Perceptual detection thresholds for numerical dispersion in binaural auralizations of two acoustically different rooms.

Room acoustic simulations using the finite-difference time-domain method on a wide frequency range can be computationally expensive and typically contain numerical dispersion. Numerical dispersion can be audible and, thus, constitutes an artifact in auralizations. There is a need to measure perceptual thresholds for numerical dispersion to achieve an optimal balance between computational complexity and audibility of dispersion. This work measures the perceptual detection thresholds for numerical dispersion in binaural auralizations of two acoustically different rooms. Numerical dispersion is incorporated into measured binaural room impulse responses (BRIRs) by the means of filters that represent the dispersion that plane waves experience, which propagate in the simulation in the direction of the worst-case dispersion error. The results show that the perceptual detection threshold is generally lower for the most reverberant room and greatly depends on the source signal independently of the room in which the threshold is measured. It is the most noticeable in the pure BRIRs, i.e., with an impulse as source signal, and almost unnoticeable with speech. The results also show that there was no statistical evidence that the perceptual thresholds for the conditions where numerical dispersion was present or absent in the direct path of the BRIRs be different.

Spherical harmonic decomposition of a sound field based on observations along the equator of a rigid spherical scatterer.

We present a method for computing a spherical harmonic representation of a sound field based on observations of the sound pressure along the equator of a rigid spherical scatterer. Our proposed solution assumes that the captured sound field is height invariant so that it can be represented by a two-dimensional (2D) plane wave decomposition (PWD). The 2D PWD is embedded in a three-dimensional representation of the sound field, which allows for perfectly undoing the effect of the spherical scattering object. If the assumption of height invariance is fulfilled, then the proposed solution is at least as accurate as a conventional spherical microphone array of the same spherical harmonic order, which requires a multiple of the number of sensors. Our targeted application is binaural rendering of the captured sound field. We demonstrate by analyzing the binaural output signals that violations of the assumptions that the solution is based on-particularly height invariance and consequently also horizontal propagation-lead to errors of moderate magnitude.

Modeling continuous source distributions in wave-based virtual acoustics.

All acoustic sources are of finite spatial extent. In volumetric wave-based simulation approaches (including, e.g., the finite difference time domain method among many others), a direct approach is to represent such continuous source distributions in terms of a collection of point-like sources at grid locations. Such a representation requires interpolation over the grid and leads to common staircasing effects, particularly under rotation or translation of the distribution. In this article, a different representation is shown, based on a spherical harmonic representation of a given distribution. The source itself is decoupled from any particular arrangement of grid points, and is compactly represented as a series of filter responses used to drive a canonical set of source terms, each activating a given spherical harmonic directivity pattern. Such filter responses are derived for a variety of commonly encountered distributions. Simulation results are presented, illustrating various features of such a representation, including convergence, behaviour under rotation, the extension to the time varying case, and differences in computational cost relative to standard grid-based source representations.

Perceptual evaluation of headphone auralization of rooms captured with spherical microphone arrays with respect to spaciousness and timbre.

A listening experiment is presented in which subjects rated the perceived differences in terms of spaciousness and timbre between a headphone-based headtracked dummy head auralization of a sound source in different rooms and a headphone-based headtracked auralization of a spherical microphone array recording of the same scenario. The underlying auralizations were based on measured impulse responses to assure equal conditions. Rigid-sphere arrays with different amounts of microphones ranging from 50 to up to 1202 were emulated through sequential measurements, and spherical harmonics orders of up to 12 were tested. The results show that the array auralizations are partially indistinguishable from the direct dummy head auralization at a spherical harmonics order of 8 or higher if the virtual sound source is located at a lateral position. No significant reduction of the perceived differences with increasing order is observed for frontal virtual sound sources. In this case, small differences with respect to both spaciousness and timbre persist. The evaluation of lowpass-filtered stimuli shows that the perceived differences occur exclusively at higher frequencies and can therefore be attributed to spatial aliasing. The room had only a minor effect on the results.

Incorporating source directivity in wave-based virtual acoustics: Time-domain models and fitting to measured data.

The modeling of source directivity is a problem of longstanding interest in virtual acoustics and auralisation. This remains the case for newer time domain volumetric wave-based approaches to simulation such as the finite difference time domain method. In this article, a spatio-temporal model of acoustic wave propagation, including a source term is presented. The source is modeled as a spatial Dirac delta function under the action of a series of differential operators associated with the spherical harmonic functions. Each term in the series gives rise to the directivity pattern of a given spherical harmonic, and is separately driven through a time domain filtering operation of an underlying source signal. Such a model is suitable for calibration against measured frequency-dependent directivity patterns and a procedure for arriving at time domain filters for each spherical harmonic channel is illustrated. It also yields a convenient framework for discretisation, and a simple strategy is presented, yielding a locally-defined operation over the spatial grid. Numerical results, illustrating various features of source directivity, including the comparison of measured and synthetic directivity patterns, are presented.

