Room Acoustic Modeling Research Articles

Spatial Encoding of Finite Difference Time Domain Acoustic Models for Auralization

A single room impulse response can reveal information about the acoustics of a given space in both objective, and, when used for auralization, subjective terms. However, for additional spatial information, or more accurate and perceptually convincing auralization, multiple impulse responses are needed. Higher order Ambisonics is a robust means of capturing the spatial qualities of an acoustic space over multiple channels for decoding and rendering over many possible speaker layouts. A method for obtaining $N$ th-order Ambisonic impulse responses from a room acoustic model, based on lower orders using differential microphone techniques is presented. This is tested using a third-order encoding of a 2-D finite difference time domain room acoustic simulation based on multiple circular arrays of receivers. Accurate channel directional profiles are obtained and results are verified in a series of listening tests comparing the localization of a sound source placed within the given simulation to the same source encoded directly. This generic encoding scheme can be applied to any room acoustic simulation technique where it is possible to obtain impulse responses across multiple receiver positions. Although the proposed method encompasses horizontal encoding only, it can also be applied directly in 3-D simulations where height information is not required in the final auralization.

Modeling incoherent reflections from rough room surfaces with image sources

Reflections at rough surfaces change the temporal structure of the reflected signal. This paper shows how to incorporate this temporal behavior in geometric room acoustics modeling. Specifically, a beam tracer is used for calculating the image sources and reflection paths. The roughness of the surfaces is taken into account in post-processing. A single reflection is assumed to distribute the energy according to an exponential function in time based on Biot's rough surface modeling theory. Multiple reflections are modeled with convolutions of exponential functions which are approximated as gamma functions.

Bayesian model-based filter design in acoustical signal processing applications

Digital filter design finds applications across many disciplines. Acoustic applications include, but are not limited to, impedance boundary conditions in time-domain wave-based room-acoustics modeling, head-related transfer functions, and loudspeaker equalization. The process of filter design can be approached in a variety of ways. To design a non-standard filter, optimization methods may be used to solve for the coefficients that minimize the difference from a specified transfer function of a prescribed order. Interpreted as a model-based inference problem, a Bayesian framework, realized through the nested sampling algorithm, provides simultaneous optimal coefficient estimates and a filter order selection criterion. The selection criterion implicitly favors simpler models over more complex models, in this case lower-order filters over higher-order filters. Many acoustical applications are well suited to this type design, where low filter order is important, and the desired system response is unattainable...

Room Acoustic Modelling Techniques: A Comparison of a Scale Model and a Computer Model for a New Opera Theatre

Today most acoustic consultants are using room acoustic computer models as a basis for their acoustic design. However, room acoustic scale modelling is still being used for the design in some major projects, although the costs and the time needed are significantly larger than those related to computer modelling. Both techniques were used by the author in a project for a new opera theatre; first the acoustical design was based on computer simulations using the Odeon software, and next a 1:20 scale model was built and tested. In the paper the results obtained with the two different modelling techniques are compared, and in general a satisfactory agreement has been found. The advantages and drawbacks related to each of the modelling techniques are discussed.

Efficient acoustic radiance transfer method with time‐dependent reflections.

Modern desktop computers are equipped with graphics cards that provide massive parallel computation power that was previously available only in supercomputers. On the other hand, there are several room acoustics modeling methods, but only some of them scale well to hundreds or thousands of parallel processors. The scalability of the acoustic radiance transfer method is examined. It is shown that it can almost fully utilize the available computing power. In simple cases, this technique achieves real‐time performance. While taking into account the limitations of the energy‐based acoustic modeling approach, the presented system can model arbitrary reflections. The reflections are presented as bi‐directional reflectance distribution functions, which depend on the incoming and outgoing directions of acoustic energy. It is also possible to add time‐dimension to such a reflection model. Some measurements are presented to show that spreading in time dimension occurs at reflections. Most of the previous room acoustics modeling techniques have ignored that phenomenon, but the acoustic radiance transfer technique can be easily modified to take this spreading effect into account.

