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DSP block diagram of source generation and injec- n tion. F denotes driving force, H ( z ) is the transfer function of
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The Finite Difference Time Domain (FDTD) method is becoming increasingly popular for room acoustics simulation. Yet, the literature on grid excitation methods is relatively sparse, and source functions are traditionally implemented in a hard or additive form using arbitrarily-shaped functions which do not necessarily obey the physical laws of sound...
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... a unipolar function, the Gaussian source exhibits a strong DC component, thus we expect solution growth. Unlike the soft source described in section 4.2, this function does not get differentiated prior to being injected in additive form, and as such, will be further referred to as an Arbitrary Soft Source . As reference, consider the same excitation function, however being filtered and injected according to the PCS principles, which is summarised in Figure 1. As will be further discussed, the PCS model acts as a natural DC-blocking filter, therefore no solution growth is expected. The result of this comparison is shown in Figure 8. Such behaviour is also sensible from a physical perspective, as a DC component in ψ ( t ) indicates that q ( t ) is not of finite length, meaning that the source endlessly generates volume velocity. Fol- lowing equation (11), the rate of fluid emergence due to the arbitrary soft source is obtained by taking the integral of equation (33) which ...
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... ρ 0 A s /X to directly obtain q . The complete signal process- i ing required for generating and injecting the source into the grid is graphically shown in figure 1. Note that the processing shown in figure 1 outputs a signal which is delayed by one-sample in comparison to the result of equation (22). In this paper we refer to the entire model presented here as a Physically-Constrained Source , abbreviated PCS hereafter. In room acoustics simulation, it is desirable to design a source function that generates a prescribed sound field. In this section we show how the Physically-Constrained Source (PCS) can be used to accomplish this task. The goal is to design an excitation signal that propagates omni-directionally and has a flat frequency response within a defined frequency range. It is not possible (nor physically practical) to implement a source mechanism with infinite bandwidth. Nevertheless, some properties of the system can be exploited to effectively band-limit the excitation signal whilst still maintaining a near-flat frequency response within its passband. The low-frequency behaviour of the source is characterised by the system resonance ω 0 and quality factor Q . The former controls the low cut-off frequency whilst the latter defines the steepness of the transition between the rolled-off frequencies and the passband. In an optimal transducer design process, the designer would specify the desired values for these parameters and the remaining electro-mechanical quantities would be engineered accordingly. In the model presented herein, it is assumed that the source has some mass, M , and the remaining damping and stiffness coefficients are then calculated from R = ω 0 Q M and K = M ω 0 2 , respectively. Since all FDTD schemes exhibit numerical dispersion, at least to some extent, it is also desired to specify a high cut-off frequency. This can be achieved by employing a driving function with a Gaussian force distribution, given ...
Context 3
... ρ 0 A s /X to directly obtain q . The complete signal process- i ing required for generating and injecting the source into the grid is graphically shown in figure 1. Note that the processing shown in figure 1 outputs a signal which is delayed by one-sample in comparison to the result of equation (22). In this paper we refer to the entire model presented here as a Physically-Constrained Source , abbreviated PCS hereafter. In room acoustics simulation, it is desirable to design a source function that generates a prescribed sound field. In this section we show how the Physically-Constrained Source (PCS) can be used to accomplish this task. The goal is to design an excitation signal that propagates omni-directionally and has a flat frequency response within a defined frequency range. It is not possible (nor physically practical) to implement a source mechanism with infinite bandwidth. Nevertheless, some properties of the system can be exploited to effectively band-limit the excitation signal whilst still maintaining a near-flat frequency response within its passband. The low-frequency behaviour of the source is characterised by the system resonance ω 0 and quality factor Q . The former controls the low cut-off frequency whilst the latter defines the steepness of the transition between the rolled-off frequencies and the passband. In an optimal transducer design process, the designer would specify the desired values for these parameters and the remaining electro-mechanical quantities would be engineered accordingly. In the model presented herein, it is assumed that the source has some mass, M , and the remaining damping and stiffness coefficients are then calculated from R = ω 0 Q M and K = M ω 0 2 , respectively. Since all FDTD schemes exhibit numerical dispersion, at least to some extent, it is also desired to specify a high cut-off frequency. This can be achieved by employing a driving function with a Gaussian force distribution, given ...
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Citations
... To this end, different source signals have been proposed in the literature enabling appropriate control over this required bandwidth (see e.g. [6,7] tionality dependent on the propagation characteristics of the FDTD grid itself. Typically however, although usually defined in terms of sound pressure, the discrete-time excitation signal itself can take any form such that it is sampled commensurate with the sampling rate of the FDTD grid used (see e.g. ...
... In addition to having little correlation with actual physical sound sources, hard sources also act as signal scatterers for any incident wave. As a consequence they become a discontinuity or perturbation in the medium, or can be considered as a sound radiating, perfectly reflecting boundary node with a size corresponding to the spatial sampling interval [6]. ...
... Ideally, as recommended in [11], the response should be measured at the source grid point and used to normalize the output at other grid points, with the suggestion that this is the reason why soft source excitation has not been extensively used in the acoustics literature. In addition, soft sources exhibit solution growth due to source-boundary interaction effects [6], requiring further pre-or post-simulation conditioning to obtain a useful IR, e.g. differentiation of the original pressure-based signal [6], or pre-filtering [8]. ...
