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To help you get started with understanding some issues relating to preference for loudspeaker directivity, I'm going to introduce selected papers from the E-Library with a small commentary on each. I'm not claiming this is the whole story, but the papers mentioned here bring up some interesting points.
Loudspeakers radiate sound in multiple directions, not always equally. There are loudspeakers that radiate in very narrow beams, and there are those that radiate almost omnidirectionally. In-between these two extremes are designs such as dipoles that radiate in a roughly figure-eight pattern. All this depends on the frequency range in question, and loudspeakers have historically tended to get more directional as the frequency gets higher. Efforts in recent times have been towards engineering a more consistent directivity pattern over the audio frequency range, in order that the reflected sound in a room integrates well with the direct sound.
How, though, does loudspeaker directivity affect listeners, and is there a "right" answer to the question of the most appropriate directivity pattern for sound reproduction? Some of this seems to come down to what aspect of sound reproduction you want to optimise, whether timbre or spatial imaging. It also has to do with how much of a role the room takes in the spatial effect of reproduction. AES papers over the years have debated the issue, and have attempted to come up with evidence for one position or another. It partly seems to come down to the intended application and how many loudspeakers there can be. In sound reinforcement for example there may be good reasons for wanting to be able to control directivity to suit particular "aiming," coverage or intelligibility requirements. In cinema sound there has been a tendency to want surround arrays to create a more diffuse or immersive effect, rather than localisable sounds, but that is changing with the advent of very large numbers of independent channels and object-based audio. There is then the question of what happens when you try to reproduce cinema-style effects in smaller rooms with only a few loudspeakers, and finally we come down to basic stereo reproduction of, say, music.
Optimum Loudspeaker Directional Patterns (James Kates, November 1980)
Back in 1980, James Kates was mainly concerned with ensuring good stereo localisation, concluding that loudspeakers should be directional over a wide frequency range, rather than omnidirectional. He also argued that directional loudspeakers would reduce the level of room reflections that might disrupt imaging.
Subjective Evaluations of Preferred Loudspeaker Directivity (Flindell et al, February 1991)
Comparing a number of different directivity characteristics, with the same on-axis response, Ian Flindell and his colleagues found no clear consensus for preference in stereo listening applications. Naive listeners slightly preferred a more omni-directional response. More omni-directional responses did not seem to impair stereo imaging.
Imaging and Loudspeaker Directivity: To Beam or Not to Beam (Roy Allison, October 1995)
Roy Allison took up the debate in 1995 with a convention paper discussing the merits of two schools of thought — the "beamers" and those who thought that loudspeakers should have as wide and uniform a directional response as possible. It partly seemed to depend on the type of music to be reproduced and whether one was trying to create a "you are there" or a "they are here" type of experience.
Subjective Appraisal of Loudspeaker Directivity for Multichannel Reproduction (Nick Zacharov, 1998)
Nick Zacharov did some experiments to look at what loudspeaker directivity might be appropriate for front and surround loudspeakers in multichannel home reproduction of TV and cinema material. He tentatively concluded that higher directivity surround speakers gave higher ratings in response to three specific questions about envelopment, directional effects and naturalness. Little difference was found between three different front directivities, but less directional designs were not found desirable.
Comments on “Subjective Appraisal of Loudspeaker Directivity for Multichannel Reproduction” and Author's Reply (Tomlinson Holman and Nick Zacharov, April 2000)
In a spirited exchange of letters between Tom Holman and Nick Zacharov, Holman took issue with the results of Zacharov's paper, saying that he had found very different results for both front and surround loudspeaker directivity. Holman also inveighed against the use of trained listening panels of audiometrically normal subjects, saying that it left little room for the opinions of real experts.
The Acoustics and Psychoacoustics of Loudspeakers and Rooms — The Stereo Past and the Multichannel Future (Floyd Toole, September 2000)
Floyd Toole emphasises the importance of accurate timbral reproduction above all else. In order to achieve this, he says that a loudspeaker must have relatively constant (or at least smoothly changing) directivity as a function of frequency. If it has, then direct, early reflected and reverberant sound can all exhibit similarly accurate timbral signatures. He points out that with multichannel audio the professional industry is committed to forward-facing direct radiator loudspeakers, so perhaps there is no cause for consumers to use anything different, now that we have independent surround channels.
Room Reflections Misunderstood? (Siegfried Linkwitz, October 2007)
Here Siegfried Linkwitz discusses the argument that room effects will be minimised if one uses highly directional loudspeakers aimed at the listener. It's practically very hard to build broadband directional loudspeakers, he says, so dipoles are the only practical directional source for typical rooms. He proposes that a pair of omnidirectional and a pair of dipole loudspeakers will sound essentially the same in a reflective listening room if their on-axis frequency response is the same. Reflections from the room can be perceptually ignored if their spectrum is similar to that of direct sound, provided they come after a suitable delay. The brain is able to separate the static listening room acoustics from the acoustics embedded in a recording presented over the loudspeakers.
Effects of Loudspeaker Directivity on Perceived Sound Quality — A Review of Existing Studies. (Evans et al, May 2009)
In 2009 William Evans and his colleagues conducted an extensive review into studies of listener response to loudspeaker directivity, finding that although the literature showed some small trends in preference during subjective tests, there was little agreement or conclusive evidence to support any point of view. Evans suggested that laboratory-style testing environments might not be representative of real listening contexts, and experiments needed to include other than "sweet spot" listening locations. Attributes being tested had varied widely in the experiments surveyed, so closer relationships needed to be established between directivity characteristics and specific perceptual attributes.
This month Francis Rumsey interviews Stavros Ntalampiras about his paper in the October issue, on sound recognition in reverberant environments.