Sound Reproduction: Loudspeakers And Rooms
Book By Floyd Tool
Loudspeakers, Rooms, and Books Aren't Perfect
Review By Howard W. Ferstler Volume 31 No. 1

  Almost for certain, anyone who has been involved in audio, as a manufacturer, a journalist, or simply a serious enthusiast, knows the name Floyd Toole.

A major researcher in audio systems and design, particularly in the realm of small-room acoustics and the relationship between measurements and listener perceptions, Toole studied electrical engineering at the University of New Brunswick, receiving a BSc in 1961. In 1965 he earned a PhD in electrical engineering from the Imperial College of Science and Technology, University of London.

Upon graduation, he joined the Acoustics and Signal Processing Group of the National Research Council (NRC; Ottawa, Canada), where he expanded his interests into the complicated interactions of room acoustics and loudspeakers, particularly as they related to the psychoacoustic relationship between what listeners hear and the technical measurements that are used in the design and evaluation of audio products. 

The research resulted in improved methods for subjective evaluations and technical measurements. For a paper on this subject he received the Audio Engineering Society (AES) Publications Award in 1988. Later work focused on one of the fundamental problems in audio, the perception and measurement of resonances, for which (with Sean Olive) he received the 1990 AES Publications Award.

In 1991, he joined Harman International Industries (Northridge, California) as Corporate Vice President of Acoustical Engineering. In 1998 he was appointed to the additional position of Senior Vice President of Acoustical Engineering for the Harman Consumer Group. He retired from Harman in 2007 and has since devoted his spare time to consulting. Toole has published papers in the journals of the AES and Acoustical Society of America (ASA), numerous AES preprints, chapters in two audio engineering handbooks, the entry on sound-reproducing systems in McGraw-Hill's Encyclopedia of Science and Technology (9th edition), and dozens of articles in consumer audio publications. He is a AES Fellow and past president, and an ASA member. In 1997 he received the AES Silver Medal Award, presented in recognition of outstanding developments in the subjective and objective evaluation of audio devices. 

Toole's latest major accomplishment is his book, tour de force — nothing else that I have read comes close to the totality of information in this one book.

The book is divided into two parts, with numerous smaller topic sections, and still more subject breakdowns within those sections. This list outlines the basic topics covered:

• Part One: Understanding the Principles (the first 267 pages), which shows how to create the best listening experience, offering practical approaches to the sound reproduction chain.

1. Sound Reproduction

2. Preserving the Art

3. Sound in rooms — matters of perspective

4. Sound Fields in Rooms

5. The many effects of reflections

6. Reflections, images and the precedence effect

7. Impressions of space

8. Imaging and spatial effects in sound reproduction

9. The effects of reflections on sound quality/timbre

10. Reflections and speech intelligibility

11. Adaptation

12. Adjacent-boundary and loudspeaker mounting effects

13. Making (bass) waves — below the transition frequency

14. A summary of part one

 

• Part Two: Designing Listening Experiences (pages 270-522).

15. Multichannel options for music and movies

16. Putting theory into practice — designing a listening experience

17. Loudspeakers I: subjective evaluations

18. Loudspeakers II: objective evaluations

19. Psychoacoustics — explaining what we measure and hear

20. Closing the loop: predicting listener preferences from measurements

21. Acoustical materials and devices

22. Designing listening experiences: recommendations

There are a multitude of charts and drawings, including numerous frequency-response graphs and room-layout diagrams. Toole dwells on some topics at length, and often refers back to them. He is obviously trying to get serious points across, and does so with enthusiasm.

I liked the book a lot, and it proffers information that I have mostly agreed with for years, particularly as it refers to the need for wide and smooth broad-bandwidth dispersion, smooth off-axis response, the importance of a center channel, the requirements of surround channels (placement and performance), and the lack of importance of combfiltering,  phase integrity, and similar artifacts in typical home listening rooms. It was good to see input regarding codecs for surround sound,  too, and it was nice to read refreshing opinions regarding tube (high output impedance) power amps and wire, although those topics are not  the main thrust.

Of course, other rational writers and research-oriented manufacturers have been saying the same things about these basic ideas and conclusions for many years, although not on so comprehensive a scale, nor with this kind of documentation.

A fair amount of what Toole says is almost self-evident from a "this  makes total sense" standpoint. For example, he lauds the idea that a speaker needs to not only be smooth on axis but also needs to be smooth off axis. Audio designers and reviewers have been on the "

And all one needs to do to prove that three channels up front is better than two is perform a quick comparison with decent multichannel source  material or a processor that can acceptably derive a solid center feed from the phantom part of a two-channel image. Ditto for wide-dispersion  monopole surround speakers vs. narrow-dispersion designs aimed at the listener.

Of course, codecs, phase behavior, plus things like amp distortion, tubes vs solid state, and speaker wire issues remain debated within the audio-tweak community. Toole does everybody a service with his analysis of those issues. He even does a very fine job of debunking the myths surrounding the so-called dipolar surround speaker design mandated in the THX home theater specifications.

