Active control of target sound fields using structural-acoustic brightness applied to a ship model's acoustic signature.

This manuscript presents a method for reproducing sound fields actively by using a vibrating submerged structure as the field reproduction source, with the target sound field to be reproduced defined in the frequency domain using the acoustic brightness approach. To balance the predetermination of a mono- or multi-zone target sound pressure field and the control effort required, singular value decomposition of the structural-acoustic system matrix is proposed. The dependency of the radiation efficiency into the target zone on the singular modes representing source and pressure modes is investigated using a wavenumber-domain approach. Furthermore, a feedforward control principle is adopted for adaptive sound-field reproduction with mode matching from the least squares perspective. Finally, an experiment is reported that involve synthesizing a tonal target underwater acoustic signature of a model of a fast attack craft (scale 1:8) at a measurement facility at Lake Plön in Germany. The results show that with the structural-acoustic brightness approach structural modes with radiation coupling into the target zone are excited and related pressure modes exhibit individual focus in the direction of hydrophones in use. Finally, a predetermined narrowband sound pressure field is actively reproduced at the hydrophone positions using inertial actuators and accelerometers on the ship model's hull.

Dynamic measuring of force-displacement-characteristics of shockmounts.

Shockmounts are widely used to isolate sensitive equipment from vibrations and mechanical shock. Despite the highly dynamic nature of shock events, the force-displacement-characteristics of shockmounts which are provided by manufacturers are gained by static measurements. Therefore, this paper presents a dynamic mechanical model of a setup for dynamic measurements of force-displacement-characteristics. The model bases on acceleration measurement of an inert loading mass, which displaces the shockmount, when the arrangement gets excited by means of a shock test machine. The influence of the shockmount's mass in measuring setup is considered as well as special needs for handling measurements under shear or roll loading. A method for allocating the measured force data on the displacement axis is developed. An equivalent of a hysteresis-loop in a decaying force-displacement-diagram is proposed. Based on exemplary measurements, error calculation and statistical analysis show the qualification of the proposed method for attaining dynamic FDC.

Predicting Failure of Additively Manufactured Specimens with Holes.

Experimental and computational studies were conducted to predict failure loads of specimens containing different-sized holes made using the additive manufacturing (AM) technique. Two different types of test specimens were prepared. Flat specimens, manufactured from polylactic acid (PLA), were subjected to uniaxial loading. Tubular specimens, made of polycarbonate (PC), were subjected to combined loading that was applied using uniaxial testing equipment. Test specimens were uniquely designed and printed to apply the combined bending and torsional loads to tubular specimens. A newly developed failure theory was applied to predict the loads that would result in the fracture of these test specimens. This theory is composed of two conditions related to stress and the stress gradient to be simultaneously satisfied to predict failure. The failure loads predicted using the new failure criteria were compared closely with the experimental data for all test specimens. In addition, a semi-empirical equation was developed to predict the critical failure surface energy for different printing angles. The critical failure surface energy is a material property and is used for the stress gradient condition. Using the semi-empirically determined values for the failure criterion provided close agreement with experimental results.

Interior near-field acoustic holography based on finite elements.

This paper deals with the mathematical framework of near-field acoustic holography based on finite elements in application to the acoustic response of a fluid within a closed cavity to the enclosure boundary conditions. The finite element method is an effective implementation of the modal approach for arbitrary geometries and provides advantages for certain wavenumber intervals in rooms. An inverse implementation of the direct problem can benefit from using generalized coordinates with modally reduced system matrices. A solution can be obtained via singular value decomposition together with Tikhonov regularization. This paper investigates acoustic mode spectrums of acoustic transfer functions, which has a major effect on the reconstruction of particle velocities from given sound pressures in a simple cavity model. It is found that the largest considered modal wavenumber in the acoustic transfer matrix should be twice the maximum excitation wavenumber. Furthermore, the relation between reconstruction errors and the detectability of evanescent waves depending on the wavenumber of excitation is considered. The proposed method is validated experimentally by reconstructing particle velocities on the inner boundaries of an Airbus A400M fuselage based on measurements of the inner pressure field. Results are compared with structural velocities measured with a laser Doppler vibrometer.

Performance evaluation of an active headrest considering non-stationary broadband disturbances and head movement.

An application of active noise control (ANC) is the active headrest, using actuators (loadspeakers), sensors (microphones), and a controller to create a local zone of quiet around the occupants head. In this paper, the attenuation performance of various ANC-algorithms for active headrests known from literature is evaluated considering non-stationary broadband disturbances and head movement. Numerical studies are performed to determine the optimal plant models and parameters for the ANC-algorithms. Based on the findings of the numerical studies, several real-time experiments are conducted with and without head tracking examining the distribution of the 10 dB zone of quiet and the attenuation at the occupants ear using either a head mounted microphone technique, the remote microphone technique, the virtual microphone technique, or the virtual microphone control method. It is found that none of the algorithms using a virtual sensing technique can produce a 10 dB zone of quiet for all considered non-stationary broadband disturbances and head movement. For the algorithm using a head mounted microphone, it is possible to form a 10 dB zone of quiet, but placing a microphone at the ear is not feasible in most situations.

Robust active noise control in the loadmaster area of a military transport aircraft.

The active noise control (ANC) method is based on the superposition of a disturbance noise field with a second anti-noise field using loudspeakers and error microphones. This method can be used to reduce the noise level inside the cabin of a propeller aircraft. However, during the design process of the ANC system, extensive measurements of transfer functions are necessary to optimize the loudspeaker and microphone positions. Sometimes, the transducer positions have to be tailored according to the optimization results to achieve a sufficient noise reduction. The purpose of this paper is to introduce a controller design method for such narrow band ANC systems. The method can be seen as an extension of common transducer placement optimization procedures. In the presented method, individual weighting parameters for the loudspeakers and microphones are used. With this procedure, the tailoring of the transducer positions is replaced by adjustment of controller parameters. Moreover, the ANC system will be robust because of the fact that the uncertainties are considered during the optimization of the controller parameters. The paper describes the necessary theoretic background for the method and demonstrates the efficiency in an acoustical mock-up of a military transport aircraft.