THE HEAR AND HOW
Perception of Reproduced Sound, edited by Soren Bech and O. Juhl Pedersen. Symposium on Perception of Reproduced Sound, 165 pp., hard back, $35. (Available from Old Colony Sound Lab, P.O. Box 243, Peterborough, N.H. 03458.) Lord Rayleigh had the right idea, in 1877, when he said that all questions concerning sound must come to the ear for decision, and from it there can be no appeal. The quest for high-fidelity sound reproduction is, at long last, turning toward subjective evaluation, with the goal of full correlation of listening tests and objective measurements.
While Americans have only tentatively dipped their toes into the waters of subjective evaluation of loudspeaker performance, the Europeans represented here seem to have enthusiastically dived in and come up with smiles on their faces. One of the contributors, Nils Kousgaard, was bold enough to state that "the inference drawn from the qualitative approach is at least as precise as the inference drawn from the quantitative approach." If properly conducted subjective tests on a speaker are as precise as properly conducted objective measurements, perhaps we are close to attaining the goal of close correlation between the two.
This book is the outgrowth of a symposium held in mid-1987 at Gammel Avernaes, Denmark. It contains 16 papers, with two more in a loose supplement slipped into the back. (One is tempted to guess that this supplement was made necessary because a couple of the authors failed to meet the manuscript deadline.) The papers are not exhaustive (nor exhausting) treatments of the various fields of specialty, but rather are relatively short summaries of theory or work in process. This makes the book especially valuable for a bird's-eye view of the complex field of psychoacoustics.
The first four chapters alone are worth the asking price. The first is "Application of Psychoacoustic Methods," by Torben Poulsen of the Acoustics Laboratory of the Technical University of Denmark. This chapter summarizes the statistical aspects of subjective measurements in a clear, understand able way. Next come "A Review of Basic Research on Timbre," "Evaluation and Scaling of Timbre in Listening Tests on Loudspeakers," and "Some Fundamentals of Auditory Spaciousness," all by noted authors. The first paper is by Reinier Plomp of the Netherlands, famous for his outstanding work on consonance and dissonance.
The second is by Henrik Staffeldt of the Danish Engineering Academy, and the third is by Jens Blauert, well known in this country for his excellent book, Spatial Hearing (MIT Press, reviewed here January 1985). This group of three papers gives one an enlightened appreciation of the importance of the concepts of timbre and auditory spaciousness. Staffeldt observes that "the reviewers of the audio magazines have developed a rich timbre-related vocabulary which sometimes is more florid than informative." He also refers to Floyd Toole's paper (delivered at the 1987 AES convention), which considers that individual listeners or groups of listeners, or even loudspeakers having ratings with 5% standard deviations may be characterized as "a subjective measuring instrument having a measuring uncertainty of less than 1 dB." The next three chapters deal with the planning of listening tests. Toole, in treating the effects of technical and environmental variables, comments that "precise localization of specific auditory images seems to benefit from placing the listener in a predominantly direct sound field" and that "a sense of spaciousness appears to benefit from fairly energetic lateral reflections in the listening room, just as it does in the concert hall." Alf Gabrielsen of Sweden's Uppsala University considers the myriad of perceptual variables which can affect the results of subjective testing of loudspeakers. For example, if judgments of "brightness" of the music are required, just what is the reference "brightness" to assure that all judges use the word in the same way? He also introduces different approaches to subjective tests: The use of descriptive ratings which require judgments of such things as clarity, fullness, and spaciousness, and the "pairwise" approach, in which judgments only of similarity between two presentations are required. Soren Bech discusses the choice of rating scale and test procedure in planning a listening test.
The next paper is "The Application of Binary Paired Comparisons to Listening Tests," by Nils Kousgaard of the University of Copenhagen. He gives reasons to support the conclusion that substitution of paired comparisons for the more common rating test carries with it no loss of precision and results in a test much easier to administer.
Michael Williams, a French sound re cording engineer and instructor, states that the phrase "carefully selected program material" can be, in fact, "a convenient way of hiding almost every vice that we are trying to get rid of." He reviews the almost infinite number of variables in the recording and reproduction chain.
Villy Hansen recounts his experience at Bang & Olufsen in developing a reliable panel of listeners for evaluating B & O's products and those of their competitors. This company would seem to be leading the way in commit ting time and money to the training of listeners. In 1984, they chose eight employees and released them from nor mal duties for fixed weekly training periods. In order for panel members to obtain experience in critical listening, the company even supplied each listener with a complete audio system and software for home use, as well as the opportunity to experience the real thing at live concerts. Such trained listeners are making great contributions to product development.
"Inadvertent Bias in Listening Tests," by S. K. Pramanik, who is also with B & 0, makes the point that listening test results may be questioned if each listener exercises the same set of faults. For instance, why are there references to "British sound" or "Japanese sound" or "American sound" in regard to the performance of equipment? National and other biases, says Pramanik, must be eliminated in the successful listening test.
Two authors are brave enough to tackle the sticky problems facing the technical reviewer who stands in the no-man's-land between the manufacturer and the consumer. Martin Colloms of Colloms Electroacoustics, London, writes on "The Technical Press, Reviewing, and the Loudspeaker Manufacturers," and Michael Madsen of the Copenhagen high fidelity magazine treats "The Technical Press and the Consumer." Floyd E. Toole draws upon the work of his own division of Canada's Nation al Research Council in the brief summary, "Correlation Between the Results of Objective and Subjective Tests." "Listening Rooms for Test of Loudspeakers," by Henrik Meller of Aalborg University in Denmark, starkly reveals the urgent need for standardization of the listening environment. This is a beautiful overview of almost every body's ideas on how to build a listening room. Meller summarizes room specifications recommended in five European organizations' publications and follows with brief descriptions of the listening rooms used by six Danish companies.
Gunther Theile of Germany's Institut fur Rundfunktechnik writes on the theory of "Stereophonic Imaging of Natural Spatial Perspective via Loudspeakers." Much of his paper is given over to objections to the summing theory of stereo localization. Theile would in stead urge the "association model," which envisions two separate processes acting on the signals received by the outer ear: First, a location-determining stage, and second, a gestalt-determining stage.
The general theme of the book is carried forward in the two papers offered in the supplement: "Psychoacoustic Problems in Listening Tests on Loudspeakers" and "Subjective Testing--A Manufacturer's Approach." The first is a statistical analysis by Staffeldt, the second a description of facilities and experiences at KEF by Laurie Fincham of KEF Electronics Limited, Kent, England.
This volume and its supplement are strongly recommended to those involved in evaluating loudspeakers or other electroacoustic transducers. To those familiar with the field, it will be a stimulating and concise review of subjective measurement problems; for those with less experience,
--F. Alton Everest
(adapted from Audio magazine, Apr. 1989)
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