However, there are a few beliefs and themes Toole works with that are  not covered in enough detail to satisfy certain rational enthusiasts, and the book leaves out or dismisses some important historical and technical  information that slightly undermines numerous conclusions he presents regarding the best sound possible from speakers. The flaws are not so  much what he says as what he does not say. While it might be esoteric hair-splitting on my part, I do want to mention some of those omissions. For example, he references some of Roy Allison's work (pg187-196)  and even reproduced one of his early JAES article diagrams (pg194; showing how Allison's Model One was configured to flatten midbass suckout), yet Toole never measured a pair.

He also might have been amazed at what the speaker could do in relation to the need for very wide and smooth broad-bandwidth dispersion.  The Allison tweeter and midrange drivers deliver exceptionally wide dispersion throughout their range, and mounted on the two angled panels of the Allison Model One, they deliver quite uniform output over a 180º-angle to above 12kHz. This means that reflections from adjacent room boundaries when employing them as front-channel systems are not only smooth (which Toole prefers), but also fairly powerful (which he seems to not come to grips with).

Interestingly, on page 137 Toole mentions that listeners appear to prefer the sound from wide-dispersion loudspeakers (not wide by Allison standards, but wide by more mainstream ones) with somewhat colored off-axis behavior, to the sound from narrow-dispersion models with less colored off-axis behavior. The only type of system illustrated that exhibits  this characteristic is the surround-channel model he discusses and illustrates on pages 195 (figure 12.13b) and 404 (figure 18.19b), both of which also have angled front panels that allow them to deliver wide dispersion  and flat power response. There is virtually no analysis concerning, and no reference to any research into, what a front-channel system having this kind of ultra-wide dispersion characteristic with minimal off-axis coloration could deliver to the listener.

On pages 340-341 Toole mentions obvious performance limitations with the AR-3, a speaker that was very highly regarded in the 1960s; its 1kHz woofer/midrange crossover point was too high, and problematic.  Yet the book never discusses whether he checked the later AR-3a model, which solved the AR-3's major defect by lowering the crossover frequency to 575Hz, by use of a new midrange driver able to handle the lower frequencies. He also mentions (page 340) that the AR-3 was considered dull by some listeners. The later AR-3a was judged by many critics to sound brighter. Given the almost cult status of that speaker, I find the lapse odd.

At one place in the book he might have the AR-3 and the AR-3a mixed up. On page 340 he states "...The Acoustic Research AR-3 (figure 17.2b) was famous for its novel acoustic-suspension woofer, and it came to be one of the reference loudspeakers of that generation. Its acoustic performance was well documented in the literature (e.g.: Allison and Berkovitz, 1972), which was a great credit to the company … " However, Allison and Berkovitz were studying the room response in Boston-area homes using the AR-3a, not the AR-3. Both speakers were references of a sort during their production runs, but the AR-3a was a better speaker. Also, on page 14 he mentions the live-vs-recorded sessions Edgar Villchur produced with the AR-3, but Toole seems to ignore their importance. All he has to say about what transpired is that numerous later design speakers from a variety of companies are better. If the AR-3 was  as dull as he says his listening panel described it, and lacking in modern quality, how did Villchur use a pair of them to impress serious audio enthusiasts and serious audio journalists, all of whom were seasoned listeners who were carefully checking for discrepancies during those sessions? Another very highly regarded speaker of the era was the AR-LST, which has achieved almost iconic status. Yet he did not audition that one  either, even though many astute individuals considered it a reference standard. Even Consumer Reports used it as a reference speaker for years, as, so I am told, did Julian Hirsch.

Both the Allison Model One and the AR-LST were designed by Roy Allison, and both used multiple drivers on angled panels for much wider dispersion than available from systems having a single, forward-facing panel. This approach should have rated serious analysis, certainly as  much text and response-diagram copy as any Quad model, two of which  are discussed with their response curves graphed. Yet virtually none of the front-channel speakers Toole dealt with in the book appear to have made use of multiple drivers on angled panels.

Given his admonition about aiming speakers at the audience in home theater installations (and perhaps music installations, too), it seems  amiss that he did not find it important to deal with designs that essentially do not need to be aimed (or at most only in the general direction of the listening area). If ultra-wide dispersing speakers are not best for L-, C-, and R-channel use, the book should have discussed their designs and explained why they are not viable.

Regarding that approach to sound radiation, it is notable that Toole makes no mention of Bose or their 901 systems. That design certainly has its detractors (and enthusiasts), but it did make a splash (Julian Hirsch went on at length about the design in his 1968 review). Yet there is no documentation in the book on why the system might be good or  bad, or in-between. That speaker is a nightmare (or legend, depending on your perspective), and should have been analyzed, especially in a book that deals so emphatically with room reflections, cross correlation, envelopment, etc.

Another speaker that made a big splash was the original two-way Large Advent, but I found no comments about it. Perhaps it was discussed and measured, but only listed as "speaker A" or "speaker Y." Popular speakers that exhibit deficiencies deserve to be analyzed, just like speakers with mostly positive attributes.

Another missing link was the Klipschorn, which certainly is an iconic item that some buffs would kill to own. While not a big seller in recent years, the speaker is an interesting enough design to warrant at least a comment or two, regardless of the conclusion.

