Physical and Computational Acoustics

  • Objective:

    The boundary element method (BEM) is an often used tool for numerically solving acoustic radiation and reflection problems. Most of the time, a formulation in the frequency domain can be used, however, for short impulses  or when the acoustic simulation is coupled with a non-linear behaviour caused by structure deformation, a formulation in the time domain is necessary.

    Method:

    The boundary integral equations and the fundamental solution necessary for the BEM in the time domain are derived by inverse Fourier transformation of the corresponding formulations in the frequency domain. These equations are then discretized using the Galerkin method in the spatial dimensions and the collocation method in the time dimension. The MOT (Marching-On-in-Time) method is used to solve the resulting system of equations. The well known stability problem of the MOT-method is handled by using the Burton-Miller approach in combination with the Galerkin method in the spatial discretization and high order temporal interpolations. It is well known that these measures enhance the stability of MOT.

    Additionally it is planned to enhance the efficiency of the method by using a modified plane wave time decomposition (PWTD) algorithm.

  • Objective

    Computational models for speech production and analysis have been of research interest since the 1960s. Most models assume the vocal tract (VT) to be a segmented straight tube, but when pronouncing  nasals like /m/ and /n/ or nasalized vowels the nasal part of the vocal tract plays an important part and a single tube model is not feasible anymore. Thus, it  is necessary to consider a branched tube model that includes an additional tube model for the nasal tract. For these branched models, the estimation of the cross section area of each segments from a given signal is highly non trivial and in general requires the solution of a non-linear system of equations.

    Methods:

    The problem is overdetermined, and we have to add additional restrictions to our solution, for example restrictions on upper and lower bounds of the area functions or smoothness assumption about the vocal tract. To that end we introduced e.g. probabilistic methods (variational Bayes) into our model estimation.

  • Objective

    Railway tunnels avoid direct acoustic annoyance by railway traffic. However, vibrations from tunnels propagate through the soil and lead to disturbances by percieved low frequency vibrations.

    The objective of this project is to develop and implement a mathematical model that takes a moving vibrating load into account. Furthermore, the surrounding soil is modeled as an anisotropic material, consisting of layers with arbitrary orientation.

     

    Methods

    The propagation of the vibrations inside the tunnel are modelled by the finite element method (FEM), where the superstructure of the tunnel and the railway are considered. Vibrations outside the tunnel, propagating through the (infiinite) soil are modelled by the boundary element method (BEM). For a detailed model of the whole system, both methods have to be coupled.

  • The rapid increase in available computing power and the fast evolution of audio interfacing and transmission technologies have led to a new age of immersive audio systems to reproduce spatial sound with surrounding loudspeakers. Many of these approaches require a precise and robust space-time-frequency analysis of sound fields. The joint project of ARI and IRCAM  combines the mathematical concepts provided by the ARI with the profound knowledge in real-time signal processing and acoustics of IRCAM. It addresses fundamental research questions in both fields and aims at developing improved methods for the target applications mentioned above.

    The main questions that his project aims at are:

    • Is it possible to apply the frame-based signal-processing tools to a predefined geometrical alignment of microphones and/or loudspeakers (e.g. to the 64-channel spherical microphone array that is currently under development at IRCAM
    • How can acoustic fields on the sphere (e.g. measured with a spherical microphone array) be represented with frames in order to have better control of the space-time-frequency resolutions on different parts of the sphere?
    • Is it possible to apply this multi-resolution space-time-frequency representation to room acoustic sensing with multichannel spherical microphone arrays (e.g. to measure the spatial distribution of early reflections with higher resolution than provided with spherical harmonic analysis)?
  • Objective:

    Acoustic holography is a mathematical tool for the localization of sources in a coherent sound field.

    Method:

    Using the information of the sound pressure in one plane, the whole three-dimensional sound field is reconstructed. The sound field must be coherent and the half-space in which the sources are situated must be known.

    Application:

    Acoustic holography is used to calculate the sound field in planes parallel to the measured plane. Normally, a plane near the hull of the structure is chosen. Concentrations in the plane are assumed to be the noise source.

  • Objective:

    The beam forming method focuses an arbitrary receiver coil using time delay and amplitude manipulation, and adds to the temporal signal of the microphones or the short time Fourier transform.

