This paper presents a survey of various audio effects that can be physically applied to a rigidly-terminated vibrating string. The string’s resonant behavior is described, and then the ability of active feedback control to “reprogram" the physics of the string is explained. Active damping, which is a direct result of applying classical control techniques, provides for an effect based on amplitude modulation (AM). Traditional electric guitar sustain techniques are elaborated upon, which suggest another approach for ensuring marginal stability of the system even in the presence of an arbitrary nonlinear and/or time-varying effect unit in the feedback loop. This approach involves placing a dynamic range limiter in the feedback loop and does not introduce significant harmonic distortion other than that due to the effect unit. The maximum RMS level of the system’s output can be easily bounded if reasonable conditions are met by the dynamic range limiter. Finally, nonlinear and time-varying feedback control loops are applied experimentally to artificially induce frequency modulation (FM) at a low rate and AM at a high rate. These effects can be interpreted musically as vibrato and as a sort of resonant ring modulation, respectively.

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In Mechanical Sound Synthesis, real mechanical devices are employed to create sound. Users can interact directly with the variables of the sound synthesis, making interactions more intuitive to both users and audience. We focus on real-time feedback control for Mechanical Sound Synthesis and provide a classification scheme using the reality-virtuality continuum. We discover an apparently novel paradigm, which is described as augmented virtuality for real-time feedback control. Exploring this paradigm, we present preliminary results from a system enabling a user to teleoperate acoustic percussion instruments with the aid of force feedback. Mechanical looping of the teleoperation trajectories and their transformations enables the synthesis of lifelike sounds with superhuman characteristics that are nevertheless produced by mechanical devices.

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Acoustic feedback is capable of driving an electroacoustic amplification system unstable. Inserting a frequency shifter into the feedback loop can increase the maximum stable gain before instability. In this paper, we explain how frequency shifting can effectively smooth out the feedback loop magnitude response and how this relates to the system stability. Then we describe measurements on real acoustic systems that we employ to study the practical performance. Although useful for stabilizing systems in reverberant environments, reasonably small amounts of frequency shifting do not provide a significant benefit for hearing aids. It can be helpful to employ a microphone with a focused directivity pattern, and we describe how the directivity pattern may affect the efficacy of frequency shifting.

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