Links

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.

• Bruno Putzeys on Amplifiers and Class D
  In Square CAD audio interview #15 Bruno Putzeys of Hypex Electronics and Grimm Audio talks about his power amplifier designs and the undeserved reputation of negative feedback. Stability of higher order and nested feedback loops is often difficult to maintain and consequently they are avoided by practically all amplifier designers. Easy to understand solutions are preferred even when their performance is limited. In some ways this is similar to loudspeaker design, where a flat on-axis frequency response and sufficient output volume are the prime design objectives. (And room response equalization with DSP will supposedly compensate for overlooked radiation issues.)
  
  
  

• Boyk on Piano
  A peek into the mind of a concert pianist and renowned teacher: http://jamesboyk.com/
  (See also James Boyk below)
  
  
  

• ETYMOTIC insights
  The Etymotic Audio File from May 2013 carries a well done educational video: Watch the Ear in Action.
  
  
  

• Linear Audio
  Publications of Linear Audio are found at www.linearaudio.net. Volume 5 is now available. Jan Didden's personal website is www.linearaudio.nl . Of particular interest to the LX521 DSP Challenge is his 6-channel volume control for the Behringer DCX2496, which could make this DSP unit a contender.
  
  
  

• Evaluation of Time-Frequency Analysis Methods and their Practical Application
  On the LISTENINC website's Literature pages I found an interesting AES Convention paper by Steve Temme et al. with the above title. It shows the development of time-frequency-energy analysis of loudspeakers from the Short Time Fourier Transform to Cumulative Spectral Decay analysis to Wigner-Ville Distribution, Smoothed WVD, and Continuous Wavelet Transform. The graphical presentations for the different analysis methods are exemplary in showing relationship and resolution of the three variables for the same test object, a wide bandpass filter with resonances, and for interpreting the graphics.
    
  
  

• KLIPPEL - Online Training
  The KLIPPEL website is a rich source of educational material about the measurement, analysis and and detailed behavior of electro-acoustic transducers in the form of papers and application notes. Now it is also possible to take online training. The first of seven sessions has been posted.
  #1  Linear Lumped Parameter Measurement
  #2  Vibration and Radiation Behavior of a Loudspeaker Membrane
  #3  Loudspeaker Nonlinearities
  #4  Loudspeaker Distortion Measurements
  #5  Predicting the Nonlinear Loudspeaker Behavior
  #6  Auralization
  #7  Micro-speaker
  
  
  

• MUSIC TO YOUR EARS
  The January 28, 2013 issue of THE NEW YORKER carries on pages 32 to 39 a most interesting article by Adam Gopnik about "The quest for 3-D recording and other mysteries of sound". It puts in perspective Edgar Choueiri's cross-talk-cancellation experiments for 3-D listening at Princeton University with the hearing and sociological research at McGill University by Albert Bregman (Auditory Scene Analysis - The Perceptual Organization of Sound), Daniel Levitin (This Is Your Brain on Music) and Jonathan Sterne (MP3: The Meaning of a Format). The article highlights the changes in music consumption from the audience in a concert hall to the audiophile sitting in front of his speakers, in the sweet spot, and to the ear-bud generation busy with other activities. "The notion of a pure musical experience is, for Sterne and his cohorts, the last sad effort of a nineteenth-century cult of attention that placed the solitary alienated (and almost always male) listener in a temple of silence, the concert hall." 
  
  
  

• MUSIC, NOISE AND HEARING: HOW TO PLAY YOUR PART
  The BBC has published "A Guide for Musicians" on how to protect their hearing. The information has relevance to the audiophile. My technical quibble is with the lack of definition for 'Sound Intensity' and 'SPL' and their use. Sound Intensity is sound power per unit area. A +3 dB change means doubling the power. The corresponding change in sound pressure level, SPL, is also +3 dB. Doubling the SPL, which is a +6 dB change, requires +6 dB or 4-times more power. Our ears respond to and can be damaged by the sound pressure level, SPL, at the eardrum. 
  
  
  

• Dynamic Space Stereophony
  I am very much interested in the spatial rendering of stereophonic program material, in the illusion created in the listener's mind. Peter Huebner goes much deeper than that when he talks about dynamic space stereophony and how music is heard in the mind of a classical composer. It is not clear to me how that could be recreated as a listening experience, but the thought is intriguing. Maybe specific multi-channel program material and a combination of multiple real and phantom sound sources with specific directional properties would approximate the composer's experience. Anyways, the pages provide a profound view of music and its possible effects upon a listener.
  