Still another speaker that made quite a psychological and technical splash was the dbx Soundfield, a speaker line that was configured to be  aimed, but not at the audience. Yet there is nothing about it or any discussion of the approach Mark Davis used to analyze distance/intensity trading and design a package that would achieve what he wanted. There is no mention of the omni-directional approach to speaker radiation patterns that we find with speakers like the Ohm/Walsh models. Mark Davis and John Strohbeen obviously went well beyond what Roy Allison pursued in the realm of super-wide dispersion. Speakers like these and the Allison models (as well as the AR-LST and AR-3a) certainly ought to have rated at least a footnote-grade comment here and there.

Many other brands and approaches were also ignored, such as Dunlavy, Waveform and Carver, each of which stands in sharp contrast to much of the competition, including those that were discussed at length. It would be nice to see just how they would have performed in some of Toole's measurement analyses — such as response-curve info from outfits like Carver, B&W, and Boston Acoustics [also David Moulton's important work culminating in the Beolab 5. DRM]. These companies have at times carried a lot of weight in the business, and it would have been instructive to see just what their assorted models could do. Of course, if Toole wrote about all of the abovementioned models in addition to those it did cover, the book would probably be 1000 pages long. In addition, just acquiring the various models and doing the work (when Toole was at the NRC and later Harman) would have been prohibitively time consuming, tedious, and expensive. In any event, I find it troubling that so many innovative designs were ignored, both in his book and in a number of his JAES articles, particularly when many of them were lionized by the audio public and their designers were considered among the best.

It is possible that some of these speakers were analyzed by Toole when he was with the Canadian NRC and later at Harman, but we are dealing with the book, and they were not included.

Omissions aside, the book offers much that is very helpful. For example, it delivers a fine analysis of subwoofer behavior in small rooms and I agree completely with Toole when it comes to smoothing the overall response by sensible placement and the use of more than one. The multiple-subwoofer issue mostly involves listening rooms that are built to accommodate a group of listeners/viewers. I am pretty sure that most home-theater enthusiasts do not have that many seats. Thus, the need for multiple subwoofers might not be all that great for most of them. Two are not a bad idea, in any case.

There is one other area where Toole and I part company: his approach to response measurements and equalization. While I agree that the 1/20-octave analysis and EQ Toole strongly favors are the most revealing approach to deal with serious issues, I am not sure that kind of refinement is necessary on a global scale. That is, most enthusiasts can get  generally acceptable results with something less sophisticated, and most do not have the resources to opt for the superb technique he outlines. Most can do well to use the 25% placement approach with subwoofer(s), and then get acceptable flatness at the primary listening location through use of a decent equalizer. Going to the extreme he outlines might demoralize some enthusiasts and turn them off to the hobby.

Toole comes down hard on 1/3-octave analysis (either within the direct field or within the reverberant field), but the fact is that it and 1/3-octave correction gets the listener 75% toward ideal performance. If they have a decent room and decent speakers, that might be more than enough. My product-reviewing work involved performing a 20-second, moving microphone measuring procedure (slowly covering a 1'x1'x5' space at listening-couch head level) while my Audio Control SA3051 1/3-octave RTA ran a cumulative analysis of the data. This does not reveal fine detail the way that a gated, fixed-microphone 1/20-octave measurement would, but by moving the microphone I get an analysis in space that can adequately simulate a true power curve. Limitations notwithstanding, I told my readers that a room curve of this kind was only a starting point. From there, I used the same battery of recordings (good ones, produced by people such as John Eargle and Michael Bishop) and carried out level-matched A/B comparisons with my reference speakers, which included the Allison IC-20s in my main system or the very different Dunlavy Cantatas in my smaller system.

Yes, my approach was deficient by Toole's standards, who appears to have little use for power or room curves, favoring instead the analysis of first-arrival signals measured over a 30º-wide window, with the signals farther off axis carrying less weight. However, I think that combining a good room-curve analysis (to cull the real dogs from speakers that are at least good) with a careful, level-matched A/B comparison series did a lot to inform my readers.

It was a pleasure to read Toole's book and I continue to recommend it (with some caveats, of course) to my friends.

[Speaker tester David Moran comments:
It is certain that Toole has hearing much more educated, indeed golden, than most of us. Still, all those who think that EQ narrower than third-octave is desirable for music playback anywhere but on paper --- and this doesn't address the matter of preferability, which would entail blind testing of 1/6th, 1/12th, or 1/20th EQ against some 1/3-octave-smoothed baseline, and then also while varying the centers of the narrower-band EQ --- should spend a day playing with a third-octave equalizer listening to familiar material over familiar speakers and with one's own head being, typically, slightly mobile, not in a vise. If they wish to make their case after that, they should then undergo David Moulton's extremely humbling EQ listening tests, contained in his Golden Ears course (www.MoultonLabs.com/full/product01). Perhaps Toole has done all this. From my own experience I would strongly suggest that spatial and temporal averaging are key to equalization. And once you grant that, then truly narrowband EQ, at least narrower than the roughly 1/6 - 1/3-octave critical bands of the human ear, is out the window.]

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