Journal of the Audio Engineering Society

2006 May - Volume 54 Number 5


Low-Frequency Optimization Using Multiple Subwoofers

Authors: Welti, Todd; Devantier, Allan

At low frequencies the listening environment has a significant impact on the sound quality of an audio system. Standing waves within the room cause large frequency-response variations at the listening locations. Furthermore, the frequency response changes significantly from one listening location to another; therefore the system cannot be equalized effectively. However, through the use of multiple subwoofers the seat-to-seat variation in the frequency response can be reduced significantly, allowing subsequent equalization to be more effective. Three methods to reduce seat-to-seat variation are described, including a novel approach based on simple signal processing. The desired result in each case is to allow the system to be equalized over a seating area rather than just one seat. Results are shown for several listening rooms.

The evaluation of the perceived spatial quality of an audio system has become more important as the technical possibilities to render spatial information increase. In recent years the field of spatial quality evaluation has been the subject of more thorough investigations than previously, one of the problems being the development of attribute scales appropriate for this purpose. The generation of attributes of spatial audio by means of elements from the repertory grid technique is investigated. In an experiment personal constructs in the form of verbal descriptors were elicited. The constructs were classified by verbal protocol analysis and reduced to a limited number of attributes by cluster analysis. The results show that the repertory grid technique enables a number of attributes of spatial sound quality to be extracted from a group of subjects and that these attributes correspond well with attributes found in other studies. The results also indicate the importance of a definition of the part—the whole or a subset—of the auditory scene to which a specific attribute is referring.

Sound field simulations at low frequencies often use finite-element or other mesh-based methods. For auralization, output data from these methods need to be converted to a format compatible with auralization methods, such as binaural reproduction, wave field synthesis (WFS), higher order Ambisonics (HOA), or vector base amplitude panning (VBAP). The mesh data can be viewed as a spatial sampling of the sound pressure distribution. A method is proposed for converting the mesh data to plane-wave components using a circular array of virtual sources centered around a reference position. Such a plane-wave decomposition (PWD) is straightforward to process further for auralization. Emphasis is put on generalized modal decompositions of sound fields through singular-value decomposition, and on the relation between modal bandwidth and the ratio of mesh width to wavelength. The special case of a decomposition into cylindrical harmonics is studied in detail. Results are presented for two-dimensional examples, and numerical issues are discussed.

In professional audio applications, small loudspeakers are often mounted on or near large solid surfaces, such as mixing consoles, desktops, and work surfaces. When mounted in this manner, the surface is in the near field of the loudspeaker and thus may detrimentally modify its frequency response by acoustic loading. In a majority of cases, analysis reveals that the magnitude response changes in a predictable and systematic way. Changes in acoustic loading raise the magnitude response in the 100–250-Hz region, and reflections cause combing effects at mid to high frequencies. A statistical study of 89 active near-field loudspeakers, located in 45 different installations, revealed that in 80% of the cases a one-sixth-octave-wide peak in the magnitude response at 141 Hz of about 4–6 dB was exhibited. The development of an active correction method is described which minimizes this low-frequency loading problem. It uses a single second-order notch filter to equalize the low-frequency loading effects.

[Feature Article] In this article we present a short update on research in automotive audio presented at recent AES conventions. Research reported here centers on the role of car doors as loudspeaker enclosures, preferred listening levels in noisy backgrounds, and the simulation of automotive audio reproduction using a binaural headphone system.

Loudness Trumps Everything

Authors: Staff, AES

[Feature Article] In this article we provide an update on the topic of program loudness, extending the overviews of recent research presented in previous issues of the Journal (p. 662 in June 2004 and p. 945 in October 2005).

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