  
  

• How sound waves become a listening experience
  An animated illustration on the Sennheiser website shows the transfer of acoustic vibrations to electrical stimuli, which are then processed in the brain. The phantom sources that we hear between the loudspeakers of a stereo system are a construct of the brain. They are derived from the superimposed sound waves of left and right loudspeakers and the room at each ear. Cross-talk is essential for producing timing cues, which the brain uses naturally for directional hearing below 800 Hz in the Interaural Time Difference (ITD) range of hearing. Frequency response shaping by the outer ear and head blockage of sound are essential for directional hearing above 2 kHz in the Interaural Level Difference (ILD) range of hearing. Cross-talk cancellation in this frequency range can improve the spatial detail of the perceived acoustic scene and particularly its proximity. With HRTF derived CTC the two loudspeakers can be heard almost as whispering into just one ear from close-up. But maintaining a spatially uniform spread of the phantom scene in front of the listener becomes problematic when CTC thins out the center of the phantom scene. I have not heard CTC processing that I prefer long-term over plain stereo. 
  
  
  

• The Science and Art of Listening by Seth S. Horowitz
  The Opinion Page of the New York Times Sunday Review carried on 11 November 2012 an article about listening vs. hearing by the author of the book. This subject is of particular interest to me, because in sound reproduction we ideally only pay attention to the direct sound from the loudspeakers, while reflections, reverberation and standing waves due the room's boundary surfaces are background sounds that can be tuned out. I am convinced that this ideal can only be achieved by using loudspeakers with frequency independent directivity. But this type of loudspeaker is not common and requires special effort to design. Instead, some people listen in acoustically dead rooms, which must be specially designed, rather than in more normal living spaces.
  See also This Is Your Brain on Music below.
  
  
  

• swissaudec GmbH
  
  

  Clemens Par, founder of swissaudec, presents at tmt27 for the first time a paper about his ground-breaking and patented new methodology for audio signal coding.   The paper will be delivered in English on Saturday, 24 November 2012, Spatial Audio, 18:00 - 19:00, Room R1.

  SA6:  Two Undiscovered Treasures in Spatial Audio: Inverse Problems and Invariant Theory

  Retrieving higher order (i.e. multichannel) signals from a lower number of input channels is a well-known problem (generally referred to as “upmixing”), which may be satisfyingly solved with stereo or multichannel content, thus providing a high level of accuracy regarding localization and sound envelopment. Such methodology, however, fails when mono input signals should be processed in real-time – prior art then will not achieve sound source separation in real-time, particularly with sound sources showing the same frequency! At the same time any upmixing algorithm implies comb filter effects when the signal order is to be lowered again (“downmixing”) – an annoying effect, for instance, with HD television broadcasts generated by means of prior art upmixing tools, which are subsequently being played back on a stereo device. This paper presents an entirely new approach by applying inverse problem theory to the world of audio (to which we subsequently refer as “inverse coding”). Inverse problems were described first by Russian-Armenian physicist Viktor Ambartsumian in 1929, then solving, for instance, complex linear integral equations by a simple numerical solution. Generally speaking, lossy data of lower order may be recovered to finally represent a data set of higher order e.g. by means of specific matrix inversions. Additionally, algebraic invariants (a property of a class of algebraic objects that remains unchanged when specific transformations are being applied) may serve as “tags” to speed up signal calibration. The result is highly efficient and comb-filter-free real-time upmixing of any given stereo, Surround or 3D audio content for the sake of restoration, troubleshooting, postproduction, broadcasting and content distribution. In particular, inverse coding may help the sound engineer to avoid colouration effects with spot microphones. In audio coding spatial bit rates may be lowered up to 200 times with multichannel content for the sake of efficient transmission over existing studio networks - and (the good news for 3D audio content providers!) future low bandwidth transcoding consumer devices will be able to cope with genuine 3D audio content of highest computational complexity.     Clemens Par (swissaudec GmbH)
  See swissaudec below
  
  
  
  

• Auro 3D - Principles in Surround Recordings with Height
  This 2011 White Paper by Theile and Wittek outlines the possible directions for 3D audio sound, where 3D means direction, distance and height. It reviews the limitations of 2.0 stereo and 5.1 surround due to phantom source instability and practical setup implementations. Described are different 3D rendering systems for playback and microphone setups for recording. This is a very interesting survey of what we have today and what might be the n-channel future. I am still amazed of what can be extracted in natural sounding spatial information from a properly done 2-channel recording. See Watson.
  A German version of the paper is available at www.hauptmikrofon.de and much more.
  