Spatial perception of sound fields recorded by spherical microphone arrays with varying spatial resolution.

The area of sound field synthesis has significantly advanced in the past decade, facilitated by the development of high-quality sound-field capturing and re-synthesis systems. Spherical microphone arrays are among the most recently developed systems for sound field capturing, enabling processing and analysis of three-dimensional sound fields in the spherical harmonics domain. In spite of these developments, a clear relation between sound fields recorded by spherical microphone arrays and their perception with a re-synthesis system has not yet been established, although some relation to scalar measures of spatial perception was recently presented. This paper presents an experimental study of spatial sound perception with the use of a spherical microphone array for sound recording and headphone-based binaural sound synthesis. Sound field analysis and processing is performed in the spherical harmonics domain with the use of head-related transfer functions and simulated enclosed sound fields. The effect of several factors, such as spherical harmonics order, frequency bandwidth, and spatial sampling, are investigated by applying the repertory grid technique to the results of the experiment, forming a clearer relation between sound-field capture with a spherical microphone array and its perception using binaural synthesis regarding space, frequency, and additional artifacts. The experimental study clearly shows that a source will be perceived more spatially sharp and more externalized when represented by a binaural stimuli reconstructed with a higher spherical harmonics order. This effect is apparent from low spherical harmonics orders. Spatial aliasing, as a result of sound field capturing with a finite number of microphones, introduces unpleasant artifacts which increased with the degree of aliasing error.

Wave field synthesis of a sound field described by spherical harmonics expansion coefficients.

Near-field compensated higher order Ambisonics (NFC-HOA) and wave field synthesis (WFS) constitute the two best-known analytic sound field synthesis methods. While WFS is typically used for the synthesis of virtual sound scenes, NFC-HOA is typically employed in order to synthesize sound fields that have been captured with appropriate microphone arrays. Such recorded sound fields are essentially represented by the coefficients of the underlying surface spherical harmonics expansion. A sound field described by such coefficients cannot be straightforwardly synthesized in WFS. This is a consequence of the fact that, unlike in NFC-HOA, it is critical in WFS to carefully select those loudspeakers that contribute to the synthesis of a given sound source in a sound field under consideration. In order to enable such a secondary source selection, it is proposed to employ the well-known concept of decomposing the sound field under consideration into a continuum of plane waves, for which the secondary source selection is straightforward. The plane wave representation is projected onto the horizontal plane and a closed form expression of the secondary source driving signals for horizontal WFS systems of arbitrary convex shape is derived.

Wave field synthesis of moving virtual sound sources with complex radiation properties.

An approach to the synthesis of moving virtual sound sources with complex radiation properties in wave field synthesis is presented. The approach exploits the fact that any stationary sound source of finite spatial extent radiates spherical waves at sufficient distance. The angular dependency of the radiation properties of the source under consideration is reflected by the amplitude and phase distribution on the spherical wave fronts. The sound field emitted by a uniformly moving monopole source is derived and the far-field radiation properties of the complex virtual source under consideration are incorporated in order to derive a closed-form expression for the loudspeaker driving signal. The results are illustrated via numerical simulations of the synthesis of the sound field of a sample moving complex virtual source.

Knowledge and use of HIV pre-exposure prophylaxis among men who have sex with men in Berlin - A multicentre, cross-sectional survey.

BACKGROUND: HIV pre-exposure prophylaxis (PrEP) has likely contributed to large decreases in HIV incidence among men who have sex with men (MSM) in several major cities. Berlin has seen a smaller decline, and affordable PrEP has been accessible through formal channels in Germany only since autumn 2017. We aimed to investigate knowledge and use of PrEP among MSM in Berlin, and factors predictive of a desire to use PrEP and history of PrEP use. METHODS: Multicentre, paper-based, self-administered survey of adult MSM whose HIV status was negative or unknown at time of participation. Data were collected from 1 October 2017 to 2 April 2018. RESULTS: 473 of 875 questionnaires were returned (response rate 54.1%; mean age 37.4 years, range 18-79). 90.0% of participants were aware of PrEP and, of these, 48.2% felt well informed about it. Among the 17.2% of participants reporting PrEP use, 59.3% indicated obtaining some or all of it from informal sources. 23.7% of those with no history of PrEP use reported having condomless anal intercourse (CAI) with two or more partners over the past six months. Worries about side effects, cost, not having a doctor who prescribes it, and a lack of information were the most frequently reported barriers to PrEP use. A desire to use PrEP and history of PrEP use were associated in our multivariable model with having multiple CAI partners. A history of PrEP use was associated with having a university degree, one or two parents born outside Germany, or friends living with HIV. CONCLUSIONS: We found high awareness of PrEP among MSM in Berlin, but also a strong need for more education on its pros, cons and proper use. The frequency of informal PrEP use was also high, raising urgent individual and public health concerns. Policy makers need to consider recent calls to improve access to PrEP and PrEP education through regular health services.