Psychoacoustic Music Sound Field Synthesis

This introduction chapter gives an overview about the book at hand, which deals with the perception of spaciousness in music, culminating in the development of a novel psychoacoustic sound field synthesis approach for a natural, spatial music listening experience. After some general remarks on the motivation and the target audience of this book the introduction guides the reader through the topics that are treated in the dedicated chapters on Spatial Concepts of Music, the Biology of the Auditory System, Psychoacoustics, the Acoustics of Musical Instruments, Room Acoustics, Conventional Stereophonic Sound, Wave Field Synthesis, and, finally, Psychoacoustic Sound Field Synthesis.

Music is spatial in many ways. Musical concepts and music perception are described by spatial terms in many cultures. This spatial thinking is reflected in music from spatial compositions to stereophonic recording and mixing techniques. Consequently, traditional music theories as well as modern music information retrieval approaches leverage spatial concepts and operations to gain a deeper understanding of music. This chapter reviews concepts of spaciousness in music psychology, provides the state of the art in spatial music composition and mixing in the recording studio, and gives an overview about spaciousness in music theory and music information retrieval. The prominence of spatial concepts in all these theoretic and practical disciplines underlines the significance of space in music. This deep relationship becomes obvious in terms of music as creative arts, an acoustical signal, and a psychological phenomenon.

The auditory system detects pressure fluctuations that propagate as waves. It can be considered as a successor of the lateral line system, which enables fish and some amphibians to detect particle accelerations. These indicate the location and swimming direction of near objects. The auditory system in fish extends the detection range of the lateral line system. Accordingly, its original function can be considered to be spatial orientation and mental representation of the acoustic surrounding, rather than communication. Starting with the lateral line system of fish and their auditory system, the human ear and auditory pathway are described. It appears that spatial information is encoded at the earliest stages of auditory processing in the brain. Spatial attributes of sound are Sound sources are localized long before they are recognized or consciously perceived.

Auditory perception is the mental representation of the acoustical outside world. Psychoacoustics describes characteristics of auditory perception and relates them to the physics of the sound field. This chapter discusses psychoacoustical foundations that play a role in spatial audio. For psychoacoustic sound field synthesis these are critical bands and masking, source localization and principles of auditory scene analysis. Their resolution, limitations, thresholds and just noticeable differences can be leveraged to present the necessary information for a desired listening experience with the precision needed for a natural, spatial listening experience.

Musical instruments create a spatial sound impression. This chapter provides an introduction to the acoustics of sound propagation from musical instruments. An overview of microphone array techniques to measure the sound radiation characteristics from the near and the far field is given. The complex point source model simplifies the physical constellation and describes what is heard by the listener. It serves as a simplification for psychoacoustic sound field synthesis for music presented in this book. Finally, the chapter illustrates strategies to visualize measured sound radiation properties.

Appropriate room acoustics are a necessity for music performance. Good room acoustics support the direct sound of musical instruments and ease ensemble playing. Minimal geometric and architectural requirements are well-established. Music experts who are familiar with multiple concert halls evaluated their acoustical characteristics from the viewpoint of performers and the audience. This inter-subjective impression can be explained to some degree by acoustic parameters derived from room impulse responses. It could be shown that the best-rated concert halls are exhibit the highest degree of spaciousness. Spatial impressions, like listener envelopment and apparent source width correlate significantly with objective parameters, like the binaural quality index and the lateral energy fraction. However, many causal relationships are still to be found.

Conventional spatial audio systems allow audio engineers and music producers to control the signal of single loudspeakers. Many stereo recording and mixing techniques established. These create loudspeaker signals that evoke a desired spatial impression for the listener to some degree. Impressions may include the perceived location and spatial extent of a sound source, or the perceived reverberance and envelopment of the room. This chapter describes the principles of established stereophonic audio systems and evaluates to what extent they fulfill the requirements for a natural, spatial music listening experience. It can be seen that technological advances in audio systems are especially spatial ones. From mono over stereo, quadraphonic sound and Dolby surround, 5.1 surround to immersive audio and binaural systems the control over source locations and the degree of apparent source width and listener envelopment increase. As a logical consequence, new spatial audio systems should keep improving aspects of perceived spaciousness.

Sound field synthesis methods, like wave field synthesis and ambisonics, aim at synthesizing a desired sound field within a listening area. The idea is that control over the sound field implies control over the spatial sound impressions of listeners. Impressions may include the perceived location and spatial extent of a sound source, or the perceived reverberance and envelopment of the room. This chapter gives a brief historic overview over sound field synthesis. Then, the sound field synthesis theory is derived from the example of wave field synthesis. Technical implementations require deviations from the theoretical core, which cause synthesis errors. Typical errors and solutions are discussed against the background of spatial sound impression. Finally, an overview of existing sound field synthesis systems in research and in the entertainment sector is given.

Sound field synthesis approaches aim at creating a desired sound field. Conventionally, these approaches are physically-motivated. Acoustical properties define the desired sound field. A perceptual evaluations of the listening experience follows the technical implementation. Psychoacoustic sound field synthesis, as introduced in this chapter, has another paradigm. Here, psychoacoustical properties determine the desired sound field. This allows for implementation of auditory  thresholds and integration times in the derivation of the sound field synthesis core. This allows for inaudible reduction of temporal, spatial, and spectral resolution. The result is a natural, spatial listening experience and a precise source localization for multiple listeners with comparably low computational efforts and inaudible synthesis errors.