Journal of the Audio Engineering Society

2007 June - Volume 55 Number 6


The nonlinear stiffness K(x) and the reciprocal compliance C(x) of suspension parts (spider, surrounds, cones) and passive radiators (drones) are measured versus displacement x over their full range of operation. A dynamic nondestructive technique is developed, which excites the suspension parts pneumatically under similar conditions as occur in a loudspeaker. The nonlinear parameters are estimated from the measured displacement and sound pressure signal. This guarantee results with highest precision as well as simple handling and short measurement times. The theoretical basis for this new technique is developed, and the practical handling, the interpretation of the results, and their reproducibility are discussed.

The problem of the bounded-input/bounded-output stability of time-varying recursive filters is discussed. While simple, well-known criteria exist for the stability of time-invariant filters, guaranteeing stability when the filter coefficients are allowed to vary is much more difficult. Better insight into the causes for instability can be gained by using the state-space representation of the filter and examining the singular values of the state transition matrix. Two simple criteria based on the state transition matrix can be derived that guarantee the stability of the time-varying filter. Moreover, in the second-order case the singular values of this matrix provide a useful estimate of the maximum and average signal gains that result from the modification of the filter coefficients. These estimates can be used in practice to keep a time-varying filter from blowing up. It is also shown that some filter topologies are better suited to time-varying filtering than others, and a few techniques are presented that can be used to stabilize an otherwise unstable time-varying filter.

Direct Acoustic Digital-to-Analog Conversion with Digital Transducer Array Loudspeakers

Authors: Mendoza-López, Jorge; Busbridge, Simon C.; Fryer, Peter A.

[Engineering Report] Different direct-converting, digitally driven loudspeaker array prototypes have been developed with moving-coil transducers. An evaluation of the radiated sound-field quality in terms of distortion figures over the three-dimensional space in relation to the main driving transducer properties is presented. The effect of key experimental factors such as transducer mismatches, array size, transducer interspacing, baffle diffraction, and transducer arrangement on the radiated sound-field properties was isolated and quantified experimentally. Values of total harmonic distortion (THD) in excess of those set by quantization distortion were found at listening distances typical of a normal-size room for prototypes made with tweeters. A smooth rolloff in the transducer frequency response resulted in improved THD figures, and a wider transducer bandwidth resulted in increased reconstruction bandwidth. Future research directions are established for the multibit direct acoustic digital-to-analog conversion, and performance bounds are set on the characteristics of transducers needed for digital reconstruction.

[Engineering Report] Directional audio coding (DirAC) is a method for spatial sound representation, applicable for different sound reproduction systems. In the analysis part the diffuseness and direction of arrival of sound are estimated in a single location depending on time and frequency. In the synthesis part microphone signals are first divided into nondiffuse and diffuse parts, and are then reproduced using different strategies. DirAC is developed from an existing technology for impulse response reproduction, spatial impulse response rendering (SIRR), and implementations of DirAC for different applications are described.

Spatial Audio

Authors: Staff, AES

[Feature] 3-D and surround audio systems continue to develop apace. Recent AES papers on the subject include coverage of motion-tracked binaural audio and the localization performance of various stereo and ambisonic systems. Furthermore there is work going on in the separation of direct and diffuse components in sound reproduction as well as the perceptual effects of decomposing surround audio signals into principal components.


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