|The Science of Domestic Concert Hall Design|
by Ralph Glasgal
Ambiophonics, 2nd Edition
Replacing Stereophonics to Achieve Concert-Hall Realism
By Ralph Glasgal, Founder Ambiophonics Institute, Rockleigh, New Jersey www.ambiophonics.org
The Blumlein Conspiracy
On December 14th, l931 the EMI sound engineer, Alan Dower Blumlein, filed a British Patent Specification 394325 entitled "Improvements in and relating to Sound-transmission, Sound-recording and Sound-reproducing systems." In the usually arcane language common to most patent applications, Blumlein's invention "consists in a system of sound transmission wherein the sound is picked up by a plurality of microphone elements and reproduced by a plurality of loud speakers, comprising two or more directionally sensitive microphones and/or an arrangement of elements in the transmission circuit or circuits whereby the relative loudness of the loud speakers is made dependent upon the direction from which the sounds arrive at the microphones."
Blumlein did not use the word "stereophonic" anywhere in his patent, but he did use the word "binaural." It was well known during the fifty years before Blumlein, that two microphones, spaced the width of the human head, feeding a remote pair of headphones, produced very realistic sound images with solid, stable, directional attributes. The problem was that the sound sources all seemed to lie within ones head or in psychoacoustic parlance, be internalized. What Blumlein sought to do was to externalize this binaural effect using loudspeakers. Externalizing the binaural effect over a full 360-degree sphere is still the Holy Grail of acoustics, particularly among those designing virtual reality video systems that also require an audio counterpart. The now dormant IMAX large screen 3d movie system uses earphones placed about an inch out from in front of the ears as well as speakers behind the screen, behind the audience, and above and below the screen to produce a full (periphonic) acoustic sphere. If home video watchers are prepared to wear earphones as well as have loudspeakers in their home movie theaters this is a very effective technology, but one that is not necessary to realistically reproduce staged musical events as opposed to movies.
Other attempts to externalize the binaural effect over a full sphere or just a circle, include, ambisonics, surround sound and the plethora of computer companies at work generating the virtual reality sound fields for the multimedia applications referred to above. Fortunately, our music problem is, and Blumlein's was, less complex since we need only consider a relatively small part of this sphere and we can assume that all direct music sound sources originate on a single flat stage in front of us (or in electronic scores a flat stage behind as well). In fact, Blumlein's first priority was to provide a better front stage sound for movies shown in theaters.
Blumlein was awarded his patent covering what we now call stereophonic sound reproduction officially on June 14th, 1933. Thus the basic stereo listening triangle is over 75 years old and just as Einstein's theory of relativity eventually refined Newtonian physics, it may be time to reexamine and modify the bedrock concepts upon which Blumlein imaging is based. And what better place to start than with Blumlein himself. Suppose one looked through Newton's treatises and found cryptic comments by Newton hinting that he knew his laws of matter, acceleration and gravity were not fully accurate at very high velocities and masses. We would then be justified in concluding that Newton had some insight into relativity but chose not to confound his contemporaries who had enough to deal with in distinguishing between mass and weight and who in any case found his formulas were always accurate enough to do jobs like getting rockets off the ground. Newton's laws still work very well today despite relativity if you are not too fussy. So it is with Blumlein. Blumlein's patent is salted with innuendoes and hints of things that should come.
Blumlein knew that his reproduction method using two widely spaced loudspeakers was flawed, but the improvement in sound reproduction over mono was so apparent that there was no need to point out in detail its theoretical imperfections, and in any case he wanted his patent to be awarded and his invention used. However, he seemingly felt compelled to indicate to his technical posterity that he really did know precisely what was right and what was wrong with the stereophonic reproduction method he was proposing. (On the recording side, he had fewer problems and proposed the coincident stereo microphone and what we now call the Blumlein shuffler, both concepts later elaborated on in Ambisonics.) Thus in a paragraph discussing the difference between low frequency phase differences and high frequency intensity differences in providing directional cues, he writes "It can be shown, however, that phase differences necessary at the ears for low frequency directional sensation are not produced solely by phase differences at two loudspeakers (both of which communicate with both ears) (parentheses Blumlein's) but that intensity differences at the speakers are necessary to give an effect of phase difference".
What Blumlein was doing here was indicating that an unavoidable defect could be a virtue in one case. That is, he could not prevent both loudspeakers from having equal access to both ears at low frequencies, (or also having a less predictable access at all higher frequencies), so he came up with a recommended coincident microphone arrangement that counted on this low frequency loudspeaker crosstalk to provide for localization in the relatively narrow low frequency band where the ear can localize only on the basis of interaural phase differences. Thus crosstalk became a necessary evil in the coincident microphoning case. What Blumlein was really saying was that if your microphones produce signals at low frequencies that don't have any phase differences, (as is the case with any coincident microphones) then the loudspeaker crosstalk could save the day but at a cost in higher frequency intensity based localization that Blumlein himself was aware of but could not fully appreciate because of the limited frequency response of the equipment he had to work with.
