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Journal of the Audio Engineering Society

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2008 November - Volume 56 Number 11

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Papers

Controlled Acoustic Bass System (CABS) A Method to Achieve Uniform Sound Field Distribution at Low Frequencies in Rectangular Rooms

Authors: Celestinos, Adrian; Nielsen, Sofus Birkedal

A new method for improving the low-frequency response in small- and medium-sized rooms uses a pair of front loudspeakers for initiating a traveling plane wave and a canceling rear pair of loudspeakers that effectively remove the acoustic reflection of the back wall. The rear loudspeakers receive a delayed and opposite phase version of the signal presented to the front loudspeakers, thereby canceling the plane wave when it reaches the rear of the room. Unlike equalization techniques that emphasize a single sweet spot, this approach covers the entire room. The upper frequency limit of the system is higher for smaller rooms. Based on an evaluation in two rooms, the system works as intended. Using this method, listeners experience a clear bass sound image, and the rear loudspeakers are inaudible.

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MPEG Surround-The ISO/MPEG Standard for Efficient and Compatible Multichannel Audio Coding

Authors: Herre, Jürgen; Kjörling, Kristofer; Breebaart, Jeroen; Faller, Christof; Disch, Sascha; Purnhagen, Heiko; Koppens, Jeroen; Hilpert, Johannes; Rödén, Jonas; Oomen, Werner; Linzmeier, Karsten; Chong, Kok Seng

Finalized in 2006, the MPEG Surround specification enables the transmission of multichannel audio signals at data rates close to those of one- and two-channel systems. This paper describes the technical architecture and capabilities of the specification. Verification tests include several operational modes as they would be used in typical application scenarios. In order to achieve backward compatibility with legacy devices that are not compliant with MPEG Surround, the spatial side information is embedded in either an ancillary part of the downmix bit stream or in a separate stream. This approach is vastly superior to a matrixed system at the comparable data rates.

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Modeling the Transient Acoustic Field Radiated by a Loudspeaker

Authors: Chappell, D. J.; Geaves, G.; Henwood, D. J.; Harris, P. J.; Chakrabarti, R.

This research concentrates on determining the transient acoustic field of a loudspeaker by applying uncoupled finite-element and boundary-element methods. The well-known problem of instability when working in the time domain was avoided by using a Burton–Miller type integral equation formulation. The accuracy of the results shows that this approach is well suited for modeling loudspeakers. Stable results were obtained in all cases and they were reasonably accurate when the time step was smaller than or equal to the experimental sampling rate. Two examples of insights are provided: differing decay rates of various loudspeaker components, and the presence of the acoustic center.

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Some Observations on Sensitivity to HRTF Magnitude

Authors: Hoffmann, Pablo F.; Møller, Henrik

The required number of head-related transfer functions (HRTFs) is determined by audible differences in the spectral characteristics as a function of separation angle. The experiment tested the ability of subjects to discriminate between changes in the minimum-phase HRTF while the interaural time difference remained constant. Subjects were able to discriminate spectral differences over a range of 3 to 17 degrees depending on the spatial locations being represented. The thresholds for detectable change in polar angle were consistently lower than changes in lateral angle. Due to the variation of the threshold, the choices of reference angles for HRTF measurements need not be uniform over the sphere.

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Concert Hall Acoustics

Authors: Rumsey, Francis

[Feature] In this month’s back-of-the-book feature article our staff writer, Francis Rumsey, reports on some of the views expressed in a recent AES workshop concerned with concert hall acoustics. This is a topic that nearly always gives rise to passionate opinions, but as Barry Blesser points out, our preference for certain hall acoustics is strongly determined by cultural and learned biases. We also learn about the acoustical characteristics of halls that are considered good for pop music performance.

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