  
  

• Noise Shaping Sigma Delta Based Dacs & Digital Jitter and Volume Controls
  Martin Mallison, Chief Technical Officer of ESS Technology, lucidly presented and explained the technical issues at RMAF 11. 
  Watch it on YouTube. 
  I do not doubt that people can hear Sigma-Delta noise shaping differences or digital jitter, though I have no personal experience. But I question his explanations as to why they are audible. Data measured in the frequency domain were not related back to the signal's nature and its amplitude in the time domain and vice versa. In general the vertical axis of the measurements was not discussed. In Fourier analysis the axis is calibrated in sinewave amplitude, but it is also used to read the amplitude of impulsive signal spectral components and of noise. Both are broadband signals that often exceed the bandwidth of the Fourier analyzer. To understand their amplitude presentation one must know the analyzer's impulse bandwidth and noise bandwidth. The spectral components of an impulse may be of very low amplitude, but in the time domain they add to a large, very short duration pulse with a repetition rate corresponding to the spectral line spacing. Similarly the amplitude of the same noise in the time domain is a function of the hearing bandwidth, e.g. 20 kHz. We hear both in the time domain and the frequency domain so we should also analyze physical processes in both domains. The audibility on the Rebecca Pidgeon track apparently is due to the impulsive signals from metal beads.
  Jitter is different. Jitter causes non-linear distortion, i.e. intermodulation of broadband signals. I doubt that this is what we hear because of its low amplitude. But jitter has a spectral distribution, which should have been shown in addition to its PDF. I believe that it is the very low frequency spectral content of the jitter, which typically has the highest amplitude, that accounts for audibility as phase modulation of the audio signal in the time domain. Again proper amplitude calibration of the analyzer is important to arrive at a meaningful interpretation of the numbers.
  
  
  

• Subjective assessment of audio quality - The means and methods within the EBU
  The European Broadcasting Union defines In section 3.2 of this EBU-Technical Review article from 1997 a set of subjective parameters, which cover the technical quality of “acoustic music”. This set should be very useful for recording engineers in assessing their work. The demonstration CD PEQS, described in EBU-TECH 3287 (October 2008), gives 66 examples of different recording quality. The LX521 Monitors readily show the differences.
  
  
  

• Less and More: The Design Ethos of Dieter Rams
  I have admired the exterior design of Braun products under Dieter Rams' leadership. The SFMOMA has currently (January 2012) an exhibition of his work and also of his influence upon other designers (e.g. Steve Jobs & team). Follow the link below to his 10 Design Principles as shown on a poster in the museum bookstore and along the walls of the exhibition. 

  
  

• Testing One, Two, Three
  Stuart Yaniger describes the methodologies and pitfalls of listening tests in Volume 2 of Linear Audio, p.161-174.
  Myself and others, notably Don Barringer, have done a lot of listening tests to fine-tune the ASP response of the ORION, which was derived from free-field acoustic measurements. This is clearly a subjective process because what I and others hear is not merely a function of the sound pressure streams at our ears, but also of our expectations and the sonic attributes we pay particular attention to. We may come to agreements, if we have the same reference sounds in our minds, but this does not necessarily guarantee that we have developed a neutral transducer. Stereo is particularly tricky because we create a phantom sound scene that only exists in our minds. It is a good magician's trick, if we can hide the loudspeakers and the listening room in the process. And when have we reached YOUR perceptual noise level, where you are no longer sure that this way or that way is more accurate? I only know when I have reached MY perceptual noise level, but it is not fixed in time and can change with new expectation and attention. Do I then start over? In the end could I fool a child or dog?
  
  
  

• MP3 and AAC explained
  Karlheinz Brandenburg of Fraunhofer Institute and MP3 fame explains the perceptual coding and decoding of sound in this 17th AES International Conference paper from 1999. Though audiophiles often malign these compressed and lossy data formats I have found that many of today's Internet Radio broadcast stations have very satisfying sound quality and provide interesting classical music and culture programming. I use a Squeezebox as signal source and an iPod to control it remotely.
    
  
  

• High Res Music
  What looks like a complete list of suppliers of high-resolution music files can be found on the auraliti website. 
  
  
  

• 2L - the Nordic Sound
  "Norway is blessed with many churches and cathedrals and most of 2L's recordings are made in these wonderful spaces. The music captured by 2L features Norwegian composers and performers, and an international repertoire reflected in the Nordic atmosphere."
  Check out www.2l.no for downloads of high-resolution sound files.
  
