Journal of the Audio Engineering Society

2005 July/August - Volume 53 Number 7/8


High-Efficiency Low-Bl Loudspeakers

Authors: Aarts, Ronald M.

Normally, low-frequency sound reproduction with small transducers is quite inefficient. This is shown by calculating the efficiency and voltage sensitivity for loudspeakers with high, medium, and, in particular, low force factors. For these low-force-factor loudspeakers a practically relevant and analytically tractable optimality criterion, involving the loudspeaker parameters, will be defined. Actual prototype bass drivers are assessed according to this criterion. Because the magnet can be considerably smaller than usual, the loudspeaker can be of the moving-magnet type with a stationary coil. These so-called low-Bl drivers have a high efficiency, however, only in a limited frequency region. To deal with that, nonlinear processing essentially compresses the bandwidth of a 20–120-Hz bass signal down to a much more narrow span. This span is centered at the resonance of the low-Bl driver, where its efficiency is maximum. The signal processing preserves the temporal envelope modulations of the original bass signal. The compression is at the expense of a decreased sound quality and requires some additional electronics. This new, optimal design has a much higher power efficiency as well as a higher voltage sensitivity than current bass drivers, while the cabinet may be much smaller.

Discrimination of Group Delay in Clicklike Signals Presented via Headphones and Loudspeakers

Authors: Flanagan, Sheila; Moore, Brian C. J.; Stone, Michael A.

Thresholds were measured for the discrimination of a click reference stimulus from a similar stimulus with a group delay in a specific frequency region, introduced using an all-pass filter. For headphone presentation the thresholds were about 1.6 ms, were independent of the center frequency of the delayed region (1, 2, or 4 kHz), and did not differ significantly for monaural and binaural listening. For presentation via loudspeakers in a low-reverberation room the thresholds were only slightly higher than with headphones and did not differ significantly for distributed-mode loudspeakers (DMLs) and cone loudspeakers. For presentation via the same loudspeakers in a reverberant room the thresholds were larger than the corresponding thresholds measured in the low-reverberation room, and this effect increased with decreasing center frequency for both loudspeaker types. For monaural listening the thresholds for discriminating group delay were significantly larger for the DML than for the cone loudspeaker, probably due to the higher ratio of reverberant-to-direct sound for the former, associated with its lower directivity. However, for binaural listening the difference between DML and cone loudspeakers became nonsignificant.

A Simple Hybrid Approach to the Time-Scale Modification of Speech

Authors: Knox, Don; Bailey, Nicholas; Stewart, Iain

Time-domain methods of time-scale modification (TSM) are attractive from the point of view of computational effort. However, they suffer from audible artifacts for larger timestretch ratios (greater than 1.3 times the original duration). The occurrence of these artifacts is often the main justification for the use of more involved analysis/synthesis methods at these ratios. For speech signals these artifacts take the form of transient repetition—causing a “stuttering” effect and roughness due to spectral mismatch at segment boundaries—most obvious during voiced signal periods. These phenomena are not addressed by existing timedomain methods. A simple hybrid algorithm utilizing both time-domain and analysis/synthesis methods is presented which illustrates how these distortions may be minimized. Results of formal listening tests illustrate an improvement in basic audio quality for timestretched speech signals when compared to equivalent samples processed by the synchronized overlap and add (SOLA) algorithm.

Directivity of Artificial and Human Speech

Authors: Halkosaari, Teemu; Vaalgamaa, Markus; Karjalainen, Matti

[Engineering Report] A study of how the directivity characteristics of artificial mouths correspond to the directivity of a real speaker is presented. The primary motivation for the research was the measurement methods applied in the telecommunications industry for the microphones used in telephones and their accessories. The responses of three artificial mouth simulators were measured in several positions. The same measurements were repeated for a group of test subjects. The measurement positions corresponded to the same positions where the microphones of telephones and their accessories, so-called headsets, would be situated. The basic mechanisms that produce the directivity patterns are discussed, and the contribution of the speech content is shown. The main contributor to the directivity is the aperture size of the mouth. The acoustical characteristics of the upper body are also a significant factor if the microphone position is not directly in front of the mouth. A greater than 10-dB difference with wide-band speech was found between artificial mouths and test subjects. It appears that the directivities of the artificial mouths are too narrow at high frequencies. To improve the correspondence of telephonometry and real speakers, a simple equalization procedure and two structural improvements are proposed.

[Feature Article] Long heralded as enabling the next generation of consumer audio-visual reproduction, optical-disk formats based on blue-laser technology are now on the verge of making their consumer debut. There have been a number of different plans for formats based on blue lasers, but the primary formats competing for a place in the high-definition entertainment market are Blu-Ray Disc and HD-DVD (formerly known as Advanced Optical Disc or AOD). In this article we summarize the primary features of the two formats, highlighting the options for audio where they are known. More details are publicly available for Blu- Ray Disc than for HD-DVD; see the relevant standards for the latest details as these may be subject to change.

[Feature Article] Chaired by Vicki Melchior, this workshop at the AES 117th Convention dealt with the topic of universal players for high-resolution audio. It was hard, Melchior said, to remember the days when there was only the CD and the CD player. Now we have formats that come from the computing, home cinema, and video fields. We have a multiplicity of different replay requirements, and consumers simply do not want to be bothered with multiple players. Universal players are a must if all these different formats are to be feasible. In a simple playback architecture, however, the question has to be asked: Can one system manage all the required tasks, particularly in the domain of high-resolution audio? Can high-resolution audio be expected to offer an improvement in the listening experience even in low-cost products or should it be reserved for only the higher end?

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