    Method:

    64 microphones are collected by a microphone array with arbitrary shape. For compatibility with acoustic holography, equal spacing and a grid with 8 x 8 microphones is used.

    Application:

    Localization of sound sources on high speed trains is a typical application. The method is used to separate locations along the train and especially the height of different sound sources. Typical sound sources on high speed trains are rail-wheel contact sites and aerodynamic areas. The aerodynamic conditions occur at all heights, especially at the pantograph.

  • Biotop Beschreibung
    Workflow Biotop

    Introduction

    Localization of sound sources plays an important role in our everyday lives. The shape of the human head, the torso and especially the shape of the outer ear (pinna) have a filtering effect on incoming sounds and thus play an important role for sound localization. This filtering effect can be described using the so called head related transfer functions (HRTFs). By calculating the distribution of the sound pressure around the head with numerical methods like the boundary element method (BEM), these HRTFs can be calculated numerically.

    Aim

    In BIOTOP the numerical calculations shall be made more efficient by using adaptive wavelet- and frame techniques. Compared to commonly used BEM basis functions, wavelets have the advantage that wavelets can adapt better to a given distribution of the acoustic field on the head. As a generalization of wavelets, frames allow for an even more flexible construction method and thus for a better adaption to the problem at hand.

    BIOTOP combines abstract theoretical mathematics with numerical and applied mathematics. It is an international DACH (DFG-FWF-SFG) project between the Philipps-Universität Marburg (Stephan Dahlke), the University Basel (Helmut Harbrecht) and the ARI. The expertise of all three research groups shall be combined to develop new strategies and numerical methods. The project is funded by the FWF: Pr. Nr. I-1018-N25

     

  • Objective:

    In order to numerically calculate individual head-related transfer functions (HRTFs), a boundary element model (BEM) was developed. This model makes it possible to calculate the sound pressure at the head that is caused by different external sound sources with frequencies up to 20,000 Hz.

    Method:

    In engineering, the traditional BEM is widely used for solving problems. However, the computational effort of the BEM grows quadratically with the number of unknowns. This is one reason why the traditional BEM cannot be used for large models, even on highly advanced computers. In order to calculate the sound pressure at the head at high frequencies, very fine meshes need to be used. These meshes result in large systems of equations. Nevertheless, to be able to use the BEM, the equations must be combined with the Fast Multipole Method (FMM). With the FMM, the resulting matrices can be kept smaller, thus allowing the numeric solving of the Helmholtz equation with feasible effort and almost no accuracy loss as compared to the traditional BEM.

    Application:

    The geometry of the head (especially the form of the outer ear or pinna) acts as a kind of filter. This geometry is very important in localizing sound in the vertical direction and distinguishing between sounds coming from the front or the back. The BEM model can be used to numerically calculate these filter functions, which are dependent on the position and the frequency of the sound source.

    Funding:

    FWF (Austrian Science Fund): Project #P18401-B15

    Publications:

    • Kreuzer, W., Majdak, P., Chen, Z. (2009): Fast multipole boundary element method to calculate head-related transfer functions for a wide frequency range, in: J. Acoust .Soc. Am. 126, 1280-1290.
    • Kreuzer, W.  and Chen, Z. S. (2008). "A Fast Multipole Boundary Element Method for calculating HRTFs," AES preprint  7020, AES Convention, Vienna.
  • Objective:

    Upon first investigation, the design of new outward-curved noise barriers has an improved noise-shielding effect if absorbing material is applied. Further investigation shall prove this ability. Numeric simulations and measurements are being processed.

    Method:

    Advanced boundary element methods (BEM) in two dimensions will prove the noise-shielding ability of the sound barrier. Different curvy and straight designs are compared to each other with respect to their shielding effect in the spectrum. Measurements at existing walls are processed and compared. Measurements are conducted without a noise barrier. A simulated softening affect of the noise barrier walls is used to simulate the noise signal behind the new barriers.

    Application:

    Calma Tec has patented the designs and will offer new designs in practice.

    List of Deliverables:

    01. Traffic Noise Recording Plan. 02. Sound Data Storage, Retrieval and Spectrographic Description. 03. Descriptive Noise Statistics. 04. Pricipal Component Analysis. 05. Sound Barrier Mesh Models. 06. Simulation, Transfer Functions & Clustering. 07. Visualization. 08. Psychoacoustic Irrelevance. 09 Modulation Effects. 10. Subjective Preference Tests. 11. Conclusions

  • Objective:

    In certain measurement setups, such as the measurement of gear mechanism behavior undergoing load reversal, the fine structure of the rotation speed function within a single rotation is interesting. In these situations, measurement errors caused by irregular cog intervals or by other failures of cogwheels are disturbing and must be corrected.