  
  

• HDtracks.com
  

  David Chesky - Composer, musician, producer, visionary

  "Founded by David and Norman Chesky of the audiophile-record label Chesky Records, HDtracks is a high-quality music download service offering a diverse catalog of independent music from around the world. HDtracks does not believe in DRM, and as a result, we offer a selection of unencrypted files that play on any computer or portable device. We have several formats available for fans of all types of music. If you would like genuine CD-quality sound, HDtracks offers both AIFF and FLAC lossless files. For those who wish to carry their music in a portable player, we have 320kbps files that far exceed the quality of standard MP3 files. Finally, audiophiles take note. HDtracks offers select titles in ultra-high resolution 96khz/24bit files. This is true DVD-audio sound quality for music lovers that demand the very best!"

  
  

• Listen - LIFE WITH CLASSICAL MUSIC
  "North America’s classical music magazine covering people, places and events; recommendations of recordings, books, and film; and all the many ways our lives are touched by classical music. Published by ArkivMusic, the Source for Classical Music."
  
  
  

• Introduction to 3-D Audio
  Fundamental aspects of spatial hearing in humans are studied by Edgar Choueiri in the 3-D Audio and Applied Acoustics Laboratory at Princeton University. One of the projects deals with optimal cross-talk cancellation of loudspeaker stereo signals at the listener's ears. The process works optimally in a room without reflections (anechoic) and for binaurally recorded material. Coincident and near-coincident 2-microphone recordings benefit also. Coherent spatial information is usually lost in mixed-down multi-microphone recording and cannot be recovered by cross-talk cancellation. I guess that playback in reverberant spaces is optimized when using loudspeakers with frequency independent radiation patterns such as omni, cardioid and dipole, together with a symmetrical placement of the loudspeakers relative to large reflecting surfaces and at least 1m away from them. This would be the same setup as for traditional stereo playback in acoustically small spaces. 
  
  
  

• Listening in on the listening brain
  A report about research into the auditory brainstem response to complex stimuli (cABR) in the June 2011 issue of Physics Today. Hearing involves a bi-directional stream of information - from inner ear to brain and from brain to inner ear. Note that the brainstem response to a 0.1 ms impulse shows activity for about 7 ms. Listen to three examples of naturally produced sounds contrasted with their brainstem responses at the end of the report.
  
  
  

• KHANACADEMY
  While not much about audio www,khanacademy.org surely has something to teach or to catch up upon for everyone. See for example his 10 minute talks about Specular and Diffuse Reflections or about Wealth Destruction.
  
  
  

• The Well-Tempered Computer
  Vincent Kars' introduction to computer audio provides a wealth of information and links to suppliers for setting up a playback system where your music collection resides in computer memory. Seeing the different acronyms and terms explained and compared is very helpful. For many audiophiles it is like learning a new language. 
  See also the dCS Guide to Computer Audio for assembling a playback system.
  
  
  

• Dipole Bass vs Monopole Bass
  Elias Pekonen measured and compared the response from a dipole bass loudspeaker to that of a monopole (box) loudspeaker in an acoustically small room using constant Q periodic wavelets. He observed that the dipole source better preserves the modulation envelope as shown very clearly in the frequency-time-amplitude data presentations on his website. 
  The work confirms my own conclusions from shaped tone-burst tests (Publications #21) and those of Holland, Newell, Mapp in "Modulation Depth as a Measure of Loudspeaker Low Frequency Performance", Proceedings of the Institute of Acoustics, Vol.26, Pt.8, 2004. 
  With the Toneburst CD you can hear changes in bass envelope modulation for different locations in your listening room.
  You can also sift through the questions, opinions, misunderstandings, irrelevancies, insights and selfassertions of the related diyaudio.com forum discussion.
  I hope that the time response in the discrete mode frequency range of a room will eventually get as much attention as the steady-state frequency response. Bass sound perception is derived from both.
  
  
  

• Linear Audio - Volume 1
  The second issue of the semi-annual bookzine has just been published and is available from www.linearaudio.net. It continues the collection of thought provoking and practical articles written by experts in various branches of audio. For example, I found the article on "Low-noise microphone preamp design" and "The F-word - or, why there is no such thing as too much feedback" particularly interesting. Check out the content. What is published here is too practical for the scientific paper ambitions of the JAES and far outside the range of Stereophile and TAS. Decades ago Wireless World was the place for this kind of audio engineering thought, but not that magazine's main topic.
  
