A listing of sites and pointers to references which are of some particular interest to electro-acoustic loudspeaker design, to sound reproduction in small spaces and to listening enjoyment.

The Physics and Psycho-Acoustics of Surround Recording (2005)
David Griesinger places emphasis on the streaming behavior of sound as he explains perception of direct and reflected sounds in a surround recording context. Intelligibility, reverberance, distance, and mud are his major concerns. Unlike for Theile and Wittek below, the placement of phantom sources in a coherent spatial continuum is of less concern. He states, though, that "Recordings stand or fall on their ability to satisfy the needs of human hearing. This is constant among people, even among recording engineers." He may not know how much is revealed about the recording technique when a loudspeaker system like the Orion is used to evaluate the plausibility of the auditory scene that is heard.
Multichannel Natural Music Recording based on Psychoacoustic Principles (2001)
Guenther Theile discusses techniques for recording music when natural reproduction over 3 front loudspeakers and 2 surround loudspeakers is desired. The perceptual effects that are generated by 5-real and n-phantom sources depend upon the microphone configurations used. Curiously, the potential influence of loudspeaker radiation pattern and room reflections upon spatial sound reproduction is not considered. Wittek below, makes the same omission.
Perceptual differences between wavefield synthesis and stereophony
Chapter 3 of the above PhD Thesis from Helmut Wittek contains a wealth of information about the properties of stereo, about phantom sources and perception models. Chapter 2 is an introduction to the perceptual attributes that apply. Dr. Helmut Wittek is the Managing Director of Engineering for Schoeps Mikrofone in Germany.
A Case for Subjectivity
Jens Blauert discusses perception, reality and virtual reality in this paper from the 2nd International VDT Symposium on 'sounddesign', 2007, in Germany. The paper is also interesting in relation to my proposed tests for the accuracy of a stereo system by concentrating on the plausibility of the reproduced Auditory Scene and the abstraction level of the character of the reference that is implied in such tests.
The Seductive (Yet Destructive) Appeal of Loud Music
Even though it is well known that prolonged exposure to loud sounds produces hearing damage, it is very common that people listen at dangerously high volume levels. Barry Blesser attempts to explain the phenomenon in this article.
Audio Musings by Sean Olive
An informative blog about loudspeaker evaluation and test methods at Harman International (JBL, Infinity, Revel). Sean is the Director of Acoustic Research.
Enjoy the Music - Review Magazine
A timely overview of what is happening in the audiophile world. Some interesting articles from BAS Speaker and other magazines. It could be worthwhile to be on the mailing list.
A sensibly designed loudspeaker
On the exhibit floor of the 129th AES Convention in Munich I saw a loudspeaker that impressed my by the logic with which it was designed, the GRIMM AUDIO LS1. I could not listen to it closely because the surrounding noise level was far too high, but what I saw and heard from the exhibitors convinced me that the LS1 is a remarkable loudspeaker design. The wide baffle yields a forward pointing and smooth polar response over a very wide frequency range with a low transition frequency region to omni-directional behavior. The inside of the box and the panels appeared to be properly designed. The speaker is active and uses intelligently applied DSP for response correction. A subwoofer can be correctly integrated if needed. One can only hope that the LS1 breaks bad habits in recording studio and home.
The "Stereo Dipole"
A very closely spaced loudspeaker pair with head related cross-talk cancellation that produces a virtual acoustic environment. This and other interesting work from the Institute of Sound and Vibration Research at the University of Southampton, UK.
Monte Kay's Home Theater
An enormous construction project that has been in the works for a number of years. I am already certain that when finished it will have outstanding sound performance.
Application of linear-phase digital crossover filters to pair-wise symmetric multi-way loudspeakers
Part 1: Control of off-axis frequency response
Part 2: Control of beamwidth and polar shape
Ulrich Horbach and D.B. (Don) Keele, Jr., AES 32nd International Conference, September 2007