    Method:

    From a reference signal, the distribution parameter of the rotation angle for each cog of the cogwheel is assigned as a cogwheel model. This cogwheel model can minimize the measurement failures caused by the cogwheel if its cog is implemented in synch with the measurement signal. If the reference signal and the measurement signal come from different measurements, the synchronicity has to be established first. The calculation of the shift between the two signals is determined by the cog index, which has the maximum correlation of the rotation angle allocation between the reference signal and the measurement signal.

    Application:

    The developed method will be used as a module in the acoustic measurement and analysis system PAK.

  • Objective:

    This project aims to develop an independent modulus for the wavelet analysis that contains a simple program interface and can be used flexibly.

    Method:

    The implementation was in C++ in the form of a wavelet analysis class and a signal queue. Features:

    • The Input/Output data format can be chosen at run time. The Input and the Output are separately configurable.
    • There are several possibilities for choosing the array and distribution of the frequency bin. The frequency bin vector can also be transferred.
    • Seven wavelets are implemented.
    • A down-sampling method can be used for the acceleration (factor: 1.2 convert frequency bins are chosen automatically).
    • Because of the disjunction in signal queue and analysis, an asynchrony Input/Output is possible.
    • Compiling an optimized numerical library can be achieved. Currently, the application of the "Intel® Signal Processing Library" (SPL) or of the "Intel® Integrated Performance Primitives" (IPP) is possible.
    • The signal queue class can be used independently of the analysis class. It also implements the down-sampling function.

    Application:

    The developed classes are used as a modulus in the acoustic measurement and analysis system PAK. The analysis class was also integrated as a signal processing atom WLLIB in STx.

  • Objective:

    The usual transformation in acoustics is the Fourier-Transformation. A fast and simple implementation is the windowed Fast Fourier Transformation. A disadvantage of the FFT is that all frequencies are equally spaced in the time frequency plane. A logarithmic spacing that allows keeps the relative resolution in the frequency plane constant is the Wavelet Transformation. This gives the possibility of a higher temporal resolution in the high frequency plane. Several types are implemented in STX and PAK.

    Method:

    A higher temporal resolution is possible, if quadratic transformations defined in the Cohen Class are used. The Windowed Pseudo Wigner Ville Distribution and a discrete version of the Choi-Williams Distribution are implemented in STX and PAK. Disadvantages of these transformations are the cross products that are reduced by smoothing in the different transformations of the Cohen class.

    Application:

    A handbook is written for or the practical use of the difficult transformations. The Handbook documents the possibilities and the limits of the transformations

  • Objective:

    In the boundary element method for infinite exterior spaces occur spurious modes that belong to the interior problem. The resonances are caused by a singularity of the matrix. As compensation additional equations are used that belong to chief points in the interior space.

    Method:

    A new method that does not depend on the appropriate selection of chief points is the Burton Miller Method. It assures good results but increases the numerical calculation and gives unsymmetrical matrices. In combination with the Fast Multipole Method (FMM) a fast calculation of non symmetric matrices is possible.

  • Objective:

    Up to now a boundary element method based on the collocation method was combined with the Fast Multipole Method to speed up the numerical calculation.

    Method:

    This approach was chosen, because the collocation is a fast method to build up the matrix and the Fast Multipole Method (FMM)is a fast method to solve large matrices. For compatibility with the software HAMS the FMM has to be ported to a Galerkin based Boundary Element (BE) approach.

  • Introduction                                                                                                                                                   

    Rumble strips are (typically periodic) grooves place at the side of the road. When a vehicle passes over a rumble strip the noise and vibration in the car should alert the driver of the imminent danger of running off the road. Thus, rumble strips have been shown to have a positive effect on traffic safety. Unfortunately, the use of rumble strips in the close vicinity of populated areas is problematic due to the increased noise burden.