A Theoretical Approach to Room Acoustic Simulations Based on a Radiative Transfer Model

A theoretical approach to room acoustic simulations based on a radiative transfer model is developed by adapting the classical radiative transfer theory from optics to acoustics. The proposed acoustic radiative transfer model expands classical geometrical room acoustic modeling algorithms by incorporating a propagation medium that absorbs and scatters radiation, handling both diffuse and non-diffuse reflections on boundaries and objects in the room. The main scope of this model is to provide a proper foundation for a wide number of room acoustic simulation models, in order to establish and unify their principles. It is shown that this room acoustic modeling technique establishes the basis of two recently proposed algorithms, the acoustic diffusion equation and the room acoustic rendering equation. Both methods are derived in detail using an analytical approximation and a simplified integral equation of the proposed method, respectively, allowing a clear definition of the underlying assumptions, limitations, advantages and disadvantages.

Parameter‐based mathematical optimization using MATLAB and CATT‐A software.

Mathematical optimization techniques have been applied to room acoustic modeling within CATT‐A. Stochastic optimization methods such as genetic algorithms, pattern search, and ant colony optimization are used via software integration between MATLAB and CATT‐A, allowing for acoustic parameter specific optimization of variable room geometry. The optimization routines rely on results from geometrical ray tracing, which informs the designer on aspects of room acoustics that empirical formulas cannot take into account. An example is presented showing an algorithm that optimizes placement of sound absorbing treatment to obtain a fixed speech transmission index using minimal sound absorbing material in a classroom setting.

Effect of diffusive and nondiffusive surfaces combinations on sound diffusion

One of room acoustic goals, especially in small to medium rooms, is sound diffusion in low frequencies, which have been the subject of lots of researches. Sound diffusion is a very important consideration in acoustics because it minimizes the coherent reflections that cause problems. It also tends to make an enclosed space sound larger than it is. Diffusion is an excellent alternative or complement to sound absorption in acoustic treatment because it doesn’t really remove much energy, which means it can be used to effectively reduce reflections while still leaving an ambient or live sounding space. Distribution of diffusive and nondiffusive surfaces on room walls affect sound diffusion in room, but the amount, combination, and location of these surfaces are still the matter of question. This paper investigates effects of these issues on room acoustic frequency response in different parts of the room with different source-receiver locations. Room acoustic model based on wave method is used (implemented) which is very accurate and convenient for low frequencies in such rooms. Different distributions of acoustic surfaces on room walls have been introduced to the model and room frequency response results are calculated. For the purpose of comparison, some measurements results are presented. Finally for more smooth frequency response in small and medium rooms, some suggestions are made.

A hybrid algorithm for selecting head-related transfer function based on similarity of anthropometric structures

Abstract As the basic data for virtual auditory technology, head-related transfer function (HRTF) has many applications in the areas of room acoustic modeling, spatial hearing and multimedia. How to individualize HRTF fast and effectively has become an opening problem at present. Based on the similarity and relativity of anthropometric structures, a hybrid HRTF customization algorithm, which has combined the method of principal component analysis (PCA), multiple linear regression (MLR) and database matching (DM), has been presented in this paper. The HRTFs selected by both the best match and the worst match have been applied into obtaining binaurally auralized sounds, which are then used for subjective listening experiments and the results are compared. For the area in the horizontal plane, the localization results have shown that the selection of HRTFs can enhance the localization accuracy and can also abate the problem of front–back confusion.

Absorption coefficient error propagation into room acoustics parameters.

It is well known that random‐incidence absorption coefficients suffer from measurement uncertainties. Whether or not these uncertainties affect the quality of results from room acoustic prediction models depends on the specific case. One might argue that the absorption of the most relevant material must be known accurately. This, however, could be the material with highest absorption or the material with the largest surface area. In this presentation, the true random‐incidence absorption coefficient and measurement uncertainties related to the current ISO 354 are briefly discussed. Then, in order to get a better view on the consequences of those uncertainties, quantitative uncertainties of some predicted parameters such as reverberation time, strength, and clarity are calculated by using an error propagation. Effects of uncertainties of absorption coefficients distributed in various room configurations are presented and discussed.

Simulation of Directional Microphones in Digital Waveguide Mesh-Based Models of Room Acoustics

Digital waveguide mesh (DWM) models are time-domain numerical methods providing computationally simple solutions for wave propagation problems. They have been used in various acoustical modeling and audio synthesis applications including synthesis of musical instrument sounds and speech, and modeling of room acoustics. A successful model of room acoustics should be able to account for source and receiver directivity. Methods for the simulation of directional sources in DWM models were previously proposed. This paper presents a method for the simulation of directional microphones in DWM-based models of room acoustics. The method is based on the directional weighting of the microphone response according to the instantaneous direction of incidence at a given point. The direction of incidence is obtained from instantaneous intensity that is calculated from local pressure values in the DWM model. The calculation of instantaneous intensity in DWM meshes and the directional accuracies of different mesh topologies are discussed. An intensity-based formulation for the response of a directional microphone is given. Simulation results for an actual microphone with frequency-dependent, non-ideal directivity function are presented.