Here is a truly ground breaking, sensible and practical application of DSP to the design of crossover filters and the polar response of large multi-way active loudspeakers. Very exciting work! It includes the vertical polar pattern.
SOUND REPRODUCTION - Loudspeakers and Rooms
Floyd E. Toole, Focal Press, 2008

In this book Floyd Toole summarizes and explains conclusions from a lifelong involvement with audio. I highly recommend it to anyone interested in factual information about loudspeakers and listening rooms, about measurements, listening observations and their practical implications. It is lucidly written in easy to understand language, extensively illustrated and referenced. It deals with the reproduction of sound - which existed in a space - inside another space. My only regret is that the potential of 2-channel playback in doing so has not been fully explored and misrepresented. This is understandable because the conventional box loudspeaker with its frequency dependent directivity index has been used for almost all of the observations that are discussed. In fact, the particular interaction of a box loudspeaker with the listening room makes it more difficult for our ear/brain perceptual apparatus to hear the recording venue's space and acoustics, provided that such information has been captured in the recording process. Floyd focuses on multiple loudspeaker surround sound. He considers this approach to spatial sound reproduction as much more rewarding and he provides extensive practical information for that. The book is very comprehensive and in my opinion a "must-read" for loudspeaker designers, recording and mastering engineers, room-acoustic consultants, audio reviewers and audiophiles. The book is about theory and praxis. It debunks much of the BS that seems to have permeated the audio industry and many of its customers.
www.amazon.com
The first open baffle loudspeaker?

Chester W. Rice and Edward W. Kellogg: Notes on the Development of a New Type of Hornless Loudspeaker,
Transactions of the American Institute of Electrical Engineers, issue 44, 1925, pages 461-475.

From page.464: "Provision was made for boxing in the instrument, and an interesting experience in this connection was that
of placing the box over the back, which had the same general effect on sound quality as applying a short horn
to the front of the diaphragm. Both helped to bring out the low tones and gave rise to some resonant effects.
Bringing out the low tones was due principally to preventing circulation of air between the front and back of the diaphragm.
The resonance was in the horn in one case, and in the box in the other.
A peculiarity of devices employing very flexibly supported diaphragms is that resonant air chambers behind the diaphragm
do about as much harm as resonant cavities in front of the diaphragm, the diaphragm usually taking part in the resonance.
Attempts to damp the interior of the box with felt were not entirely successful.

A happy solution of the problem of preventing circulation was obtained by employing a flat baffle-board,
at the suggestion of Rice, who was the first of the group to recognize the importance of the circulation factor
in preventing the radiation of low tones. With the flat baffle, no air resonance occurs and both sides of the diaphragm
give useful radiation, the total power radiated for a given diaphragm amplitude being nearly four times as great
as that radiated when the back of the diaphragm is enclosed."

This is probably the first description of an open baffle loudspeaker.
The laboratory model is shown on page 474 and is described as:
".... cabinet set containing rectifier, amplifier and loud speaker. The front of the cabinet acts as a baffle.
To prevent air resonance in the box, the sides and back are vented by inserting panels of perforated brass."
The first active loudspeaker as well?

(Source: Ralf Gerhard Ehlert, Musikwissenschaftliches Institut der Universitaet Koeln, Medienstimmen)

H-frame loudspeaker

Lenzola-Doppelstrahler »Janus«
"Resonanzfrei durch dicke mit Mineralwolle isolierte Doppelwände.
Die Leitflächen vor und hinter der Membran vermeiden akustischen Kurzschluß
und ergeben eine günstigere Schallabstrahlung" Abb. 61 aus [Günther (Hg.) 1936]

Dipole PA loudspeakers from Telefunken at the 1936 Olympics.

Lautsprecher auf der Berliner Funkausstellung 1939
"Die Entwicklung des dynamischen Lautsprechers scheint schon seit mehreren Jahren so ziemlich zum Abschluß gekommen zu sein. Man sieht deshalb wenig Neues. Nach den Angaben der Aussteller bemüht man sich zur Zeit vor allem darum, die Feldstärke im Luftspalt zu erhöhen, um die Empfindlichkeit zu steigern und um, was erheblich wichtiger erscheint, die Haupteigenschwingungen der Membranen zu dämpfen. ...."

