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About this book

This open access book provides a concise explanation of the fundamentals and background of the surround sound recording and playback technology Ambisonics. It equips readers with the psychoacoustical, signal processing, acoustical, and mathematical knowledge needed to understand the inner workings of modern processing utilities, special equipment for recording, manipulation, and reproduction in the higher-order Ambisonic format. The book comes with various practical examples based on free software tools and open scientific data for reproducible research.

The book’s introductory section offers a perspective on Ambisonics spanning from the origins of coincident recordings in the 1930s to the Ambisonic concepts of the 1970s, as well as classical ways of applying Ambisonics in first-order coincident sound scene recording and reproduction that have been practiced since the 1980s. As, from time to time, the underlying mathematics become quite involved, but should be comprehensive without sacrificing readability, the book includes an extensive mathematical appendix. The book offers readers a deeper understanding of Ambisonic technologies, and will especially benefit scientists, audio-system and audio-recording engineers.

In the advanced sections of the book, fundamentals and modern techniques as higher-order Ambisonic decoding, 3D audio effects, and higher-order recording are explained. Those techniques are shown to be suitable to supply audience areas ranging from studio-sized to hundreds of listeners, or headphone-based playback, regardless whether it is live, interactive, or studio-produced 3D audio material.

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Table of Contents

Frontmatter

Open Access

Chapter 1. XY, MS, and First-Order Ambisonics

Abstract
This chapter describes first-order Ambisonic technologies starting from classical coincident audio recording and playback principles from the 1930s until the invention of first-order Ambisonics in the 1970s. Coincident recording is based on arrangements of directional microphones at the smallest-possible spacings in between. Hereby incident sound approximately arrives with equal delay at all microphones. Intensity-based coincident stereophonic recording such as XY and MS typically yields stable directional playback on a stereophonic loudspeaker pair. While the stereo width is adjustable by MS processing, the directional mapping of first-order Ambisonics is a bit more rigid: the omnidirectional and figure-of-eight recording pickup patterns are reproduced unaltered by equivalent patterns in playback. In perfect appreciation of the benefits of coincident first-order Ambisonic recording technologies in VR and field recording, the chapter gives practical examples for encoding, headphone- and loudspeaker-based decoding. It concludes with a desire for a higher-order Ambisonics format to get a larger sweet area and accommodate first-order resolution-enhancement algorithms, the embedding of alternative, channel-based recordings, etc.
Franz Zotter, Matthias Frank

Open Access

Chapter 2. Auditory Events of Multi-loudspeaker Playback

Abstract
This chapter describes the perceptual properties of auditory events, the sound images that we localize in terms of direction and width, when distributing a signal with different amplitudes to one or a couple of loudspeakers. These amplitude differences are what methods for amplitude panning implement, and they are also what mapping of any coincident-microphone recording implies when reproduced over the directions of a loudspeaker layout. Therefore several listening experiments on localization are described and analyzed that are essential to understand and model the psychoacoustical properties of amplitude panning on multiple loudspeakers of a 3D audio system. For delay-based recordings or diffuse sounds, there is some relation, however, it is found to be less stable for the desired applications. Moreover, amplitude panning is not only about consistent directional localization. Loudness, spectrum, temporal structure, or the perceived width should be panning-invariant. The chapter also shows experiments and models required to understand and provide those panning-invariant aspects, especially for moving sounds. It concludes with openly-available response data of most of the presented listening experiments.
Franz Zotter, Matthias Frank

Open Access

Chapter 3. Amplitude Panning Using Vector Bases

Abstract
This chapter describes Ville Pulkki’s famous vector-base amplitude panning (VBAP) as the most robust and generic algorithm of amplitude panning that works on nearly any surrounding loudspeaker layout. VBAP activates the smallest-possible number of loudspeakers, which gives a directionally robust auditory event localization for virtual sound sources, but it can also cause fluctuations in width and coloration for moving sources. Multiple-direction amplitude panning (MDAP) proposed by Pulkki is a modification that increases the number of activated loudspeakers. In this way, more direction-independence is achieved at the cost of an increased perceived source width and reduced localization accuracy at off-center positions. As vector-base panning methods rely on convex hull triangulation, irregular loudspeaker layouts yielding degenerate vector bases can become a problem. Imaginary loudspeaker insertion and downmix is shown as robust method improving the behavior, in particular for smaller surround-with-height loudspeaker layouts. The chapter concludes with some practical examples using free software tools that accomplish amplitude panning on vector bases.
Franz Zotter, Matthias Frank

