Physical and Computational Acoustics

  • Orthobem: Simulation of Vibrations in Tunnels

    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.

  • PAAB

    Introduction

    Railway vehicles passing through tight curves can produce a high pitched noise called curve squeal. Curve squeal is a very salient type of noise located in the high frequency range that can range between a tonal narrow band and a wide band noise. The reason for the tonal noise is lateral creepage on the top of the rail, which excites wheel vibration at frequencies corresponding to their modes. Wide band noise, however, is caused by wheel flanges touching the rail.

    Aims

    The project PAAB aims at investigating the effect on the perceived annoyance of such noises using in a perception test. Using the resulting perceptual characterization of curve squeal should aid in more adequately considering this type of noise in noise mapping.

    Methods

    Based on previous conventional large-scale emission measurements as well as new measurements at immission distances using a head-and-torso-simulator representative samples for curve squeal will be derived and used in a perception test. This will also be aided by using synthetic well defined curve squeal noise.

    PAAB is funded by the FFG (project 860523) and the Austrian Federal Railways (ÖBB). The project is done in cooperation with the Research Center of Railway Engineering, Traffic Economics and Ropeways, Institute of Transportation, Vienna University of Technololgy (project leader), Kirisits Engineering Consultants, and psiacoustic Umweltforschung und Engineering GmbH.

     

     

  • PASS - Psychoacoustic Analysis of Noise Emissions Induced by Railway Traffic

    The project PASS, which is processed in cooperation with the IEW of the TU Vienna and psiacoustic GmbH, deals with the psychoacoustic evaluation of noise. The project is a continuation of the project RELSKG and deals with high and low noise barriers that are simulated with the 2.5 D boundary element method (BEM) assuming incoherent line sources. The comparison of the 2.5 D BEM with measurements resulted in a good agreement. Additionally measurements with rail dampers were taken into account in the psychoacoustic tests. The evaluation was done in two tests with 40 test persons. The first test determines the relative annoyance and the second the just noticeable difference in annoyance. The results ware that freight trains at the same A-level are less annoying than passenger trains and that at the same A-level the noise behind a noise barrier is a little bit more annoying than without a measure. The project started in 2013 and lasts until the end of 2014.

  • Principal Component Analysis (PCA) for the Estimation of the Acoustic Far-Field Level

    Objective:

    If measurements are possible only at the hull of a machine, a tool is needed to separate the dominating near-field components from the far-field components. This, in turn, allows the far-field levels to be estimated. The separation is often not possible using spectral methods, because both components have nearly the same frequency. Using a limited number of microphones, a modal separation is also impossible. Instead of a modal analysis, a principal component analysis is applied.

    Method:

    The narrow-band Fourier transform method is used, and a separate analysis is conducted for each frequency. The cross-power matrix spanning all microphone positions is used. The components are then calculated using the PCA. As long as the modes at the microphone positions have different relative values, PCA can be used to separate them. In an initial test, the far field is observed and the transfer function for every component from the near field to the far field is estimated. These transfer functions are assumed to be constant in time. They are used for the estimation of the overall far-field level.

    Application:

    Observation of the far-field level of machines.

  • RAARA - Residential Area Augmented Reality Acoustics

    Objective

    We thank the Austrian Science Fund (FFG) for funding this project, grant number 873588. Principal Investigator is the AIT. Noise means trouble. In addition to traffic and industry, it is mainly emitted by heating or cooling appliances: Air heat pumps, recoolers and fans. In order to minimize noise immissions to the population in urban areas, the project is developing methods that enable simple, intuitive and at the same time accurate handling of noise emissions and their reduction.

     

    Methods

    The aim is to virtually place the noise sources in a real environment VOR ORT using augmented reality before they are installed and to visually display the sound emissions in colour and make them audible. Obstacles or soundproofing measures such as walls, fences and walls are detected automatically or can be added virtually. In order to achieve these goals, comprehensive method developments for efficient acoustic calculation are required: frequency-dependent and time-dependent behaviour, absorption and reflection. This unique approach facilitates the planning of renewable heating and cooling appliances, increases the acceptance and thus the share of renewable energies and lowers the noise level in cities.

     

     

     

     

     

  • RELSKG: Development of a computational method for noise barriers with a complex geometry

    Objective:

    Standard noise mapping software use geometrical approaches to determine insertion loss for a noise barrier. These methods are not well suited for evaluating complex geometries e.g. curved noise barriers or noise barriers with multiple refracting edges. Here, we aim at deriving frequency and source- as well as receiver-position dependent adjustments using the boundary element method. Further, the effect of absorbing layers will be investigated as a function of the geometry. Results will be incorporated into a standard noise mapping software.

    Method:

    The cross-sections of different geometries are first parameterized and discretized and then evaluated using two-dimensional boundary element simulations. The BEM code was developed at our institute. Different parameter sets are evaluated in order to derive the adjustments for the specific geometries compared to a straight noise barrier. To make the simulations more realistic, a grassland impedance model is used instead of a fully reflecting half plane. Simulations will also be evaluated using measurements from actual noise barriers.