From a German website that has collected historical information on loudspeakers, amplifiers and microphones. http://www.medienstimmen.de/ela/lautsprecher/modelle/main.htm

This Is Your Brain on Music - The Science of a Human Obsession

Dan Levitin was the Keynote Speaker at the 123rd AES Convention in New York, 2007. He gave a fascinating talk on why we can get goose bumps when we listen to music. What is the true impact of powerful music on the human brain? As a former rock musician, producer and now professor at McGill University Laboratory for Musical Perception, Cognition and Expertise in Canada, he combines art and science in this most stimulating and easy to read book. (Dutton, 2006)
Spaces Speak, Are You Listening? - Experiencing aural architecture

A book about the science, art and culture of acoustic spaces written by Barry Blesser and Linda-Ruth Salter. In light of my observations with the ORION+, for attempting to reproduce one acoustic space inside another one, chapters such as "2 - Auditory Spatial Awareness", "6 - Scientific Perspectives on Spatial Acoustics" and "8 - Auditory Spatial Awareness as Evolutionary Artifact" are of particular interest. (MIT Press, 2007)
Multidimensional Audio

A paper about audio measurements by Henning Moller of test instrument maker Bruel & Kjaer. The information is technically sound, but the measurement applications are dated. The paper is very educational, particularly for those in the DIY community who lack measurement capability, especially for free-field acoustic measurements, and instead rely on computer simulation and their ears. That is a very limiting method for designing loudspeakers and guarantees sub-optimal results if the goal is accurate sound reproduction.

Loudspeaker Placement for Optimized Phantom Source Reproduction

A joint paper by J. Gerhard, B. Theiss, M. O. Hawksford from the 1996 AES Convention in Copenhagen (Preprint 4246).

Their loudspeaker layout is quite different from the ORION++, but similarly based on psychoacoustic observations. I suspect that their listening experience could have been even more convincing, if omni-directional loudspeakers had been used. Those would have increased the reflected energy from the wall in front of the listener over the energy coming from the side walls. That would have reduced the need for micro placement of speakers and listener to 0.5" (12 mm) accuracy without affecting imaging precision.

A setup of loudspeakers parallel to the long wall of the room and close to the listener would seem to be well suited for small rooms and the PLUTO. It could also easily fulfill the minimum distance requirement of 3' (6 ms) from any wall. Close listening distance to the speakers also reduces the magnitude of floor and ceiling reflections if that should matter.

For general background see: Jens Blauert, Spatial Hearing - The Psychophysics of Human Sound Localization, MIT Press, 1996

