Because many elements of audio systems display nonlinearities, it is important to understand and model their behavior. The usual way to model such nonlinearities uses Volterra series and its subclass, polynomial Hammerstein model(s), which is a cascade of M parallel branches composed of a static monomial nonlinearity followed by linear filters. Unfortunately, the cascade of Hammerstein models manifests a large sensitivity to implementation inaccuracies, especially when the order M increases. This paper highlights different kinds of errors and shows how they can affect emulation accuracy. The authors propose a way to get around each problem thereby leading to an accurate emulation of three nonlinear systems: the Tubes-Screamer like overdrive effect, the TSA15h tube amplifier, and the Engl Retro Tube 50 E762 amplifier. A solution is also provided for cases of input amplitude dependence, which appears valid for some devices. The described method appears to be suitable for real-time emulation of audio nonlinear systems.
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Electric guitars use sensors based on magnetic induction to convert string vibration into electrical signals. This research explores a model of the induced voltage when the field of the permanent magnetic is assumed to be uniform. This approximation allows for the derivation of an analytical expression where the influence of different parameters, such as coil, magnet, and string are explicitly represented. The theoretical response of the pickup to a harmonic motion of the string at low frequencies is compared to experimental data obtained from a dedicated experimental bench. Some of the more interesting results are as follows: (1) the pickup is a velocity sensor and most sensitive to z polarization; (2) the furthest turns of the coil have little or no contribution to the output voltage; (3) for plain strings, the output level is proportional to the cross section of the string; (4) a wound string has the effect of decreasing the level of the fundamental and, thus, increasing the distortion; (5) the string materials appear to have no influence on the output voltage; (6) distortion depends mostly on the amplitude of the string displacement but weakly on the distance coil/string at rest.
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Flat-panel loudspeakers, often called Distributed-Mode Loudspeakers (DMLs), are created by using one or more force exciters to induced vibrations in an elastic flat surface. This report describes a lumped element equivalent model of a flat-panel loudspeaker including multiple bending modes of an elastic plate, the dynamical influence of affixed inertial exciters, and the contribution of the enclosure. Unlike a classical loudspeaker driver, flat panels have many more degrees of freedom. The panel’s natural resonant frequencies are shifted by the dynamical coupling to the exciters and the enclosure. Three important design aspects are illustrated through simulations: (1) the effective low-frequency cutoff of the flat-panel speaker is determined by the higher of either the exciter resonant frequencies or the lowest panel resonant mode; (2) rigidly backed exciters can be employed to avoid mode-splitting at low frequencies, and (3) achieving resonances below 100 Hz with a panel mounted near a wall requires the use of panels with considerable mass. Experimentally, the models were shown to accurately predict the vibrational behavior of a panel with resonant exciters and a rear enclosure.
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Previous research showed that when identical noise signals are presented from two loudspeakers equidistant from the listener, the resulting phantom image is perceived as being elevated in the median plane. In this study, listening tests used eleven natural sources and four noise sources with different spectral and temporal characteristics reproduced with seven loudspeaker base angles between 0° and 360°. While the degree of perceived elevation depends on the base angle of the loudspeakers, the spectral and temporal characteristics of the sound source also play a significant role in determining perceived elevation. Results generally suggest that the effect is stronger for sources that have a transient nature and a flat frequency spectrum as compared to continuous and low-frequency sources. It is proposed that the perceived degree of elevation is determined by a relative cue related to the spectral energy distribution at high frequencies and also by an absolute cue associated with the acoustic crosstalk and torso reflections at low frequencies. A novel hypothesis about the role of acoustic crosstalk and torso reflection at low frequencies is explored. At frequencies below 3 kHz, the brain might use the first notch in the ear-input spectrum, which is produced by the combination of acoustic crosstalk and torso reflection, as a cue for localizing a phantom source at an elevated position in the median plane. These results may prove useful for 3D sound panning, recording, and mixing without elevated speakers.
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Ambisonics is a production format that can be used for 3D audio. It is based on the representation of the sound field by decomposing it into orthonormal basis functions, called spherical harmonics. This representation allows for a flexible production process that is independent of the target playback system, be it loudspeakers or headphones. The concert night at the International Conference on Spatial Audio 2015 employed the Ambisonics format to distribute the concert to different venues and broadcasts in real time: a concert venue with a 29-channel loudspeaker system, a monitor venue with a 25-channel loudspeaker system, an outside broadcasting van with a 23-channel loudspeaker system, as well as a 5.1 mixdown and a binaural headphone mixdown for national terrestrial and satellite radio broadcasting. This flexibility is achieved by the computation of suitable decoder matrices and weightings. The success of the sound system at ICSA 2015 encouraged the authors to carry out a live 3D concert with Al Di Meola in July 2016 that included spatial real-time effects and transmission to a neighboring venue.
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Engineering Briefs summarized here show that there is a valuable medium by which more practical work can be exposed to the broader community. People working with recording methods and various forms of practical audio processing can present their work in progress, and share their ideas about recording, mixing, and reproduction.
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