Dynamic Spatial Responsiveness in Concert Halls
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acoustics Article Dynamic Spatial Responsiveness in Concert Halls Evan Green * and Eckhard Kahle Kahle Acoustics, 1050 Brussels, Belgium * Correspondence: [email protected]; Tel.: +32-2-345-10-10 Received: 30 May 2019; Accepted: 2 July 2019; Published: 22 July 2019 Abstract: In musical perception, a proportion of the reflected sound energy arriving at the ear is not consciously perceived. Investigations by Wettschurek in the 1970s showed the detectability to be dependent on the overall loudness and direction of arrival of reflected sound. The relationship Wettschurek found between reflection detectability, listening level, and direction of arrival correlates well with the subjective progression of spatial response during a musical crescendo: from frontal at pianissimo, through increasing apparent source width, to a fully present room acoustic at forte. “Dynamic spatial responsiveness” was mentioned in some of the earliest psychoacoustics research and recent work indicates that it is a key factor in acoustical preference. This article describes measurements of perception thresholds made using a binaural virtual acoustics system—these show good agreement with Wettschurek’s results. The perception measurements indicate that the subjective effect of reflections varies with overall listening level, even when the reflection level, delay, and direction relative to the direct sound are maintained. Reflections which are perceptually fused with the source may at louder overall listening levels become allocated to the room presence. An algorithm has been developed to visualize dynamic spatial responsiveness—i.e., which aspects of a three-dimensional (3D) Room Impulse Response would be detectable at different dynamic levels—and has been applied to measured concert hall impulse responses. Keywords: auditorium acoustics; dynamics; spaciousness; spatial response; auditory perception 1. Introduction 1.1. (Dynamic) Spatial Responsiveness Musicians and conductors consider the concert hall part of their instrument—it is a tool for musical expression, with the best concert halls enhancing the range of expression. The deliberate variation of intensity—in other words, changes in musical dynamics—is a key ingredient in most musical composition and performance. Without both strong and subtle dynamic changes, much musical and emotional intensity is lost. Playing a musical instrument with more intensity generates not only an increase in objective sound level, but also changes in the frequency spectrum of the sound. The connection between musical dynamics and spatial impression was observed in some of the earliest research into psychoacoustics. Marshall [1] writes in 1966 that “as a property of the hall, [spatial impression] relates to loudness attributes ::: for the listener, it generates a sense of envelopment in the sound and of direct involvement with it”, with similar observations being made by Keet [2] and Barron [3]. Writing in 1978, Kuhl [4] states that (translated from German) “[spatial impression] is only, if at all, present in forte passages. Musical dynamics therefore not only influence the loudness, but with a sufficient spatial responsiveness, they increase the involvement of the room in the music. For the lay-listener, who is perhaps not even aware of this effect, this spaciousness is an unconscious yet pleasant experience.” In certain conditions therefore, increases in dynamics can generate an additional dimension of spatial impression for the listener—changes in the spatial impression due to changes in dynamics will be referred to here as “dynamic spatial responsiveness”. 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Research by Wettschurek 1.2. Research by Wettschurek In Wettschurek’s thesis [15], he describes an experiment to determine the perception threshold In Wettschurek’s thesis [15], he describes an experiment to determine the perception threshold of of a test reflection dependent on overall listening levels. The experiment is shown diagrammatically a test reflection dependent on overall listening levels. The experiment is shown diagrammatically in in Figure 1. Figure1. FigureFigure 1. 1. DiagrammaticDiagrammatic representation representation of of an an experiment experiment to to determine determine the the perception perception threshold threshold of ofa reflectiona reflection E Ein inthe the presence presence of of direct direct sound sound D, D, a alateral lateral reflection reflection R, R, and and reverberation reverberation H. H. After After WettschurekWettschurek [15]. [15]. 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