Bob Pease is a legend in electronic design. In the Pease Porridge column of Electronic Design magazine he often talks about audio related electronics. For example, "What's all this best stuff, anyhow?" shows an easy way to measure operational amplifier distortion and noise by analyzing the amplified error voltage. In "What's all this Sallen-Key stuff, anyhow?" he points to the influence of the frequency dependent output impedance on filter roll-off performance. Bob highlights many design issues and deepens understanding of circuit and component behavior.
The Acoustics and Psychoacoustics of Loudspeakers in Small Rooms
Floyd Toole presented a tutorial on this subject at the New York AES Convention in 2005. Here is a summary as taken from the March/April issue of Multimedia Manufacturer Magazine. It appears that his latest findings match more closely my own observations and experiences which had convinced me that uniformity of directivity over a very broad frequency band, such as from omni-directional and dipole radiators, is optimal for accurate sound reproduction in typical living rooms. See also related earlier publications below and my Letter to the Editor of TAC.
A comprehensive review of this subject has now been published in the June 2006 issue of the Journal of the Audio Engineering Society by Floyd Toole, "Loudspeakers and Rooms for Sound Reproduction - A Scientific Review". Highly recommended study material for what is known and what needs further investigation.
Auditory Scene Analysis
A relatively new field of study which should lead to further understanding of the listening room's role in audio playback. I highly recommend to take a close look at the work of Albert S. Bregman, "Auditory Scene Analysis - The Perceptual Organization of Sound", MIT Press 1990. In a different application this material has already led to greater audio data compression rates but with fewer artifacts. There is an Audio CD available with 41 demonstrations of Auditory Scene Analysis. The included booklet gives a very clear introduction to the subject and detailed descriptions of the demonstrations.
Quality in the Age of "Good Enough"
Blair Jackson, MIX Magazine's senior editor writes about the recording business in the May 2005 issue.
Loudspeaker and listener positions for optimal low-frequency spatial reproduction in listening rooms.
David Griesinger of Lexicon argues and demonstrates that realistic bass reproduction requires more than a flat steady-state frequency response in the room. He points out the importance of the randomly varying pressure gradient between the ears, the velocity field vector, for spatial envelopment. The now widespread listening experience with dipole woofers, which set up strong velocity fields, confirms his observations. See also my Publications (21), (1) and Issues in speaker design - C.
Do You Hear What I Hear?
Learning to listen in a mediated world. An interesting article in the June 2005 issue of MIX magazine which talks about our auditory references.
Ripple Tank (2-D Waves) Applet
Paul Falstad has written and collected many interesting Applets. His Ripple Tank Applet shows numerous acoustic and electric wave propagation phenomena in an infinite plane and in a bounded plane. Of particular interest are visualizations of the wave fields for point and dipole sources when walls introduce reflections. Note that for acoustic waves the Fixed Edges check box must stay unchecked at all times. Select a very low source frequency so that the wavelength is in the order of the wall length to see what happens in a room at low frequencies. Place the source in different locations. Vary the distance between the (+) and (-) dipole sources. Focus on a fixed point in the plane - the listening position - and observe the sound pressure variation with time relative to that of the source. Check out the 3-D View and rotate the plane. All this should give you an idea about the complexity of sound propagation in acoustically small spaces where room modes dominate. Then increase the Source Frequency and Resolution to their maximum to see the essentially random reverberant field behavior above the Schroeder frequency.
Putting the Science Back into Loudspeakers - John Watkinson
A thought provoking article about the information capacity of loudspeakers and how it might be measured. The mp3 encoded tracks on my Demo CD give you an idea about your own sound system's information capacity and/or your hearing ability.
The books by John Watkinson are highly recommended. 'The Art of Sound Reproduction' should be required reading for every audio equipment reviewer.
Music and the Brain - Norman M. Weinberger
The November 2004 issue of the Scientific American carries an article about what happens in the brains of listeners and musicians.
How we Localize Sound - W. M. Hartmann
A very readable and thorough review from Physics Today , November 1999, of how we use our ears and mind to localize sounds.
Mastering Audio - The art and the science - Bob Katz
If you are interested in knowing what can be done to the recorded sound before the bits are committed to a CD, then you should read a few chapters in this book. It was written by Bob Katz, a well known and respected mastering engineer. Critical technical content has been checked by Jim Johnston, formerly of Bell Labs/AT&T. We tend to judge speakers by our favorite recordings without fully realizing how much the signals may already have been processed to suit commercial interests. Highly recommended reading and very educational.
