etc., and divided the cavity into 8 small way by adding the sound source, sound source of energy on the cavity. By comparison we found that a single sound source, adding the energy source of the whisper chamber above the source of the whispers not to join the chamber, and the farther away the tune of energy, sound pressure level are smaller, which is not conducive to the generation and diffuse sound field measured absorption coefficient. When we add more sound source (usually three), and whispered in the cavity energy and sound pressure level has been significantly improved, at low frequencies, non-cavity sound pressure level is lift by an average of about 5dB. This makes a better spreadness of sound field of a small reverberation chamber, and of course will lead to a practical accurate measurement.Finally, we analyze the sound insulation capability of small chamber walls. I adopted theoretical analysis, got the mass law of the single wall -- when the material is constant, sound insulation wall thickness and the frequency with the increase of Thickness or frequency for each doubling of capacity to improve sound insulation 6dB. However, environmental requirements for the higher acoustic acoustic instruments, such as our small chamber, by increasing the wall thickness to achieve higher sound transmission loss is obviously not the way the economy is also very heavy, so we introduce the double-wall insulation Problems, the analysis we found that when we use double-wall insulation, you can greatly improve the small reverberation chamber and off the acoustic sound environment, and allows us to design a small chamber within the sound field will not receive significant outside interference . Ensure the accuracy of measurement. Same article also noted that the existing theory of the double wall insulation there is a large theoretical error, making the application and the actual access to them is very large. Based on the analysis found that for double-wall, we should consider its sound insulation between the double walls connected together and double the amount of noise when the single-wall and wall of sound insulation between independent. When the air layer thickness is small, it should be seen as connected to a small double-wall spring. Because of its existence, the ratio of double-wall insulation is much lower than the separate one, but still larger than the case that they are connected together.From the above argument and simulation, we have got a clear mind and an intergate method to constuct an Alpha Cabin, and apply it into the practical engineering更多还原