Sound radiation of complex mode shapes

要有效地抑制噪声污染，需要对噪声产生源有准确的了解。如今，对振动声学有了深刻而巩固的基础知识。噪声源的主要特征是振动幅度、频谱含量和频率相关的辐射能力。由于结构的动力响应由振型决定，因此从结构振型的角度对后者进行了详细的研究。相关研究主要基于结构阻尼均匀分布的假设和简单的边界条件，简化了基本的微分方程组。然后，相关的本征问题产生实正则模。这一假设对许多应用都是有效的，并且提供了足够准确的结果。然而，由纤维增强塑料制成的高性能轻质结构的应用越来越多，同时也增加了对声学的要求，从而增加了声学减振措施。后者出于减轻重量的原因位于关键结构点，这违反了结构阻尼均匀分布的假设。因此，结构的共振是由复杂形状的振型决定的，即前向波而不是驻波。边界条件也可能导致这些复杂的振型。复振型的存在提出了它们对声辐射影响的问题。该项目的基本假设是，非比例阻尼引起的复振型的复杂程度对符合频率以下的声辐射有显著影响。声辐射特性可以用一个复辐射系数来描述。该项目将系统地研究振型复杂性与声辐射之间的关系。

The effective suppression of noise pollution requires the precise knowledge of the noise generating sources. Nowadays a profound and consolidated basic knowledge on vibroacoustics exists. Sources of noise are mainly characterized by vibration amplitude, spectral content, and frequency dependent radiation capability. The latter was investigated in detail in terms of structural mode shapes because the dynamic response of structures is determined by mode shapes. The related research is mainly based on the assumption of homogenously distributed structural damping and simple boundary conditions which simplifies the underlying differential equations. Then the related eigenproblem yields real normal modes. This assumption is valid for many applications and delivers sufficiently accurate results. However, the increasing application of high performance light weight structures made from fiber reinforced plastics simultaneously increase the acoustic requirements and, in consequence measures for acoustic damping. The latter are located at critical structural points for weight-saving reasons which violates the assumption of homogenously distributed structural damping. Consequently the structural resonances are determined by modes of complex shapes, i.e. forward waves instead of standing waves. Boundary condition may cause these complex mode shapes as well. The existence of complex modes raises the question of their influence on sound radiation.The basic hypothesis of the project is that the grade of complexity of complex mode shapes caused by non-proportional damping has a significant influence on the sound radiation below the coincidence frequency. The sound radiation characteristics should by describable by a complex radiation coefficient. The project will systematically investigate the relation between the mode shape complexity and the sound radiation.