Journal of the Audio Engineering Society

2009 June - Volume 57 Number 6


Methods and Limitations of Line Source Simulation

Authors: Feistel, Stefan; Thompson, Ambrose; Ahnert, Wolfgang

A computational model for loudspeaker arrays using the complex directivity data of point sources can predict the radiating pattern of any continuous line source. A line source of finite size can be decomposed into a set of smaller sources that can be measured and then treated as point sources that contribute to the far field. Any linearly extended loudspeaker with discrete elements can be simulated with this computational model, even more accurately when accounting for the acoustic support of adjacent cabinets. A comparison of measured and predicted performance of different arrays demonstrates the utility of this approach. Other approaches of comparable accuracy can be computationally expensive and are often of limited applicability.

With the widespread interest in spatial audio reproduction, this study explores an innovative way of creating and distributing spatial information corresponding to a mono or stereo archival recording in a real space. An arbitrary number of room impulse responses can be generated from the time structure of one or two reference responses by extracting an approximation to the geometric structure of a reference space. The early reflections are examined to extract the initial reflections from the floor, ceiling, and walls. The remaining scattered and diffused sound is modeled as scattered point sources. Estimates of such parameters as room volume, source–receiver distance, and incident direction of the direct sound are possible.

The extensive research on the perceptual attributes of analog filters used for loudspeaker crossover networks does not necessarily apply to digital filters. In this study finite-impulse response (FIR) and Linkwitz–Riley (LR) digital crossover filters were examined for their perceptual artifacts. Subjective tests with headphones and loudspeakers showed that for LR filters the audibility of phase distortion can be predicted by group delay errors. But FIR filters of high order produce audible artifacts because of time smear created by extensive ringing. LR filters of order 8 or less and FIR filters of about 600 were without problems. These safety limits should be respected.

In augmented reality audio (ARA) applications, the natural sound environment is enriched with the addition of virtual sounds, and both are presented to the listener over headphones. A typical configuration has microphones outside of the headphones, and these signals are combined with a virtual source rendering of other sources. However, for listeners to be comfortable being immersed in an ARA space, the sound of the environment should be unchanged by the system, especially localizing sound cues that warn of potential danger. Subjects will only wear an ARA system for extended intervals if it does not disturb normal functioning. Other social issues need to be addressed, such as signaling that normal dialogue is possible even when there is visual indication of headphones, which are expected to block environmental sound.

Correction to “Virtualized Listening Tests for Loudspeakers”

Authors: Hiekkanen, Timo; Mäkivirta, Aki; Karjalainen, Matti

In the above report the classification on top of pages 237 – 251 should have read Engineering Reports (not Papers).

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[Feature] The recent AES 35th International Conference, Audio for Games, held in London in February, illustrated the way in which sounds and music for games are evolving in response to user expectations and changing game genres. The traditional film score, such as written by Prokoviev for Alexander Nevsky, was an essentially static creation designed to match the screen action. While the composer might have worked closely with the director to create a combined music/film entity that satisfied the viewer on a number of aesthetic levels, the sound track was fixed for all time in a single form. Games have many things in common with films but differ in the essential fact that they are unpredictable to some degree, and yet still have music and effects as an accompaniment. For this reason the sounds that accompany games cannot be entirely fixed and need to be allowed to change or evolve according to the current game state. How this has been dealt with to date, and what might be achieved in future, are tackled in a number of interesting papers from the conference.

[Feature] There have been many technological breakthroughs in the area of fiber optic technology, allowing an easier migration into the professional audio arena. Following the recent rise of copper prices in worldwide markets, there has been an increased usage in fiber-optic-based equipment, cables, and connectors deployed for pro audio and video. The timing seems right for presenting this tutorial (an update and expansion to Ajemian and Grundy article in JAES 1990 March), to bring the professional audio community up to date with the fiber optic cables and connectors currently being deployed in pro audio.

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