Virtual Acoustics: Localization Model & Numeric Simulations

Localization of sound sources is an important task of the human auditory system and much research effort has been put into the development of audio devices for virtual acoustics, i.e. the reproduction of spatial sounds via headphones. Even though the process of sound localization is not completely understood yet, it is possible to simulate spatial sounds via headphones by using head-related transfer functions (HRTFs). HRTFs describe the filtering of the incoming sound due to head, torso and particularly the pinna and thus they strongly depend on the particular details in the listener`s geometry. In general, for realistic spatial-sound reproduction via headphones, the individual HRTFs must be measured. The currently available acoustic measurement is a technically-complex process, involves placing microphones into the listener`s ears, and lasts for tens of minutes. As a first step towards an easily accessible method for virtual acoustics, we visually scanned listeners` heads, obtained a geometrical representation of the heads (3-D mesh), and calculated HRTFs using numerical algorithms (part of the FWF-project P18401-B15). While our results showed that the numerical method is generally capable to calculate HRTFs, several questions remained open, especially in the interpretation from the perceptual perspective. The perceptual evaluation of the effect of the systematic parameter variation turned out to be a challenging task because of the large number of tests conditions and the lengthy psychoacoustic tests. It became clear that a sound- localization model predicting the performance is required to reduce the number of conditions for further behavioral evaluations in experiments. In LocaPhoto, we propose to develop such a sound-localization model and to continue our work on numerical HRTF calculations. Thus the first goal of LocaPhoto is to create a functional localization model which is able to predict localization performance for sound sources positioned in 3-D free field. The model will include recent neuro-physiological findings and is aimed as a tool for further research on modeling the mechanisms behind spatial hearing by the scientific community outside of LocaPhoto. In LocaPhoto, the model will be used to evaluate HRTFs from the perceptual perspective. This will allow to achieve the second goal of LocaPhoto: the development of a method to numerically calculate perceptually valid HRTFs based on the visually retrieved geometry of a listener. Based on the results from our previous project, we plan to simplify the geometry acquisition and to increase the mesh quality by generating 3-D meshes from 2-D photos with photogrammetric-reconstruction algorithms. Given the huge amount of parameters in the numerical calculations, we expect hundreds of calculated HRTF sets. The localization model will be used to select the promising HRTF candidates, all of which will be subjectively tested in localization experiments. Working in parallel on both goals will allow for synergy, substantially increasing the chances of success.

Localization of sound sources is an important task of the human auditory system. Much research effort has been put into the development of audio devices reproducing spatial sounds via headphones. Even though the process of sound localization is not completely understood yet, it is possible to simulate spatial sounds via headphones by using the so-called head-related transfer functions (HRTFs). HRTFs describe the filtering of the incoming sound due to head, torso and particularly the pinna and thus they strongly depend on the particular details in the listener's geometry. In general, for realistic spatial-sound reproduction via headphones, the individual HRTFs must be measured. Before the start of LocaPhoto, the available acoustic measurement was a technically-complex process, involves placing microphones into the listener's ears, and lasting for tens of minutes. Also, the evaluation of the measured HRTFs was possible only by means of extensive behavioral experiments.In the project LocaPhoto, we followed two goals: an improved method for the acquisition of HRTFs and a model-based evaluation of HRTFs. For the HRTF acquisition, we have developed a procedure which uses photographs of a listener. In our method, many photographs of the listeners head and ears are taken. In a process called photogrammetric reconstruction, these 2-dimensional representations of listeners geometry are used to calculate a 3-dimensional geometry, a mesh. Then, virtual microphones are placed at elements corresponding to the ear canal in the mesh and loudspeakers are positioned around the mesh. With such a mesh, numerical simulation of the sound is performed. As a result, HRTFs ready to be used to reproduce spatial sound sources via headphones are obtained.For the HRTF evaluation, we have developed a model of human auditory system. It simulates the human sound-localization behavior for sound sources placed in vertical planes. In the progress of our project, this model was used to evaluate the numerically calculated HRTFs and refine the calculation procedure. But this model is important for further research on modeling the mechanisms behind spatial hearing by the scientific community outside of our project. Our model is implemented in the free and open-source software package Auditory Modeling Toolbox available to others. Further, in our project, we have developed SOFA, a file format to store HRTFs. SOFA is dedicated to simplify the HRTF exchange between laboratories and to allow a more efficient HRTF implementation in devices offering spatial audio. SOFA has been accepted as a standard of the Audio Engineering Society. All these efforts consolidated the relevance of HRTFs in the audio-engineering community.