Book Review: Sound Reproduction: Loudspeakers and Rooms

Sound Reproduction: Loudspeakers and Rooms
By Floyd E. Toole. Focal Press (Oxford, England, UK, www.elsevier.com, footnote 1), 2008. Paperback, 550 pages, ISBN 978-0240520094. $49.95.

The author of this book, Dr. Floyd E. Toole, is one of the most influential scientists in modern audio. After studying electrical engineering at the University of New Brunswick and receiving a PhD from the Imperial College of Science and Technology, University of London, in 1965 Toole joined Canada's National Research Council (NRC), in Ottawa, Ontario, where he attained the position of Senior Research Officer in the Acoustics and Signal Processing Group. During his tenure at the NRC he oversaw a facility for acoustical research and testing that has nurtured the development of many of Canada's most successful loudspeaker companies, including Axiom, Energy, Mirage, Paradigm, and PSB.

In 1991, Toole became Corporate Vice President–Acoustical Engineering of Harman International Industries. There he created and directed the Harman Research and Development Group, a central resource for technology development and subjective measurements for the Harman International companies, most especially the divisions producing the Infinity, JBL, and Revel loudspeakers. Toole retired from Harman in 2007, and in 2008 was awarded the Lifetime Achievement Award of the Custom Electronic Design & Installation Association (CEDIA).

Sound Reproduction: Loudspeakers and Rooms is a relatively nontechnical compendium of Toole's work on small-room acoustics, loudspeakers, and listening. "Part One: Understanding the Principles" comprises nine chapters analyzing the basic principles of acoustics in live musical performances and in domestic listening rooms, and, most important, in what ways those two listening environments differ. Part and parcel of the studies, drawn from many sources as well as Toole's own experiments, are efforts to relate measurements to the perceptions of listeners in each environment. The topics addressed include the influences of listener position, room dimensions, and room surfaces on real and perceived frequency response, and on the listener's perceptions of imaging (direction and distance), spaciousness, and immersion with one, two, or more loudspeakers.

Most of the discussion is based on the results of logical, well-designed experiments, such as simulating the positions and angles of incidence of boundary reflections by the use of discrete sound sources in an anechoic environment. However, Toole approaches the problem of reaching a consensus from the results of different experiments in a number of ways. Sometimes, he undertakes to rescale the original data to demonstrate that the underlying phenomena, described differently in different studies, are actually similar. Sometimes, he simply discusses how different studies might draw the reader to the same conclusion. This approach is surprisingly satisfying; it draws the reader's attention away from the abstract and toward the real world.

Toole concludes Part One with a description of some "typical" domestic listening arrangements and how they conform or, more often, fail to conform to the parameters he has deemed conducive to good sound reproduction. He also provides a tutorial in how common graphical representations of frequency/time/amplitude performance can be misleading, and takes to task some electronic "Band-Aids"—such as the use, in the bass range, of the once-popular 1/3-octave graphic equalizer.

All of this lays the groundwork for the seven chapters of "Part Two: Designing Listening Experiences," in which Toole recounts the modern history of sound reproduction, describes the relevant parameters of a contemporary listening room, critically addresses loudspeaker evaluation, and discusses system setup. At the heart of these chapters is his historic work on the subjective and objective evaluation of loudspeaker performance. The former, based on blind tests of trained listeners, grew from a seminal experience in 1965, when Toole and a few colleagues, as part of an effort to select a reference speaker for studies of sound localization and imaging, subjected themselves to a subjective listening test of four respected speakers. "The results surprised all of us," he writes. "The audible differences were absolutely enormous, but there was general agreement about which ones seemed to sound good." The ensuing studies give the reader an appreciation of Toole's sense of discovery, and opportunities (for those of us who've been around a while) to recall our familiarity with the actual speakers.

What followed from this experience was Toole's increasing sophistication in subjective testing to eliminate visual and other biases, and culminated in the work done at Harman by Toole and Sean Olive. This included computerized training for listeners, double-blind protocols, and Harman's amazing mechanized listening room, in which pairs of speakers are rapidly moved into identical positions behind acoustically transparent screens, to eliminate room bias and associated listener cues.

Toole then turns this approach on its head, using subjective but controlled studies to determine how the results correlate with objectively measured properties of loudspeaker performance. To paraphrase his conclusions almost simplistically, Toole defines five measurements as being statistically predictive of subjective satisfaction:
1) On-axis Frequency Response (FR)
2) Listening Window: a spatial average of FRs in the ±10° vertical and ±30° horizontal windows
3) Early Reflections: a spatial average of the FRs of the early reflections from the four walls, ceiling, and floor
4) Sound Power: a weighted average of all of the above to assess the total acoustic energy radiating from the source
5) Directivity Index: the difference between the on-axis response and sound power

According to Toole, optimization of these parameters should result in a speaker that is musically satisfying to the ears of most listeners, including audiophiles, assuming those listeners have been disabused of or isolated from their other biases. Unsurprisingly, although these measurement parameters were chosen to predict subjective enjoyment, they also make a rather good case for objective linearity. In an ideal world, loudspeaker manufacturers would offer such standardized and revealing information about their products, and we, instructed by this book, could more informedly select those we most want to audition.

Toole concludes with guidelines for the selection of audio equipment and for optimizing the setup of a practical listening room, based on his insights into auditory psychophysics and the acoustics of speakers and rooms. This is very helpful in terms of practical guidance for speaker placement, particularly of subwoofers, and for the physical and electronic correction of the room acoustic.

Reading this book will give you an immense appreciation for Floyd E. Toole's efforts to find where the subjective and the objective overlap, and to introduce coherence to a field still fraught with fantasy and conjecture (footnote 2). Sound Reproduction: Loudspeakers and Rooms won't only change the way you understand audio; it will change the way you listen.—Kalman Rubinson



Footnote 1: Kalman Rubinson is an author of a physiology textbook published by Elsevier, the parent company of Focal Press.

Footnote 2: Though I suffer a brickbat or two in Floyd's book, I heartily recommend it to anyone who has even the slightest fascination with what a loudspeaker is supposed to do and why.—John Atkinson

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