Audio feedback, in which sound is generated through the connection of a microphone, signal processing components and a loudspeaker, is one of the popular structures used in computer music. Since audio feedback has a circular structure using the output signals as re-input signal, beyond the one-way relationship, it results in unique features and a lot of attempts have been made to take advantage of an audio feedback in music composition and sound arts.
This dissertation practically explores audio feedback systems by developing control methods and applications according to environmental openness. Environmental openness, degree of contact with external energy and information, enables development of various musical applications depending on the structure. Since audio feedback features nonlinear characteristics, sonic control methods are required to create the feedback-based applications for specific purposes (e.g., performance, composition and exhibition). These are the reasons why this thesis includes control methods as well as artistic uses. By generating sounds using the feedback structure according to environmental openness and presenting control methods for certain sonic properties, we propose creative utilization of feedback systems in the fields of musical interactions while preserving the unique nonlinear characteristics. Classification of these methods also builds a new framework of audio feedback systems regarding sonic control, which would explore a wide range of potential applications.
We present several systems in three types of environmental openness: closed, semi-open and open. Closed feedback structure is a typical model of Karplus-Strong algorithm and digital waveguide synthesis, and we propose synthesis models and instrument software for the Geomungo, a Korean traditional plucked-string instrument, with a time-varying filter for vibrato effects. Semi-open feedback system installed in a specific shape space (e.g., a pipe) generates resonant sounds, and we propose two types of feedback-based pipe interfaces. Open system could generate various timbres depending on the internal and external conditions, and we explore the methods for intentional control of audio feedback through a context-based control and an evolutionary control.