Decoupling a midrange driver from the cabinet - Andrew Jones of TAD/Pioneer
Different techniques have been used to reduce the transfer of vibration energy from a loudspeaker driver to its cabinet. I like to mount the driver by its magnet and to let the driver basket rim touch the baffle only lightly so that little force is transmitted. It is effective and easier to implement reliably than compliant mounting from the rim. The PHOENIX and ORION Revision 0.1 driver attachments are examples of this. In a box design I had rested the magnet in a cradle and clamped it with a metal strap, which could be tightened from the outside using long screw-bolts.
Designing Loudspeakers at Revel - Interview with Kevin Voecks
Note the description of the listening tests. Trained listeners are used, but what is their reference? Neutral sounding pink noise? Live acoustic sounds? Speakers are compared against each other in double blind preference tests. How accurate is the preferred speaker? Note the listening room with diffusers. Certainly not what most people will use. Kevin Voecks clearly states the importance of timbre due to direct frequency response, 1st reflection and power response, the importance of polar response to 60 degrees off-axis horizontally. He points to the listening room as the biggest problem. The need for multiple woofers to combat modes. These are areas where an open baffle speaker like the ORION has clear advantages due to the figure-of-eight polar response down to the lowest frequency. Thermal compression in drivers and the associated response changes due interaction with the passive crossover elements are non-issues in an active speaker. No mention is made of other nonlinear distortion tests. The importance of reducing resonances (stored energy) is highlighted, but no mention of how Revel tames the cabinet vibrations or the sound inside the cabinet. Revel certainly has all the resources one could ever wish for and their top speakers probably define the performance plateau that can be reached within the box speaker paradigm.
See also http://www.reed-electronics.com/tmworld/article/CA475937.html for related speaker design targets of the Harman team.
Capturing Music: The Impossible Task - James Boyk talks from a performing musician's view point about the difficulties of recording music and reproducing it accurately. (See also James Boyk below)
The psychoacoustics of multichannel audio - Robert Stuart of Meridian Audio Ltd gives a tutorial about the different psychoacoustic parameters that influence sound perception in single channel and multichannel sound reproduction. Little of the material is immediately useful other than to point out the complexity of the processing that happens between our two ears. It can be expected that multichannel sound has less masking than mono or stereo, though this depends heavily on how the different channels were recorded and processed. Given the great variability of stereo recording, which should be a stable technology by now, it needs to be seen how well multichannel will do when there are no standards for recording technique nor playback equipment performance. For example, neither the SACD or the DVD-A consortium, nor any multichannel receiver manufacturer, specifies the low frequency roll-off characteristic of the multiple loudspeakers to be used, which is absolutely necessary to know for proper bass management. Following merely the installed Home Theater practices will hardly reveal the full potential of the new sound recording media. See also my comments and Publication 19.
Ambiophonics - Interesting material on acoustics and psychoacoustics that is well presented by Ralph Glasgal. This type of reproduction system requires a dedicated room and is capable of a convincing sonic experience. Yet, it does not fit my life style and there is something about it that I find tiring upon extended listening.
Reference earphones - My motto is "True to the original", which means true to what has been placed on the storage medium be it CD, DAT or whatever. Every electro-acoustic transducer introduces distortion, with loudspeakers being the worst offenders. Etymotic Research ER-4S earphones come the closest to reproducing what was recorded and they can serve the very useful purpose of identifying the areas where a given loudspeaker is failing. The accuracy of these earphones can be significantly improved by inserting a simple passive equalization network into the electrical signal path. It is very illustrative, but possibly disconcerting to hear, how a loudspeaker is distorting in frequency, time or dynamics. For example, take track 2 of Deedee Bridgewater's Live at Yoshi's CD and listen to the range of her voice, the drum set, the background sounds, etc. If you are not using dipole bass speakers you will be surprised to notice the sound that your boxes and room contribute to what you hear. The earphones can be a reliable reference for comparing frequency response, transient response and dynamics. I have some trouble with unnatural in-head localization, the lack of any tactile sensation of sound, and pushing a plug into my ear canal, but at 10% of the cost of the ORION they are a most worthwhile investment in accurate sound reproduction.
MP3 interview with John Sunier for Audiophile Audition.
Hear me talk about some of the loudspeaker features that are most important for realistic sound reproduction.

