Near-field Acoustical Holography - a new sensor concept for methods of active noise reduction

本课题的研究假设包括一种基于近场声学全息的传感器原理的开发过程，该原理适用于带有结构致动器的主动降噪系统。主要目的是通过声场重建的逆方法将传感器原理从主动噪声和振动控制领域扩展开来。特别地，声场量的逆投影对自适应信号处理稳定性的影响将被研究。与现有的方法相比，期望科学知识的扩展。其原因是声学传感器与现代反声学方法的结合使用使得任意形状结构的近场声场量与声功率成正比的非接触可观测性成为可能。因此，避免了由于结构传感器如加速度计不能按要求固定而产生的问题。采用近场声学全息技术，避免了昂贵的传感器组间传递函数测量。为此，仅使用声学模型，与使用振动声学模型相比，这使得模型质量的不确定性更低。实验是在一个传输测试设施中进行的，首先是可以解析描述的结构部件，然后是频率范围高达1khz的复杂结构部件。这个频率范围是典型的主动降噪方法，作为被动降噪措施的补充，在更高的频率下更有效。在自由场条件下，基于傅里叶变换的近场声学全息作为一种传感原理，在简支板上主动降噪。利用粒子速度在平板表面的傅里叶变换与辐射声压在远场的方向性规律之间的关系，实现了辐射声功率的最小化。在了解逆声场重构方法作为一种传感器原理对主动减声方法的基本作用后，将该方法推广到具有更复杂几何形状的部件上进行实验。将所研究的概念应用于汽车或飞机舱室等空腔的基本原理也有待开发。因此，在项目的最后阶段，通过引入反射面，将基于近场声全息的传感器原理扩展到室内声场。最后，将与传统的主动降噪传感器原理进行比较。

The research hypothesis of the project comprises the development process of a sensor principle based on near-field acoustical holography for active noise reduction systems with structural actuators. The main objective is to extend sensor principles from the field of active noise and vibration control by inverse methods for sound field reconstruction. In particular, the influence of the inverse projection of sound field quantities on the stability of adaptive signal processing will be investigated. Compared to already existing approaches, an extension of the scientific knowledge is expected. The reason for this is that the use of acoustic sensors in conjunction with modern inverse acoustics methods enables non-contact observability of the sound field quantities proportional to the sound power in the near-field of arbitrarily shaped structures. Consequently, problems are avoided which arise because structural sensors such as accelerometers cannot be fixed as required. With near-field acoustical holography, a method is chosen which avoids costly measurements of transfer functions between sensor groups. For this purpose, only acoustic models are used, which results in lower uncertainties with respect to the model quality compared to the use of vibroacoustic models. Experiments are carried out in a transmission test facility, first on structural components that can be described analytically and then on complex structural components in the frequency range up to 1 kHz. This frequency range is typical for active noise reduction methods as a complement to passive measures, which are more effective at higher frequencies. Under free-field conditions, the Fourier transform-based near-field acoustical holography serves in the first step as a sensing principle for active noise reduction on a simply supported plate. With the relationship between the Fourier transform of the particle velocity on the plate surface and the directivity pattern of the radiated sound pressure into the far-field, the minimization of the radiated sound power is achieved. With the knowledge gained about the fundamental effects of inverse sound field reconstruction methods as a sensor principle on methods of active sound reduction, the experimental extension of the method to components with more complicated geometries will be carried out. The basic principles for the application of the investigated concept to cavities such as car or aircraft cabins are also to be developed. Therefore, in the last project phase the extension of the near-field acoustical holography-based sensor principle to sound fields in rooms is carried out by introducing reflection surfaces. Finally, comparisons will be made with conventional sensor principles for active sound reduction.