  
  

• Vector Wavefront Format (VWF) 
  Graeme Huon from HUONLABS in Australia presents a sound reproduction system that conveys not only direction but also distance of acoustic sources. It is based on the premise that we hear the curvature of the arriving wavefront. I am not convinced, but here is a series of short articles that explain the concept. Also shown is a loudspeaker design with omni polar response obtained from controlled diffraction plate (CDP) structures, which are claimed to accurately combine the outputs from multiple drivers on a spherical surface. 
      See also: Graeme Huon and Zeljko Velican, 'Spatially consistent reproduction of the reverberant sound field', 116th AES Convention, Berlin, 2004, Preprint 6109
     Graeme Huon and Zeljko Velican, 'Low frequency optimization and non-bass masking effects for sound field re-creation', 115th AES Convention, New York, 2003, Preprint 5989
  
  
  

• The Inaudible Loudspeaker - Unmasking audio sound information by applying monaural stereophonics
  Leo de Klerk, a Netherlands artist and recording engineer, proposes a vertical array of 4 loudspeakers, (L_bottom + L_top) & (R_bottom + R_top), to produce a center phantom scene from a stereo recording that will not shift for off-axis listening angles. The loudspeakers should be omni-directional. Bottom and top loudspeakers receive the same input signal. I have not heard such configuration. An excited listener to my Pluto 2.1 demo at the TMT26 was trying to get Leo de Klerk to come and meet me, but our schedules prevented it from happening.  
  
  
  

• Orion Gateway
  Eric Weitzman started a new website and blog describing his Orion experience and modification, his Hubble Super Regulator, Balrog Turntable, and Curcio Phono Preamp to which you can leave replies.
  
  
  

• Open Baffle Dipoles - How they work and how to use them well
  Rudolf Finke has on his website DIPOLE+ an article "Open baffle 1.pdf" that shows calculated on-axis and off-axis frequency responses of drivers on differently shaped baffles. In particular much attention has been paid to the transition region from 'dipole' response to 'dipolar' (beaming) behavior of practical size drivers. It is a well done study of trends and limitations. 
  
  
  

• RCA Victor presents 'Sound and the Story' (1956) 
  Here is a historic video that clearly states RCA's sonic goals for their recordings in the days of mono and early stereo:
      At 02:35: “A planned illusion … the best seat in the concert hall”
      At 23:10: “ It’s traveled a long way – for an illusion. The last stop home, and the best seat in the concert hall.”
  No wonder the Living Stereo series got such legendary reputation. Check out the placement of the listeners and especially of the monitor speakers in the recording studio, well away from all walls (for example at 06:10 to 06:50). The entire room, which features dimensions larger than the typical studio of today, resembles acoustics more characteristic of a large living room than an over-damped studio (but often with lots of very early reflection from the huge mixer console in front of the typically flush mounted wall speakers).
  RCA placed the main microphones above and in front of the orchestra. What were the problems with recording directly from "the best seat"? Could such recording be done with modern microphones like a pair of Schoeps SuperCMIT and capture the timbre, localization and spaciousness as heard from "the best seat"? What is the recording engineer's goal today?
  
  
  

• Mikrofonaufnahmetechnik und Tonstudiotechnik (Recording & Studio Techniques)
  Eberhard Sengpiel was honored in 2010 by the Verband Deutscher Tonmeister (VDT) for his contibutions to the Tonmeister (~recording engineer) profession. His website contains a very extensive compendium of lecture notes (example) for recording engineers. But I found nothing about issues or solutions to stereo playback in reflective spaces, which the vast majority of consumers have to deal with. Why are there no lectures about the all-important "Wiedergabetechnik" (Playback Techniques)? Why no discussion of the influence of the "Mono Recording Angle", which captures sound outside of the stereo recording angle?
   
  
  
• swissaudec 
  Clemens Par creates professional three-dimensional sound from one single audio channel. He thereby saves 50% up to 88% of the audio signal bandwidth and, vice versa, enhances any audio data to a professional three-dimensional sound image. I was impressed by his demo at TMT26, but how could he do it?
  
