Mcj Maarten Hornikx

The 2.5-dimensional equivalent sources method for directly exposed and shielded urban canyons.

When a domain in outdoor acoustics is invariant in one direction, an inverse Fourier transform can be used to transform solutions of the two-dimensional Helmholtz equation to a solution of the three-dimensional Helmholtz equation for arbitrary source and observer positions, thereby reducing the computational costs. This previously published approach [D. Duhamel, J. Sound Vib. 197, 547-571 (1996)] is called a 2.5-dimensional method and has here been extended to the urban geometry of parallel canyons, thereby using the equivalent sources method to generate the two-dimensional solutions. No atmospheric effects are considered. To keep the error arising from the transform small, two-dimensional solutions with a very fine frequency resolution are necessary due to the multiple reflections in the canyons. Using the transform, the solution for an incoherent line source can be obtained much more efficiently than by using the three-dimensional solution. It is shown that the use of a coherent line source for shielded urban canyon observer positions leads mostly to an overprediction of levels and can yield erroneous results for noise abatement schemes. Moreover, the importance of multiple facade reflections in shielded urban areas is emphasized by vehicle pass-by calculations, where cases with absorptive and diffusive surfaces have been modeled.

A scale model study of parallel urban canyons

Shielded urban areas are of importance regarding urban citizens’ annoyance and adverse health effects related to road traffic noise. This work extends the existing knowledge of sound propagation to such areas by a scale model study, rather than by model calculations. The scale model study was executed for two parallel urban canyons at a 1 to 40 scale, with a point source located in one canyon. Cases with acoustically hard facades and absorption and diffusion facade treatments were in vestigated. To correct for excess air attenuation of the measurements, a wavelet-based method has been applied. The measurement results in the shielded canyon show that, in contrast to the directly exposed street canyon, the levels and the decay times are quite constant over the length of the canyon. The energy-time curve in the shielded canyon is characterized by a rise time, which can be related to the sound pressure level. The rise times and decays can be explained by separate reflection, diffraction and diffusion processes. A closed courtyard situation enlarges the level difference between acoustically hard facades and applied facade absorption or diffusion treatments at both the directly exposed and shielded side. A comparison between measurements with two different diffusion mechanisms, horizontal and vertical diffusion, reveals that vertical diffusion yields lower levels at the shielded side compared to horizontal diffusion for the investigated situations.

Acoustic modelling for indoor and outdoor spaces

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Acoustic modelling of sports halls, two case studies

To achieve a preferable sound field in traditionally shoebox-shaped sports halls, the sound absorbing material is often applied in the upper part of the hall. The applicability of predicting the acoustics of sports halls by three different acoustic calculation methods is investigated: a diffuse field method, a geometrical acoustics method and a full wave-based method. The predicted reverberation time and sound pressure level are compared to measured data for two sports halls in the low frequency range up to the 630 Hz ⅓ octave band. From the three methods, results from the wave-based method agree best with the measured results. Results indicate the importance of the chosen material properties in the prediction methods used. Sound pressure levels resulting from the diffuse field method are comparable with results from the other prediction methods, but the reverberation time prediction is not reliable using this method.

OpenPSTD:The open source pseudospectral time-domain method for acoustic propagation

Abstract An open source implementation of the Fourier pseudospectral time-domain (PSTD) method for computing the propagation of sound is presented, which is geared towards applications in the built environment. Being a wave-based method, PSTD captures phenomena like diffraction, but maintains efficiency in processing time and memory usage as it allows to spatially sample close to the Nyquist criterion, thus keeping both the required spatial and temporal resolution coarse. In the implementation it has been opted to model the physical geometry as a composition of rectangular two-dimensional subdomains, hence initially restricting the implementation to orthogonal and two-dimensional situations. The strategy of using subdomains divides the problem domain into local subsets, which enables the simulation software to be built according to Object-Oriented Programming best practices and allows room for further computational parallelization. The software is built using the open source components, Blender, Numpy and Python, and has been published under an open source license itself as well. For accelerating the software, an option has been included to accelerate the calculations by a partial implementation of the code on the Graphical Processing Unit (GPU), which increases the throughput by up to fifteen times. The details of the implementation are reported, as well as the accuracy of the code. Program summary Program title: openPSTD v1.1 (v1.0 is the version without the GPU acceleration) Catalogue identifier: AFAA_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AFAA_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: GNU General Public License, version 3 No. of lines in distributed program, including test data, etc.: 45339 No. of bytes in distributed program, including test data, etc.: 4139815 Distribution format: tar.gz Programming language: Python (as comes with Blender 2.72). Computer: Variable. Operating system: Windows, Linux, Mac OS X. RAM: From 250 MB for a typical geometry up to 1 GB for large geometries (with about 4M grid points) Classification: 4.3, 12. External routines: Blender 2.72, NumPy, SciPy, PyFFT, PyOpenCL, PyCUDA Nature of problem: Sound propagation Solution method: Fourier pseudospectral time-domain method Restrictions: Structured grid, two dimensions, real-valued boundary conditions only Unusual features: Implementation of code using Blender/Python including GPU acceleration; subdomain modelling within Fourier pseudospectral method Running time: Depending on the dimension of the problem, calculation times take minutes up to hours

Numerical investigation of the effect of crosswind on sound propagation outdoors

The effect of wind on sound propagation in the atmosphere has been studied extensively before with an emphasize on downwind sound propagation, typically representing worst-case scenarios. However, the influence of oblique and crosswind on propagation from various types of sources raises some questions in acoustic literature. In this work, the effect of a logarithmic wind speed profile at different wind directions has been studied for sound emitted by a point source, a coherent line source and an incoherent line source. For this purpose, a full three-dimensional wave-based method was used. For the incoherent line source simulation, the Harmonoise engineering approach based on a summation of source segments was considered as well and shows to be in satisfying agreement with the latter. While for a point source and coherent line source crosswind shows to have an insignificant effect, it is important in case of an incoherent line source. Also, the stretch of the incoherent line source contributing to the noise level at a receiver close to this line differs strongly depending on the wind direction.

