Journal of the Audio Engineering Society

2005 March - Volume 53 Number 3


Vertical Localization of Sound from Multiway Loudspeakers

Authors: Ferguson, Sam; Cabrera, Densil

Practical wide-range loudspeakers are usually implemented with multiple drivers, but the systematic effect of the signal frequency upon the vertical localization of sound is scarcely used for loudspeaker enclosure design. Tendencies in vertical localization for the frequency bands characteristic of woofers and tweeters in loudspeakers are shown. Using vertical arrays of individually controlled loudspeakers, synchronous and asynchronous bands of noise were presented to subjects. The frequency of the source affected the vertical position of the lowand high-frequency auditory image pairs significantly and systematically, in a manner broadly consistent with previous studies concerned with single auditory images. Lower frequency sources are localized below their physical positions whereas high-frequency sources are localized at their true positions. This effect is also shown to occur for musical signals. It is demonstrated that low-frequency sources are not localized well when presented in exact synchrony with high-frequency sources, or when they only include energy below 500 Hz.

The effect on audio quality of controlled multichannel audio bandwidth limitation and selected down-mix algorithms was quantified using one generic attribute (basic audio quality) and three specific attributes (timbral fidelity, frontal spatial fidelity, and surround spatial fidelity). The investigation was focused on the standard 5.1 multichannel audio setup (ITU-R BS.775-1) and was limited to the optimum listening position. The results obtained from a panel of experienced listeners indicate that the basic audio quality of multichannel recordings is more affected by timbral fidelity than by spatial fidelities. Therefore it can be concluded that in the case of broadcasting multichannel audio under highly restricted transmission conditions, it is better, in terms of basic audio quality, to sacrifice spatial fidelity by downmixing original multichannel audio material to a lower number of broadcast audio channels than to sacrifice the timbral fidelity by transmitting all channels with limited bandwidths.

Intercomparison measurements of room acoustical parameters and speech intelligibility measures were performed in a 1500-m3 room containing a sound system. The parameters included the octave-band reverberation times T20 and T30 averaged over the entire room, the speech intelligibility indices STI, STI(mod), STI(male), STI(female), RASTI, and %ALcons, the frequency response in one-third-octave bands, and the room acoustical parameters EDT, C80, C50, D50, and Ts at various positions in accordance with ISO 3382. The measurements carried out by 15 participants from Switzerland were compared. As far as can be ascertained, two main factors appear to be responsible for the spread in results. On the one hand, the noise disturbances, at times quite strong, seemed to pose difficulties for the participants. Frequently too little attention was paid to achieving an adequate signal-to-noise ratio. Furthermore, for some measurements the results of certain participants diverged greatly from those of others, partly as a result of incorrect settings of the instruments. In the measurement analysis the standard deviations were compared to the perception thresholds. When the data from a few conspicuous participants were eliminated, the standard deviations were often within the region of the perception thresholds.

[Communications] Table look-up (or fixed-wavetable) music synthesis has recently received new interest in the context of mobile applications such as personalized ring tones and pager notification signals: the market for consumers purchasing polyphonic ring tones exceeded US $4 billion worldwide in 2004. Table look-up (or fixed-wavetable) synthesis methods have been widely used for many years in music synthesizers. The limited amount of storage and transmission bandwidth available in such mobile devices makes the use of compressed synthesizer data desirable. The important considerations for implementing a wavetable music synthesizer in a small portable device are described. In particular, an efficient compression/decompression method for the synthesizer data is used in order to reduce the size of the synthesizer data bank with good fidelity while still maintaining the loop continuity constraints necessary for looped wavetable synthesis.

[Feature Article] Low bit-rate coding standards are always evolving, leading to solutions offering better quality and more flexibility at lower bit rates than previously available. Particular challenges in this evolution have arisen in recent years, owing to the proliferation of multichannel audio and digital radio systems whose standards were set in stone before the most recent low bit-rate coding systems became available (see the article on Digital Radio in the December 2004 Journal). Research efforts have centered on trying to find compatible ways of adding the enhancements offered by modern developments without penalizing those using legacy equipment.


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