The way the loudspeaker crosstalk helps in the low frequency case is as follows. At low frequencies it can be assumed that any sound from one speaker will produce the same sound pressure at both ears since the head is not an effective barrier to long wavelength sounds. But the signal will be slightly delayed in getting to the more remote ear. If now there is a second loudspeaker emitting the same low frequency signal, then when this second pair of soundwaves meets the first pair it will combine with the first pair to form a new soundwave. When two waveforms, that have the same shape, but differ in amplitude and also have a fixed time delay between them, are added together, the result is a new wave shifted in phase. At one ear the louder signal combines with the delayed softer signal. At the other ear the softer signal combines with the delayed louder signal. The results are identical amplitudes but different phase shifts at each ear and thus an interaural phase difference between the ears is created that is proportional to the original intensity difference between the microphones.
Of course, if you use a more common, non-coincident microphone technique, such as a head spaced array, this crosstalk can cause localization blurring. That Blumlein understood that this unavoidable crosstalk caused imaging problems at higher frequencies is clear from some of the other quotes below. He clearly seemed preoccupied with this issue as he prepared his text. In point of fact, we know today that this loudspeaker communication with both ears makes it impossible for standard stereo or its surround sound relatives to create a fully realistic and lifelike stage image. But wait. There is much more to be gleaned from Blumlein. Blumlein's hints to his audiophile posterity continue with "the sense of direction of the apparent sound source will only be conveyed to a listener for the full frequency range for positions lying between the loudspeakers" Thus Blumlein certainly understood that the width of the stage he could create with loudspeakers was limited by crosstalk to the space between those loudspeakers, a serious defect, but one that was not crucial to Blumlein since he was largely concerned with widely spaced loudspeakers in large movie theaters or halls that had fairly narrow screens or stages in comparison to the depth of the theater.
In the context of a patent application however, this is not the sort of observation one would ordinarily include. It is easy to understand why the maximum width of the stereophonic sound image is limited to the angle the speakers subtend at the listening position. Let us assume that a single sound source such as a trumpet is located stage right at 80-degrees. Let us further assume that under these circumstances the sound reaching the left microphone in a stereo recording setup is negligible and therefore no audible sound comes from the left speaker during playback. The trumpet sound blares forth from the right loudspeaker at normal intensity. If the right speaker is at the usual 30-degree angle from the centerline of the normal stereo playback triangle, then the trumpet will appear to be sounding from that position instead of from 80-degrees. This is of course the everyday real life situation where we can easily locate the source of any discrete sound that reaches both ears without impediment.
Many of us have, however, heard recordings of stereo systems that do sometimes produce images that come from beyond the speakers and some audiophiles believe that if they could only get perfect recordings, speakers, cables and electronics, the image would open out. Blumlein was also loath to admit defeat on this point. He writes "but if it is desired to convey the impression that the sound source has moved to a position beyond the space between the loudspeakers the modifying networks may be arranged to reverse the phase of that loudspeaker remote from which the source is desired to appear, and this will suffice to convey the desired impression for the low frequency sound." (hang on to that word "low") This suggestion makes sense in a particular movie scene where you could briefly reverse the phase of one speaker to move dialog or a sound effect off screen, but we know that leaving one speaker out-of-phase all the time does not work for music reproduction via the stereo triangle.
What Blumlein was suggesting is a primitive form of logic steering thus foreshadowing Dolby Pro-Logic. But he has explained why sometimes images do appear beyond the position of the loudspeakers. Any inadvertent phase reversal of a spot microphone in the recording mix or an out-of-phase driver, or a large phase shift in the crossover network of a three or four way loudspeaker system or a reflection from the wall behind a dipole loudspeaker can convince even experienced listeners that wider stages can be achieved, somehow, using normal stereo technology. Unfortunately, logic steering, surround coding and even multi-channel recording methods cannot achieve the binaural ideal that Blumlein was striving for.
So far, Blumlein himself has told us that the stereophonic reproduction method has two inherent flaws. There is a third problem that Blumlein seems to have been aware of because of his use of the word "low" in the last quote. This is the image position distortion caused by higher frequency sounds that hit the pinnae from angles that do not correspond to the actual angles of the recorded source. Thus, perhaps Blumlein had trouble moving a birdcall off stage using his phase reversal trick. A related issue is the question of recorded ambience and here Blumlein appears to be struggling with the problem of reproducing such recorded hall ambience from the proper direction. "The reflected sound waves which arise during recording will be reproduced with a directional sense and will sound more natural than they would with a non-directional system. If difficulties arise in reproduction, they may be overcome by employing a second pair of loudspeakers differently spaced and having a different modifying network from the first pair." While the vocabulary may be a bit different, this is a pretty good description of surround sound or Ambisonics and is also the basic starting point for the ambience and imaging system I have called Ambiophonics.