    Aims

    The aim of the project LARS (LärmArme RumpelStreifen or low noise rumble strips) was to find rumble strip designs that cause less noise in the environment without significantly affecting the alerting effect inside the vehicle. For this purpose, a number of conventional designs as well as three alternative concepts were investigated: conical grooves to guide the noise under the car, pseudo-random groove spacing to reduce tonality and thus annoyance, as well as sinusoidal depth profiles which should produce mostly vibration and only little noise and which are already used in practice.

    Methods

    Two test tracks were established covering a range of different milling patterns in order to measure the effects of rumble strips for a car and a commercial vehicle running over them. Acoustic measurements using microphones and a head-and-torso-simulator were done inside the vehicle as well as in the surroundings of the track. Furthermore, the vibration of the steering wheel and the driver seat were measured. Using the acoustics measurements, synthetic rumble strip noises were produced, in order to get a wider range of possible rumble strip designs than by pure measurements.

    Perception tests with 16 listeners were performed where the annoyance of the immissions as well as the urgency and reaction times for the sounds generated in the interior were determined also using the synthetic stimuli.

    LARS was funded by the FFG (project 840515) and the ASFINAG. The project was done in cooperation with the Research Center of Railway Engineering, Traffic Economics and Ropeways, Institute of Transportation, Vienna University of Technology, and ABF Strassensanierungs GmbH.

  • Objective:

    The Acoustic Research Institute was mandated to do measurements with the acoustic 64-channel microphone array using the beam forming method to derive a source model for high speed trains according to the new guideline CNOSSOS-EU.

    Method:

    The beam forming method was used, because the train is a fast moving vehicle and therefore a transient acoustic source. Five heights were used in the evaluation based on the CNOSSOS-EU and additionally five heights were evaluated that fit to the geometry of the trains.

    Application:

    Speeds from 200 km/h up to 330 km/h were tested for the ICEs and from 200 km/h up to 250 km/h for the Railjet. At the same speed both trains had the same acoustic level.

  • Objective:

    In Cooperation with National Instruments an implementation of MPEG4 features in the software package DIADEM is planned.

    Method:

    The application of MPEG4 features to noise is proven. Now the implementation of MPEG4 features into DIADEM is planned. In preparation of the project additional features were implemented into STX. The implementation into DIADEM is projected in the future.

    Application:

    DIADEM is a database that allows for a rapid search of measurement recordings. New search indexes can be generated based on the MPEG4 features of the recordings.

  • Objective:

    The Multilevel Fast Multipole Method, when used in combination with the Boundary Element Method (BEM), is a tool to significantly speed up the simulation of large objects almost without loss in accuracy.

    Method:

    The Fast Multipole Method subdivides the Boundary Element mesh into different clusters. If two clusters are sufficiently far away from each other (i.e. they are in each other's far field), all calculations that would have to be made for every pair of nodes can be reduced to the midpoints of the clusters with almost no loss of accuracy. For clusters not in the far field, the traditional BEM has to be applied. The Multilevel Fast Multipole Method introduces different levels of clustering (clusters made out of smaller clusters) to additionally enhance computation speed.

    Application:

    The MLFFM is used for the simulation of head related transfer functions. The diagram above compares the result of a classical BEM with the MLFMM.

  • Objective:

    An important difficulty of ray-tracing and boundary element method is the fine grid, which is needed in the high frequency region.

    Method:

    By means of new alternating shape functions e.g. wavelets at the boundary it could be possible to define a grid on the boundary that is independent from the wave number.

  • Objective:

    Methods to predict the propagation of vibrations in soil are relatively undeveloped. Reasons for this include the complexity of the wave propagation in soil and the insufficient knowledge of material parameters. During this project a method was developed to simulate the propagation of vibrations that are caused by a load at the base of a tunnel.

    Method:

    When dealing with the model of a tunnel in a semi-infinite domain like soil, the boundary element method (BEM) seems to be an appropriate tool. Unfortunately it cannot be applied directly to layered orthotropic media, because of the lack of a closed form of the Greens function, which is essential for BEM. But by transforming the whole system into the Fourier domain with respect to space and time, it is possible to numerically construct an approximation for this function on a predefined grid. With this approximation the boundary integral equation, that describes the propagation of waves caused by a vibrating load at the base of a tunnel can be solved.

    Application:

    Models that can help to predict the propagation of vibrations inside soil layers are of great interest in earthquake sciences or when constructing railway lines and tunnels.