Comparison of Simple Room Acoustic Models Used for Industrial Spaces

In order to select the most cost-effective noise control solution, the influence of the noise control measures on noise level should be reliably predicted. Reasonably accurate commercial ray-tracing, image source and finite element method modeling programs are available. However, the creation of a model and sometimes the calculations are time-consuming and their use in preliminary room acoustic design is not always cost-effective. These sophisticated room acoustic modeling programs demand special expertise which limits their use in practice. Several simple room acoustic models have been developed for noise control but information on their usability and prediction accuracy is rather hard to find. The purpose of this study was to examine simple room acoustic models developed for predicting noise levels and reverberation times in octave bands. Seven simple sound pressure level models, seven simple reverberation time models and a validated ray-tracing program (ODEON 3.1) were tested in four industrial workplaces, in which major noise control measures were implemented. The prediction results were compared to measurement results which were performed using an omni-directional sound source before and after the implementation of noise control measures. Accuracies of predicted sound pressure levels were determined as differences between measured and predicted sound pressure levels in octave bands of 125―4000 Hz. Accuracies of predicted reverberation times were similarly determined. The accuracy of the ray-tracing model was the most acceptable, as expected. The accuracy of simple sound pressure level models developed by Kuttruff, Osipov et al., and Thompson et al. was comparable to the accuracy of the ray-tracing models. The accuracy of the simple reverberation time model developed by Heerema and Hodgson was almost comparable to the accuracy of the ray-tracing model. The above-mentioned simple sound pressure level models provided sufficient accuracy for predicting average insertion loss, e.g. by using acoustic tiles or sound-absorbing materials. For more detailed or complex room acoustic design, sophisticated room acoustic modeling programs are recommended.

Comparison of auralized and measured sound in moderately reverberant spaces.

This research performs an in‐depth comparison of sound from computer auralizations with measured binaural room impulse responses and recordings taken in actual spaces. Binaural measurements and recordings have been gathered in two church naves with midfrequency reverberation times of about 1.5 and 2.5 s. The results from these measurements are compared with auralizations from computer models of the spaces in two different room acoustic modeling programs. The source and receiver positions used for the auralizations are the same as those measured in the real spaces. One of the naves was previously investigated [R.C. Coffeen and G. Caunt, J. Acoust. Soc. Am. 105, 1174 (1999) (A)] with results from computer auralizations compared to measured binaural recordings in the space. As numerous advancements have been made to computer modeling programs since that time, this project revisits and expands the previous research using updated modeling and measurement techniques. The results of this research not only portray the developments in computer modeling that have occurred over the past decade, but also depict the relationship between current computer auralizations and actual sound in moderately reverberant spaces.

An Empirical Diffusion Model for Acoustic Prediction in Rooms with Mixed Diffuse and Specular Reflections

In this paper, a modification to the room-acoustic diffusion model is proposed to take different amounts of wall scattering into account. An extensive set of numerical simulations using a cone-tracing software has first been carried out, in order to highlight the impact of the scattering coefficient on the diffusion process in rooms, in terms of sound pressure levels. An iterative method is then proposed to identify, for a given value of the wall's scattering coefficient, the diffusion constant that allows the stationary sound field to be governed by a diffusion process, regardless of the room's geometry. Using this method, an empirical law can be proposed between the diffusion constant and the scattering coefficient. The empirical diffusion model is then compared to scale model experiments, as well as to other models from the literature, with a satisfactory agreement for the sound pressure level. However, the empirical diffusion model fails to predict the sound decay for rooms with perfectly specularly reflecting surfaces, due to the inherent concept of a diffusion process.