    Noise reduction of a T-type barrier at 800 Hz

    Project partners:

    • TAS Schreiner (measurements)
    • Soundplan (implementation in sound mapping software)

    Funding:

    This project is funded from the VIF2011 call of the FFG (BMVIT, ASFINAG, ÖBB)

  • Spectral Transform of Sound Fields (STSF)

    Objective:

    The Spatial Transform of Sound Fields (STSF) is an extension of the acoustic holography that enables the handling of incoherent sound sources.

    Method:

    The Karhunen Loeve Expansion or Principal Component Analysis (PCA) method is used to separate the random field recorded at different microphone positions into coherent components. Again, acoustic holography is used to transform every component from the measurement plane into a plane in arbitrary depth. If needed, the total incoherent sound field in the chosen depth can be reconstructed.

    Application:

    Localization of incoherent sound sources near the hull of the structure.

  • Stochastic Transformation Methods (Acoustics and Vibration)

    Objective:

    In the past a FWF project dealing with the basics of Stochastic Transformation Methods was executed at the ARI. Explicitly the Karhunen Loeve Expansion and the Transformation of a polynomial Chaos were applied in the wave number domain. The procedure is based on the assumption of Gaussian distributed variables. This assumption shall be generalized to arbitrary random variables.

    Method:

    The assumption of a wave number domain limits the model to a horizontally layered half space. This limitation shall be overcome by Wavelets kernels in the transformation instead of Fourier kernels. The aim is the possibility to calculated one sided statistical distributions for the physical parameters and arbitrary boundaries with the new method.

  • SysBahnlärm: creating a handbook on the systemic reduction of railway noise

    Objective:

    SysBahnLärm was a joint project of the ARI with the TU Vienna the Austrian Railways and industrial partners funded by the FFG as well as the ÖBB. Aim of the project was to create a handbook on the systemic reduction of railway noise. The ARI was responsible for the psychoacoustic evaluation of the effects of noise from wheels with different roughness and of different noise reduction systems e.g. rail damping systems. Further, the ARI investigated the emission pattern of the rail-wheel contact using our 64-channel microphone array.

    Method:

    Using measured train pass-by signals, a psychoacoustic testing procedure was developed and stimuli for this test were selected. Subjects had to rate the relative annoyance of different trains or different noise reduction systems with respect to each other.
    For investigating the rail-wheel contact, a beamforming technique was used in order to determine the point of the maximal emission relative to the top of the rail.

    Application:

    The handbook should act as a guideline for the different noise reduction measures and their respective advantages and problems.

  • Vibrations in Random Layers

    Objective:

    One of the biggest problems encountered when building numerical models for layers is the lack of exact deterministic material parameters. Therefore, stochastic models should be use. However, these models have the general drawback of overusing computer resources. This project developed a stochastic model with the ability to use a shear modulus in conjunction with a special iteration scheme allowing efficient implementation.

    Method:

    With the Karhunen Loeve Expansion (KLE), it is possible to split the stochastic shear modulus, and therefore the whole system, into a deterministic and a stochastic part. These parts can then be transformed into a linear system of equations using finite elements and Chaos Polynomial Decomposition. Combining the KLE and the Fourier Transformation in combination with Plancherel's theorem enables decoupling of the deterministic part into smaller subsystems. An iteration scheme was developed which narrows the application of "costly" routines to only these smaller deterministic subsystems, instead of the whole higher dimensional (up to a dimension of 10,000) system matrix.

    Application:

    As concerns about vibrations produced by machinery and traffic have increased in past years, models that can predict vibrations in soil became more important. However, since material parameters for soil layers cannot be measured exactly in practice, it is reasonable to use stochastic models.

  • Wavelet Approaches in the Fast Multipole Method

    Objective:

    Beside the Fast Multipole Method wavelet based approaches are of increasing interest for the fast calculation of large matrices.

    Method:

    The first part of the project is the implementation of the wavelet method for the compression of the data. Next step is the investigation whether wavelet and FMM approaches can be used together and whether an additional speed up is possible.

    Application:

    The aim of the project is the development of an algorithm that allows for a fast calculation of large matrices. The final aim is the possibilities to handle large acoustic problems numerically.

  • WiABahn - Acoustic Effect of Shielding Edges Near the Rail and Roofs Above Railway Platforms

    Introduction

    Railway platforms are located very close to the track and thus are assumed to alter the sound propagation. The degree of this effect, however, has not yet been investigated in detail

    Aims

    The aim of the project WiaBahn was to investigate the shielding effect of railway platforms. One of the main questions was how to properly deal with the vicinity to the track, the platform’s large reflecting horizontal surface, and the often present canopy. It is unclear whether standard noise propagation prediction methods can be applied without modifications.

    Methods

    Based on measurements directly at the platform as well as in the distance the acoustic effect of low railway platforms was investigated and suitable source models for the 2.5D boundary element method (BEM) as well as for standardized prediction methods were derived. The advantage of the 2.5D method which was also used in the project PASS is, that a constant cross-section can be combined with point sources or incoherent line sources which is not possible with pure 2D methods. 3D BEM is not feasible for such large structures.

    WiaBahn was funded by the FFG (project 845678) and the ÖBB. The project was done in cooperation with the  Austrian Institute of Technology (AIT, project leader) and Kirisits Engineering Consultants.