Journal of the Audio Engineering Society

2004 October - Volume 52 Number 10


Subjective Investigations of Inverse Filtering

Authors: Norcross, Scott G.; Soulodre, Gilbert A.; Lavoie, Michel C.

Inverse filtering is the concept that one can "undo" the filtering caused by a system such as a loudspeaker or room. This approach strives to correct both the magnitude and the phase of the system. Inverse filtering has been proposed for numerous applications in audio and telecommunications, such as loud speaker equalization, virtual source creation, and room deconvolution. When inverting the impulse response (IR), undesired audible artifacts may be produced. The severity of these artifacts is affected by the characteristics of the IR of the system, and the method used to compute the inverse filter. When the IR is nonminimum phase, the artifacts tend to be more severe and become distinctly audible. The artifacts produced by the inverse-filtering process can actually degrade the overall signal quality rather than improve it. Formal subjective tests were conducted to investigate and highlight potential limitations associated with several inverse-filtering techniques. Time-domain and frequency-domain methods were implemented, along with several types of regularization and complex smoothing to help reduce the level of audible artifacts. The results of the subjective tests show that the various inverse-filtering techniques can sometimes improve the subjective quality and in other cases degrade the audio quality.

Analysis of a Folded Acoustic Horn

Authors: Bright, Andrew; Holland, Keith; Fahy, Frank J.

A boundary-element model is used to analyze the acoustic behavior of a folded horn. Results from the boundary-element model are compared with measurements of the throat radiation impedance and the far-field acoustic response. Further analysis shows how a oneparameter model can be derived from the boundary-element results and used to gain insight into the behavior of the folded-horn loudspeaker system. It is shown that a one-parameter model can be used to describe the behavior of the folded horn over a useful frequency range.

The Distributed Edge Dipole (DED) Model for Cabinet Diffraction Effects

Authors: Urban, Marcel; Heil, Christian; Pignon, C.; Combet, C.; Bauman, P.


A simple model is proposed to account for the effects of cabinet edge diffraction on the radiated sound field for direct-radiating loudspeaker components when mounted in an enclosure. The proposed approach is termed the distributed edge dipole (DED) model since it is developed based on the Kirchhoff approximation using distributed dipoles with their axes perpendicular to the baffle edge as the elementary diffractive sources. The DED model is first tested against measurements for a thin circular baffle and then applied to a real-world loudspeaker that has a thick, rectangular baffle. The forward sound pressure level and the entire angular domain are investigated, and predictions of the DED model show good agreement with experimental measurements.

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Discussions about subjective loudspeaker preferences often ignore the need for a rating scale to evaluate accuracy. Preferences and accuracy are different perceptual qualities. Accuracy relates to the degree to which a reproduced sound field is subjectively equivalent to a corresponding live presentation of the same source. There is no proof that preferences among different individuals will match accuracy.

[feature] Prior to the 116th Convention in Berlin, held in May this year, the AES ran a symposium on multichannel audio for radio broadcasters. The symposium attracted a large number of interested delegates working primarily in the European broadcasting industry, as well as visitors from Asia and the United States.


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