Acoustic quality, proximity, and localization in concert halls: The role of harmonic phase alignment.

Abstract

Human perception of source distance and direction is predominantly visual. At a music performance, with eyes open, we are usually sure we are hearing the precise location of each musician. But this impression can change dramatically when the eyes are closed. When working on hall acoustics, we find it very useful to walk slowly away from a small group of musicians or actors while looking at the floor. In practice, we often use a small version of the virtual orchestra (Patynen & Lokki, 2011), which plays tirelessly on stage with no visual reference for who is playing which musical line. Close to the group, each performer or musical line can be localized precisely by ear, and the lines, played or spoken, can be easily distinguished separately from the others. As we walk back, very little changes at first. The spread of azimuth diminishes, but the ability to localize and follow the lines is unchanged. The performers are still present.At a particular distance, everything changes. The sound of the ensemble collapses into a fuzzy ball, and the sense of closeness of the performers is lost. We will refer to the distance from the sound sources found in this way as the limit of localization distance, or LLD. The difference in the distance between a position in the hall where the sound is sharp and clear to a position where the sound is fuzzy can be less than a meter or two. One can learn to hear the difference in sound by walking back and forth around the LLD-a new experience for most people. Immediately switching between binaural recordings in front of and behind the LLD can also be startlingly different, even when played through loudspeakers. There is a high degree of agreement between individuals for the distance from the stage of the LLD. The property that creates it appears to be a property of the sound field, not of the individual.It is the sound, the component of a sound field that travels directly from a source to a listener, that contains the information needed to localize the source and to perceive its closeness. We hypothesize that the peaks in the pressure envelope created by the alignment of harmonic phases in the direct sound are essential for the sense of closeness and also enable the ability to separate sounds. We infer from the experience described above that the ability of the ear to separate direct sound from reflections works well down to a certain value of the direct to reverberant ratio, D/R, in a particular venue. Below this value, localization and separation of individual sources become difficult, and the sense of closeness disappears.We are learning that the sense of closeness has a major role for in concert venues. Patynen and Lokki (2011) have developed a laboratory system that can instantly A/B compare different seats and different halls without a moving visual image. They have collected and tested enough hall data to discover a hierarchy of sonic factors that influence preference. Surprisingly, a factor they identified as of great importance to all groups of listeners has been previously unknown. They named the factor proximity, or the impression the listener is sonically close to the performer. To quote from Lokki, Patynen, Kuusinen, and Tervo (2012): An interesting fact is that neither Definition and Reverberance nor early decay time (EDT) and C80 explain the at all. In contrast, the is best explained with subjective Proximity and with Bassiness, Envelopment, and Loudness to some extent. Further there is no objective measure that correlates to Proximity and overall average of preference (p. 3159). In our view, this is a powerful and damning statement. Why do commonly thought-of acoustic qualities such as definition and reverberance, as well as the measures EDT and C80, have no consistent effect on preference, and why has one of the most important determinants of been both previously unknown and unmeasurable?We believe that understanding the acoustics of music venues has been stymied by two problems. …