This paper considers source excitation strategies in finite difference time domain room acoustics simulations for auralization purposes. We demonstrate that FDTD simulations can be conducted to obtain impulse responses based on unit impulse excitation, this being the shortest, simplest and most efficiently implemented signal that might be applied. Single, rather than double, precision accuracy simulations might be implemented where memory use is critical but the consequence is a remarkably increased noise floor. Hard source excitation introduces a discontinuity in the simulated acoustic field resulting in a shift of resonant modes from expected values. Additive sources do not introduce such discontinuities, but instead result in a broadband offset across the frequency spectrum. Transparent sources address both of these issues and with unit impulse excitation the calculation of the compensation filters required to implement transparency is also simplified. However, both transparent and additive source excitation demonstrate solution growth problems for a bounded space. Any of these approaches might be used if the consequences are understood and compensated for, however, for room acoustics simulation the hard source is the least favorable due to the fundamental changes it imparts on the underlying geometry. These methods are further tested through the implementation of a directional sound source based on multiple omnidirectional point sources.
... When considering the type of source to use, an important design consideration was to improve lowfrequency accuracy without introducing DC increments. The physically constrained source proposed by Sheaffer et al [29] [30] and made available for download by the author at [31] fits this criterion and is relatively straightforward to implement into the simulation routine. ...
Low-frequency acoustic effects have been well documented in archaeological studies for nearly two decades. However, to date, specialist acoustic input into the field of archaeoacoustics has been in the minority of research effort put into this emerging branch of scientific study. This project aims to investigate the initial findings regarding low-frequency responses of chambered Neolithic tombs as reported by Devereux et al[1] carrying out a computer simulation study of a case study monument through the application of a Finite Difference Time Domain (FDTD) method. The open source acoustic simulation package WaveCloud served as the basis for the simulations. An emulation of the physical soundfield within a reference room has been undertaken to validate the implementation proposed herein. Implementation shortcomings and improvements have been addressed, which informed the case study site study. Despite a degree of space-time accuracies and subsequent validation requirements, partial success in simulating the acoustic properties of the site was achieved. A tentative analysis of the correlation of the identified resonant peaks and subjective perception threshold of changes in low frequencies as proposed by Avis et al. has also been carried out.
... A 3x3x3 room model was calculated using the standard rectilinear FDTD grid, with an object approximating the listener placed at the centre of the domain. A Physically-Constrained (PCS) sound source [20] with a high cutoff frequency at 10kHz, low frequency resonance at 100Hz and Q = 0.7 was placed at 1m radial distance away from the object and the resulting sound pressure was recorded on the surface of the object, at the positions of the ears. The simulation was repeated 360 • around the object at 5 • intervals. ...
***AUTHOR'S NOTE: This is a POMA (ICA) conference paper not a JASA paper. This cannot be changed in ResearchGate(!)******
Binaural room impulse responses are important for auralization as well as for objective research in room acoustics. In geometrical room simulation methods, obtaining such responses is easily achieved by convolving each computed reflection tap with a corresponding pre-measured angle-dependent head-related impulse response. Unfortunately, employing such an approach in wave based methods is challenging due to temporal overlap of room reflections in the calculated response. One alternative is to physically embed a listener geometry in the grid. Whilst this method is straightforward, it requires voxelization of a geometrically complex object. Furthermore, with non-conformal boundary conditions, the voxelized geometry is sample-rate dependent, meaning that numerical consistency is compromised. In this paper, we discuss the merits and drawbacks of embedding different listener geometries in the grid, ranging from a simple rigid sphere to a fully featured laser-scan of a Kemar mannikin. We then introduce a parametric model of a human listener whose head related effects are structurally approximated by digital filters. The model is applied to simulated results in order to extrapolate a binaural response from a single pressure-velocity receiver, without the need to embed any objects in the grid. A comparative analysis of the two methods is presented, and results are discussed in light of room acoustics modeling.
Human localisation of sound in enclosed spaces is a cross-disciplinary research topic, with important applications in auditory science, room acoustics, spatial audio and telecommunications. By combining an accelerated model of 3D sound propagation in rooms with a perceptual model of spatial processing, this thesis provides an integrated framework for studying sound localisation in enclosed spaces on the horizontal plane, with particular emphasis on room acoustics applications. The room model is based on the finite difference time domain (FDTD) method, which has been extended to include physically-constrained sources and binaural receivers based on laser-scanned listener geometries. The underlying algorithms have been optimised to run on parallel graphics hardware, thus allowing for a high spatial resolution, and accordingly, a significant decrease of numerical dispersion evident in the FDTD method. The perceptual stage of the model features a signal processing chain emulating the physiology of the auditory periphery, binaural cue selection based on interaural coherence, and a final decision maker based on supervised learning. The entire model is shown to be capable of imitating human sound localisation in different listening situations, including free field conditions and at the presence of sound occlusion, diffraction and reflection. Results are validated against subjective data found in the literature, and the model's applications to the fields of room acoustics and spatial audio are demonstrated and discussed.
