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

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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.