Audio Artistry - The company builds a line of open baffle, dipole loudspeakers that I designed.
The Beethoven-Elite and Beethoven-Grand are 4-way active systems that incorporate my latest thinking in speaker design. The two models evolved from the bi-amplified Beethoven. A review of the Beethoven is available in the Stereophile Archives.
The beginnings of the Dvorak series are described in my 1992 AES paper about a compact dipole design.
The Vivaldi is a 3-way system and demonstrates the high level of performance that can be obtained even in a fully passive dipole design. The frequency dependent cancellation of front and rear radiation is a good challenge when designing for a flat on-axis and off-axis response down to the bass region.

Audio Artistry Team

Siegfried Linkwitz, R&D
Marshall Kay, AA President, Operations, Marketing, Finance, Purchasing, Personnel
Tom Hoffman, Distribution, Sales
Kurt Pasquale, Manufacturing

Vivaldi Loudspeaker introduction, 1996

Shannon Dickson

Shannon Dickson interviewed me for Stereophile Magazine, (Vol.19, No. 4, 1996). I give a little history of how I got involved in audio and what I am trying to accomplish. His article is reprinted on the Audio Artistry web site.

Here he is in his home in Hawaii with his Audio Artistry Beethoven-Grand.

Gradient - Here are loudspeakers developed by Jorma Salmi. His observations and thinking about the room-speaker interaction are very parallel to mine.
KLIPPEL - Measurement and modeling of large signal parameters of drivers and simulation of their effect upon distortion. Highly educational material that applies especially to drivers for use in open baffle woofer designs, which require large volume displacements. See, for example, Wolfgang Klippel's 109th AES Convention paper [58], "Diagnosis and Remedy of Nonlinearities in Electrodynamical Transducers".
"Rules of the Game" by James Boyk - Lecturer in Music in Electrical Engineering, California Institute of Technology.
Is Digital Audio perfect? This article is an excellent look at human perception when trying to evaluate new versus old technology.
"....Then we must judge the New, not against the Old, but against the ultimate purpose of both. In audio, this means either a bypass test with a high-quality direct feed identified to the listener, or, if possible, a comparison with the live sound...."
"The Music of Sound" is about differences in listening and a reminder that a loudspeaker must be able to reproduce the most subtle sonic details in order to convey music's emotional message.
David Griesinger's work on sound envelopment is required reading for anyone interested in a music surround system. The popular home theater setups for movie viewing provide mostly pan-potted five channel mono sound, which works effectively because of the dominant cues from the picture screen. Music surround requires different recording techniques and playback processing to create a convincing illusion of spatial relationships. For an introduction to the issues involved study the preprint of Griesinger's talk at the Nov. 2000 Surround Conference in Paris.
Also, study the insightful MS PowerPoint slide presentation, "Perception of mid frequency and high frequency intermodulation in loudspeakers, and its relationship to high-definition audio - A physicist meets the twilight zone". In addition to the ultrasound findings, I was particularly interested in his discussion of distortion in loudspeaker reproduction of massed chorus and orchestra, beginning with slide 39. These are the two types of signals that I find most critical for assessing speaker performance. They involve naturally occurring distortion in the ear, which is also present at a live performance.
Are You On The Road To ... Audio Hell? The problem of reviewing and comparing audio components in the absence of a reliable reference is discussed in this article from Enjoy The Music.com.
I maintain that an accurate system will give you sonic surprises. Too many systems, though, impart a sameness to everything played through them and the sound experience becomes predictable.
Acoustics Animations - Here are nice illustrations of the sound fields radiated by monopole and dipole sources.
Open baffle, full-range circular array source - A box-less speaker construction for those with extra room for the rear radiation. I have no doubt that this approach has the potential for very accurate sound reproduction. Look at the many pictures if you do not understand Swedish.
Subwoofers: Optimum number and location - Based on the assumption that a smooth steady-state frequency response in the room is most desirable this investigation by Todd Welti from Harman International develops practical monopole subwoofer layouts. In my experience the steady-state frequency response does not explain the qualitative difference in bass reproduction between monopole and dipole woofers. The low frequency transient behavior due to the active number of modal resonances in the room response must not be ignored.
Additionally, the white paper "Getting the bass right" by Floyd E. Toole will give insight into practical ways for dealing with the steady-state response of the room.