Open Access

Chapter 4. Ambisonic Amplitude Panning and Decoding in Higher Orders

Abstract
Already in the 1970s, the idea of using continuous harmonic functions of scalable resolution was described by Cooper and then Gerzon, who introduced the name Ambisonics. This chapter starts by reviewing properties of first-order horizontal Ambisonics, using an interpretation in terms of panning functions. And the required mathematical formulations for 3D higher-order Ambisonics are developed here, with the idea to improve the directional resolution. Based on this formalism, ideal loudspeaker layouts can be defined for constant loudness, localization, and width, according to the previous models. The chapter discusses how Ambisonics can be decoded to less ideal, typical loudspeaker setups for studios, concerts, sound-reinforcement systems, and to headphones. The behavior is analyzed by a rich variety of listening experiments and for various decoding applications. The chapter concludes with example applications using free software tools.
Franz Zotter, Matthias Frank

Open Access

Chapter 5. Signal Flow and Effects in Ambisonic Productions

Abstract
This chapter presents the internal working principles of various Ambisonic 3D audio effects. No matter which digital audio workstation or processing software is used in a production, the general Ambisonic signal infrastructure is outlined as an important overview of the signal processing chain. The effects presented are frequency-independent effects such as directional re-mapping (mirror, rotation, warping) and re-weighting (directional level modification), and frequency-dependent effects such as widening/distance/diffuseness, diffuse reverberation, and resolution-enhanced convolution reverberation.
Franz Zotter, Matthias Frank

Open Access

Chapter 6. Higher-Order Ambisonic Microphones and the Wave Equation (Linear, Lossless)

Abstract
Unlike pressure-gradient transducers, single-transducer microphones with higher-order directivity apparently turned out to be difficult to manufacture at reasonable audio quality. Therefore nowadays, higher-order Ambisonic recording with compact devices is based on compact spherical arrays of pressure transducers. To prepare for higher-order Ambisonic recording based on arrays, we first need a model of the sound pressure that the individual transducers of such an array would receive in an arbitrary surrounding sound field. The lossless, linear wave equation is the most suitable model to describe how sound propagates when the sound field is composed of surrounding sound sources. Fundamentally, the wave equation models sound propagation by how small packages of air react (i) when being expanded or compressed by a change of the internal pressure, and to (ii) directional differences in the outside pressure by starting to move. Based there upon, the inhomogeneous solutions of the wave equation describe how an entire free sound field builds up if being excited by an omnidirectional sound source, as a simplified model of an arbitrary physical source, such as a loudspeaker, human talker, or musical instrument. After adressing these basics, the chapter shows a way to get Ambisonic signals of high spatial and timbral quality from the array signals, considering the necessary diffuse-field equalization, side-lobe suppression, and trade off between spatial resolution and low-frequeny noise boost. The chapter concludes with application examples.
Franz Zotter, Matthias Frank

Open Access

Chapter 7. Compact Spherical Loudspeaker Arrays

Abstract
This chapter introduces auditory objects that can be created by adjustable-directivity sources in rooms. After showing basic positioning properties in distance and direction, we describe physical first- and higher-order spherical loudspeaker arrays and their control, such as the loudspeaker cubes or the icosahedral loudspeaker (IKO). Not only static auditory objects, but such traversing space by their time-varying beam forming are considered here. Signal dependency and different practical setups are discussed and briefly analyzed. This young Ambisonic technology brings new means of expression to sound reinforcement, electroacoustic or computer music.
Franz Zotter, Matthias Frank

Backmatter

Additional information