Sachau, Delf

The use of inertial actuators to control the radiated sound pressure of a steel ship model at a lake measurement facility is examined. Therefore, methods of active vibration control as well as active control of target sound fields are applied using a fixed configuration of twelve accelerometers, eight control actuators, and five hydrophones. A narrowband feedforward active control system is used to manipulate the sound pressure at hydrophone positions, focusing not only on reducing but also on adding spectral lines in the radiated signature. The performance is assessed using measured data by additional accelerometers inside the ship model as well as by hydrophones surrounding the measurement facility. It is found that less control effort is necessary for the generation of additional tones compared to the control of a present disturbance at hydrophones. In the frequency range considered (below 500 Hz), the actively induced change in the mean structural velocity is not necessarily proportional to the change in the radiated sound pressure. In contrast to the vibration velocity, no unwanted amplification of the sound pressure is found for the frequencies observed.

Sources for underwater sound with large dimensions l, such as a coating on a boat structure, can be suitable for generating low-frequency underwater sound. In this context, low-frequency refers to wavelength λ in water that fall within the range of 40 l>λ>1,5 l . Such low-frequency sources can be applicable for a wide variety of long-range applications, such as communication, navigation and sonar. In this study, an array of 18 circular piezoelectric actuators embedded in a potting compound create an active surface for generating underwater sound. The coating is attached to a plane plate of glass-fiber reinforced plastic, typical for maritime applications. The acoustic propagation is investigated in an underwater test range with free field conditions, specifically for low frequencies. This study focuses on sound radiation and beam steering behaviour (in terms of propagation and vibroacoustics). On the one hand, the test results show the strongly frequency-dependent performance of the coating as an underwater sound source. On the other hand, a significant influence of the vibroacoustic behavior on the sound radiation can be recognized. The beam steering experiment reveals the phase shifts, up to which the coating still emits sound without major losses.

To test methods for active noise and vibration compensation, the mathematical model of a rectangular simply supported plate in a rigid baffled is reproduced experimentally in a laboratory. In order to verify the accuracy of this replication, the assumed boundary conditions are examined by using harmonic signals for excitation. The investigated frequency range comprises the first 20 natural frequencies of the plate and ranges in the first quartile below the coincidence frequency. The acoustic boundary conditions are tested by comparing the calculated sound field of the circular piston radiator in a rigid baffle with the measured sound field generated by a loudspeaker of the same size mounted in a large wooden wall. The structural boundary conditions are verified by comparing the normal vibration velocity calculated using the model of the simply supported plate and the velocity field measured on a plate using a laser vibrometer.

Reducing the sound reflected by objects under water and thus their ”visibility” is a design challenge and requires detailed information about the geometry- and material-dependent backscattering behavior of the structure. Using various numerical methods and measurements, the radiation-relevant areas of objects are to be determined using generic model examples, taking into account different types of sound sources. The contribution briefly presents the methods used (boundary element method, ray tracing and hotspot detection), analyzes the results of numerical simulations and compares them with corresponding measurements.

From a strategic point of view, it is necessary to protect underwater vehicles from reconnaissance. Modern sonar systems are capable of determining the exact position of those vehicles. They do this by emitting signals that reflect off the surface of the vehicles, which are then detected and evaluated. At low frequencies, reflection can be reduced by using an active system. The incoming sound waves are measured in front of the surface and then processed by a control system. The system drives an actuator applied to the surface that minimizes the reflection. Three different actuator concepts are compared in this study: First, a 3-3 mode piezoceramic patch transducer attached to steel; second, a bending actuator embedded in foam; and third, a multilayer structure made of Poly(vinylidenfluorid-Trifluorethylen) (PVDF-TrFE) film. Measurements in laboratory tests show the different characteristics of the actuators under the influence of sonar-like signals and under hydrostatic pressure from 1 to 40 bar.

Investigations of a test implementation of the “simple” Kirchhoff approximation method (KIA) for GPU graphics adapters (using OpenCL) have shown that, depending on the GPU hardware available, significant performance improvements can be achieved. Therefore, as part of the “Computational Acoustics” research project, the BEAM ray tracing method, originally implemented in C++, was ported to address the required acoustic problems (bistatic, monostatic calculations and their combination). This article discusses the challenges encountered during this process, shares the practical experiences implementing the complex parallelized algorithm in OpenCL and shows the results achieved. It also highlights the speed advantages compared to conventional high-performance PC workstations using examples of underwater objects.

Layered media, in form of both fluid layers and solid structures, play an important role in underwater acoustics. Typical applications are acoustic windows for sonar systems or absorption layers for submarines. However, the either desired total absorption, total reflection or total transmission is not achieved in practice. The vibro-acoustic behaviour of an object plays a particularly important role in the resulting target echo strength. Active surfaces, for example made of piezo-ceramics, increase the possibilities of influencing the target echo strength beyond the geometry, material properties and material composition of the layer.\nThis study compares the target strength behaviour of different layered media including active surfaces for underwater applications. An analytical model is used to estimate the reflection and transmission behaviour. When the reflection of a surface is minimized by an active coating, the transmission increases significantly. This is a particular problem in layered media, as the transmitted sound waves can cause subsequent layers and structures to vibrate and thus increase the total target echo strength. Various countermeasures are being investigated to prevent this.

Sources for underwater sound have a wide range of applications, such as active sonar, navigation, and underwater communication. Particularly, sources with large dimensions l, such as a coating on a boat structure, can be suitable for generating low-frequency underwater sound. In this context, low-frequency refers to the range of 0.15 ≤ He ≤ 4.5 with Helmholtz number He = kl. In this study, a sample of an active surface for generating underwater sound is created by attaching an array of 18 circular piezoelectric actuators to a glass-fiber reinforced plastic (GRP) plate as substrate for the coating. The array is coated with an impedance matching layer. The radiation characteristics of the active surface are investigated in an underwater test range with nearly free field conditions. The setup shows suitability for generating low frequency sound, although the vibroacoustic behaviour of the plate has a significant influence on the generated sound pressure. Depending on the frequency range, the sound pressure is either exponentially or linearly dependent on He. The active coating on the GRP-plate proved to be working as expected. For further investigations and improvements, the deviations between the mathematical model and the real behavior of the array must be investigated more intensively.