Framework for Real-Time Auralization in Architectural Acoustics

Auralization is the process of making audible the acoustics of complex virtual architectural spaces in a realistic and accurate manner. This paper presents a novel real-time auralization software environment comprising a room acoustic modeler, a spatial renderer for auralization, and a visualization and scene graph unit for interactivity. The computation of early reflection paths within the geometric model is based on an efficient beam tracing algorithm capable of real-time detection of specular reflection paths in a static geometry with one or several moving listener(s). For “simple" rooms, the real-time performance is maintained even with dynamic geometries and sources. Results of the room acoustic model consisting of visible reflection paths and their accumulated material attenuation are sent to the audio renderer. From this geometrical and acoustical data, listener position-relative 3D room impulse responses are generated using a hierarchical-order Ambisonics approach. Finally, the spatialized audio output is presented to the listener via multiple loudspeakers or binaurally rendered over headphones. As higher order reflections are more diffuse in nature, they are encoded using lower Ambisonic orders, thereby reducing computational load. The environment combines high quality audio with visual rendering realized using the open source platforms Pure Data and VirChor respectively. This open source auralization framework provides direct audio-visual interaction in real-time and is suitable for VR environments.

Measurement of diffracted pressure fields using an automated measurement system.

The theory of sound diffraction has been studied widely. Different modeling methods for sound diffraction have been proposed. Some of these modeling techniques are also used in the room acoustic modeling. In this study, diffracted pressure fields from a thin panel were measured. In the measurements, a loud-speaker was used as a sound source and diffraction from a thin panel was measured. The measurements were performed in small semianechoic chamber using a three-dimensional 3-D automated position system. This system consists of a 3-D position system, a data acquisition hardware, and a control unit. With this system, it is possible to measure diffracted pressure fields with high-spatial accuracy. In addition, numerical simulations are used in the modeling of sound diffraction from a thin edge. Finally, the simulated pressure fields are compared to the measured fields.

Early energy decays in two churches in Hong Kong

Acoustical measurements based on impulse responses have been made in a Protestant and a Catholic church in Hong Kong. Analysis has concentrated on reverberation time, early decay time and C 80, the early-to-late ratio, as well as integrated decays. The measured results have been compared with predictions according to two different theoretical room acoustic models: a model for a single diffuse space and one for a set of coupled-rooms. The comparison has shown that treating the churches as a series of coupled spaces is more valid. However the clarity, C 80, at different receiver positions was found to be either under- or over-estimated by the classical coupled-room model. Following this observation, a delayed coupled-room model was developed in the hope of achieving more accurate predictions. In this model, the initial condition under the impulse excitation was modified by introducing a time delay for sound propagated from the source to achieve diffuse sound fields in each subspace. The predicted results of the coupled-room model agree well with the measured results.

Spatial rendering of audiovisual synthetic speech use for immersive environments

Synthetic speech is usually delivered as a mono audio signal. In this project, audiovisual speech synthesis is attributed to a virtual agent moving in a virtual three-dimensional scene. More realistic acoustic rendering is achieved by taking into account the position of the agent in the scene, the acoustics of the room depicted in the scene, and the orientation of the virtual character's head relative. 3D phoneme dependant radiation patterns have been measured for two speakers and a singer. These data are integrated into a Text-To-Speech system using a phoneme to directivity pattern transcription module which also includes a phoneme to viseme model for the agent. In addition to the effects related to agent's head orientation for the direct sound, a room acoustics model allows for realistic rendering of the room effect as well as the apparent distance as depicted in the virtual scene. Real-time synthesis is implemented in a 3D audio rendering system.

Diffraction modeling in acoustic radiance transfer method

The room acoustic radiance transfer method is a solution to recently presented room acoustic rendering equation which formulates the mathematical basis for all the ray‐based (geometrical) room acoustic modeling algorithms. The basic acoustic transfer method gives as accurate results as the state‐of‐the‐art commercial room acoustic modeling software. However, the basic method still lacks, e.g., diffraction modeling and modeling of complex reflections from surfaces. In this paper we discuss different diffraction modeling methods in the light of the acoustic radiance transfer method. The problems as well as benefits of each diffraction modeling method are summarized to understand which one of them can be implemented together with acoustic radiance transfer. Finally, some implementation examples are given.

A new symphonic hall, Musikhuset Aarhus, Denmark

During the past 3 years Musikhuset Aarhus in Denmark has been enlarged by an area of approximately 17.000 m2 to a new total area of approximately 34.000 m2. The new part of the complex contains a symphonic concert hall for Aarhus Symphonic Orchestra with 1200 seats, a hall for rythmic contemporary music with 600 seats and a hall for classical music with 160 seats. The complex also houses the new premises for Aarhus Academy of Music. Important design considerations will be highlighted in the paper and the results of the preliminary and final computer modeling of room acoustics will be presented using examples of auralisation. In conclusion the room acoustic parameters (measured according to ISO 3382 standard) will be discussed, also in respect of the initial reaction from ochestral musicians.