The effective air absorption coefficient for predicting reverberation time in full octave bands

A substantial amount of research has been devoted to producing a calculation model for air absorption for pure tones. However, most statistical and geometrical room acoustic prediction models calculate the reverberation time in full octave bands in accordance with ISO 3382-1 (International Organization for Standardization, 2009). So far, the available methods that allow calculation of air absorption in octave bands have not been investigated for room acoustic applications. In this paper, the effect of air absorption on octave band reverberation time calculations is investigated based on calculations. It is found that the approximation method, as described in the standard ANSI S1.26 (American National Standards Institute, 1995), fails to estimate accurate decay curves for full octave bands. In this paper, a method is used to calculate the energy decay curve in rooms based on a summation of pure tones within the band. From this decay curve, which is found to be slightly concave upwards, T20 and T30 can be determined. For different conditions, an effective intensity attenuation coefficient mB ;eff for the full octave bands has been calculated. This mB ;eff can be used for reverberation time calculations, if results are to be compared with T20 or T30 measurements. Also, guidelines are given for the air absorption correction of decay curves, measured in a scale model.

Hybrid Fourier pseudospectral/discontinuous Galerkin time-domain method for wave propagation

The Fourier Pseudospectral time-domain (Fourier PSTD) method was shown to be an efficient way of modelling acoustic propagation problems as described by the linearized Euler equations (LEE), but is limited to real-valued frequency independent boundary conditions and predominantly staircase-like boundary shapes. This paper presents a hybrid approach to solve the LEE, coupling Fourier PSTD with a nodal Discontinuous Galerkin (DG) method. DG exhibits almost no restrictions with respect to geometrical complexity or boundary conditions. The aim of this novel method is to allow the computation of complex geometries and to be a step towards the implementation of frequency dependent boundary conditions by using the benefits of DG at the boundaries, while keeping the efficient Fourier PSTD in the bulk of the domain. The hybridization approach is based on conformal meshes to avoid spatial interpolation of the DG solutions when transferring values from DG to Fourier PSTD, while the data transfer from Fourier PSTD to DG is done utilizing spectral interpolation of the Fourier PSTD solutions. The accuracy of the hybrid approach is presented for one- and two-dimensional acoustic problems and the main sources of error are investigated. It is concluded that the hybrid methodology does not introduce significant errors compared to the Fourier PSTD stand-alone solver. An example of a cylinder scattering problem is presented and accurate results have been obtained when using the proposed approach. Finally, no instabilities were found during long-time calculation using the current hybrid methodology on a two-dimensional domain. A method for wave propagation in fluid domains with complex geometries is presented.The method combines Fourier PSTD in the bulk of the domain and DG at boundaries.The main application of interest is atmospheric sound propagation in cities.The method shows no significant additional error compare with a Fourier PSTD solver.No indication of instability in long-time calculations has been detected.

Acoustic properties of tongue clicks used for human echolocation

In active human echolocation, it is common for visually impaired persons to produce tongue click signals for extracting information from the physical environment around them. The purpose of this paper is to provide a comprehensive dataset of original detailed experimental data on human tongue clicks. The methods used for analysis are described in order to encourage further work to complement the data, which is available online and open for contributions. Series of six different click signals have been recorded from blind persons in an anechoic room. The different click signals all resemble damped oscillations and have in common that the dominant frequency content is located between 1,000 Hz and 8,000 Hz. The standard deviation in click series of the broadband click sound level is between 2.2 and 4.3 dB. The total duration of the clicks, which is mainly determined by the drop-time, ranges between 2.0 ms and 10.0 ms, for a 20 dB decay range. Average standard deviations of 57% in rise-time and 32% in drop-time have been found. The horizontal and vertical directivity of the tongue clicks show a close agreement with literature data from speech signals, but click signals are generally more directive. The minimum and maximum deviations in the 1/3 octave bands from 1,250 Hz to 4,000 Hz are less than 6 dB, for 0 to 120 deg. azimuth, and for –27 to 127 deg. elevation.

Noise disturbance in open-plan study environments: a field study on noise sources, student tasks and room acoustic parameters

Abstract The aim of this study is to gain more insight in the assessment of noise in open-plan study environments and to reveal correlations between noise disturbance experienced by students and the noise sources they perceive, the tasks they perform and the acoustic parameters of the open-plan study environment they work in. Data were collected in five open-plan study environments at universities in the Netherlands. A questionnaire was used to investigate student tasks, perceived sound sources and their perceived disturbance, and sound measurements were performed to determine the room acoustic parameters. This study shows that 38% of the surveyed students are disturbed by background noise in an open-plan study environment. Students are mostly disturbed by speech when performing complex cognitive tasks like studying for an exam, reading and writing. Significant but weak correlations were found between the room acoustic parameters and noise disturbance of students. Practitioner Summary: A field study was conducted to gain more insight in the assessment of noise in open-plan study environments at universities in the Netherlands. More than one third of the students was disturbed by noise. An interaction effect was found for task type, source type and room acoustic parameters.