  
  
• BBC Research & Development Reports
  Many interesting reports about loudspeaker design and test & evaluation methods from 1957 to 1999
  
  
  
• Some factors in loudspeaker quality
  H. D. Harwood from the BBC Research Department talks in this landmark Wireless World article from 1976 about observations and unresolved issues related to loudspeaker design. On page 48 he questions the optimum on-axis frequency response and mentions a shallow dip between 1 kHz and 3 kHz. On page 54 he pleads for a non-uniform axial frequency response. Experiences with equalizing an initially flat on-axis response to obtain the realistic sounding ORION-3.2 have convinced me that 'flat' is not correct for a stereo system. Here we must create believable phantom sources from two real sources at +/-30 degree angle to the listener, at the 'sweet spot', in the room. Nor is the BBC dip the answer. Instead, the response must be flat above 100 Hz and then gradually drop to a lower level at high frequencies. There are general physical and psychoacoustic requirements for such a response, which I will explain at a later time. A specific on-axis frequency response curve is usually the secret behind a good sounding loudspeaker. The marketing department likes to label this curve 'flat'. 
      For Stereo, neither the two monitor loudspeakers in the recording studio, nor the playback loudspeakers in the home, should have a flat on-axis frequency response. A corresponding power response and placement of the acoustically small loudspeakers in the room at least 1 m away from large reflectors, hides both loudspeakers and room acoustically from the listener in the 'sweet spot'.
  
  
  
• Acoustics Handbook
  Application Note 100 from Hewlett Packard Co. marks the company's short-lived entry into the acoustic measurement field in 1968. The flagship product was the Zwicker Analyzer for loudness measurements.  AN100 contains fundamental and useful information in its 116 pages (9MB) such as physical acoustics, physio-psychological acoustics, survey of measurement methods, instrumentation, calibration of microphones, sound level meters, filters, frequency analysis, standards. 
  
  
  
• Application of linear-phase digital crossover filters to pair-wise symmetric multi-way loudspeakers
  Part 1: Control of off-axis frequency response, PP presentation 1 
  Part 2: Control of beamwidth and polar shape, PP presentation 2
  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 avoids lobing of the vertical polar pattern.
  The Audio Toolbox by Dr. Ulrich Brüggemann (((acourate))) now includes these crossover filters. 
  
  
  
  Putting the Science back into Loudspeakers
  John Watkinson from Celtic Audio Ltd. presents his observations and theories in a thought-provoking paper. I see many parallels to my work, in particular his emphasis on minimal diffraction, i.e. omni-directional radiation and the consistency of the reflected with the direct sound spectrum. He emphasizes the necessity for linear phase response and waveform fidelity for localization and spatial accuracy of phantom sources. I have not found or heard convincing evidence for this, if phase/ group delay distortion is kept low. Linear phase can only matter for instantaneously broadband transient signals. The highpass nature of loudspeakers, though, requires 12 dB/oct low frequency response for minimum group delay build-up, not vented enclosures. The Cabar stereo monitor loudspeaker system bears many similarities to a pair of Pluto 2.1. I designed Pluto because I wanted to compare a low diffraction, acoustically small source to Orion, which led to the rear tweeter addition in Orion+.
  
  
  
  
  
• Constant beam-width transducer (CBT)  
  D. B. (Don) Keele has done extensive work on line array loudspeakers for sound reinforcement applications, where it is important to control the vertical radiation pattern and to avoid lobes, while maintaining wide horizontal dispersion to cover an audience. In a curved array with proper shading the delay of individual drivers is obtained mechanically and the output level from groups of drivers in the array is changed independent of frequency and only according to their placement in the array. The joint paper with Douglas J. Button describes straight and curved arrays and compares their vertical radiation pattern to that of a 2-way loudspeaker over a ground plane. The paper is rich in graphical illustrations. 
  An introduction to the CBT and its application is given in Don's Boston AES-ASA-BAS presentation from January 2010 with overheads and accompanying audio/video (Don Keele.mov).
  I have heard Monte Kay's center speaker CBT and was quite impressed with its acoustic performance in his home theater application. It seems the CBT in its various forms presents a business opportunity for highest quality sound reinforcement, where especially the mid and high frequency portions of the spectrum must be properly dealt with. It would be the end of the horn and compression driver in quality critical applications.
   
  
  
• Microphones and acoustic measurements
  The Bruel & Kjaer Library has a collection of their Application Notes, Technical Reviews, Primers, Handbooks, Conference Papers and a Dictionary. It is a treasure chest of information about acoustic and vibration measurement methods and tools. I desired but could not justify a product purchase from B&K, but I was an avid collector of their publications. Here now is everything in one place and conveniently accessible. 
  
  
  

• 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 be aware 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 me 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 frequency transition 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. 
  
  
  
  
  
• 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.