Numerische und experimentelle Untersuchungen zur Reduktion der Körperschalltransmission

Abstract

The use of compounds and lightweight structures further emphasises the problem of sound transmission and radiation. Vibration, caused by the engine of an aircraft for instance, results in sound emission into the cabin. An new approach in noise reduction is taken by the use of structural intensity as a measure for the vibrational power per unit area. Using structural intensity instead of velocity as a cost function for control, the power flow is interrupted und there is a global attenuation of the downstream structure. This thesis gathers the necessary basis for the measurement and control of structural intensity using the example of a one-dimensional structure. The first part deals with different methods for measurement and calculation proposed in literature. These differ in the number of sensors used. By neglecting near field components of the vibration a reduction of the number of sensors is possible. Hence the error implied by this simplification is observed. Further limitations for the sensor spacing, as well as errors caused by displaced sensors, are calculated. Finally the methods using accelerometers are tested in an experiment. In the course of numerical investigations on the basis of a finite element model, it is shown that the simplified methods allow a qualitative analysis of the intensity. Especially in near field areas there is a significant error in intensity level. Only the full methods considering all field components are applicable for control. The methods using pure accelerometers or the corresponding method with combined accelerometers and angular rate sensors are capable of exactly calculating the structural intensity in near and far field areas. Due to the direct measurement of the angular rate, the method using combined sensors shows less errors caused by increasing sensor spacing or sensor displacement. Hence the control of structural intensity is based on these two methods. In order to use linear filters, the structural intensity is separated into its linear terms, being the basis for the calculation of the filter coefficients. Numerical analyses on a state space model show a tonal as well as a broadband attenuation of the energy flow and velocity. This barrier effect is observed globally downstream of the control. In comparison, a pure velocity control can only achieve global attenuation at resonance frequencies.