Being the trials and tribulations of a stereophone and loudspeaker designer
during his quest for the ideal pair of stereo phones.
Article by Jacob C. Turner [Vice President-Engineering & Research, Koss Corporation]
For longer than man has known about the spherical shape of the planet on which he lives, he has argued about the relationship between subjective and objective reality. Stereophone makers and users alike are presently stewing in that same juice.
Let me attempt to bring this issue into perspective and show how it is acutely relevant both to the audio engineer and consumer.
This is no new conflict. Over 2300 years ago the Greek philosopher Democritus wrote: "Sweet and bitter, cold and warm, as well as all the colors; all these things exist but in opinion and not in reality."' Many centuries after, the German mathematician Leibnitz wrote: "I am able to prove that not only light, color, heat and the like, but motion, shape, and extension too are mere apparent qualities.'" Albert Einstein carried this train of logic further by showing that even space and time are forms of intuition, which can no more be divorced from consciousness than can our concepts of color, shape or size.
These seeming subtleties of philosophy have had a quite profound bearing on modern science. For along with the philosophers' reduction of objective reality to a shadow-world of perceptions, scientists later became alarmingly aware of the limitations of man's sensory ability and subsequently the equally limited scope of "objective" knowledge about our world, since it can only be perceived through this sensory veil.
In spite of our apparent sensory imprisonment, however, a curious order seems to run through our perceptions, as if indeed there might be an underlayer of objective reality which our senses are able to translate.
Therein lies the dilemma.
The aim of science is to describe and explain objectively the world we live in. And yet, even the instruments and tools that the scientist creates and employs in his search for objective reality are, in fact, presuppositions of what that reality is. Ironically, the vast bulk of technology involved in modern electronic equipment (and in audio) is directly related to two natural phenomena which we are not yet able to explain, electricity and magnetism.
We know how to use some of their effects but we don't know what they really are, or indeed if they are really "something." In other words, guided by our perceptions of reality, we have amassed an enormous and constantly expanding body of knowledge and technology that we have learned to utilize in controlling and changing our lives.
It seems reasonable to conclude two points from the above. First, that man's primary understanding about the nature of his world is the result of sensory perception; second, that the discipline of modern scientific practice provides organization, integration and extension of all man's perceptions into a more manageable and consistent body of knowledge and technology which would otherwise be unthinkable. Viewed together, they create a highly synergistic combination of forces that is properly descriptive of modern man; viewed in isolation, each one individually is medieval and parochial.
Nowhere is this necessary relationship displayed more dramatically than in our own brain, where the objective scientist in all of us (left hemisphere)
is joined together with the subjective intuitor/artist in all of us (right hemisphere) to form a complete functional mind.
Few areas of modern industrial society present greater opportunities for implementing this complete view of knowledge than is presented to today's audio component designer, and particularly the stereophone engineer. Concerned primarily with equipment design and production, men in this area of the audio business are still very intimately involved with both the performing arts and the recording industry, that is the music business. And yet, unlike the performing musician, the audio designer is at once equally the artist and the scientist-the artist in that the designer's objective is musical pleasure and abstract communication; the scientist in that the designer's means to the foregoing objectives, of necessity, involve high technology.
Herein, however, lie several problems which hinder our ability to judge the accuracy of our efforts definitively.
Measuring With Music
To begin with, the entire structure of musical sounds and the manner in which they are produced are manmade and arbitrary, differing from culture to culture and from age to age. There are no fixed musical references in nature against which objective and subjective standards can be drawn, either for instrumental quality or musical interpretation.
As a result, the instruments which produce the musical sounds have such individual sonic personalities that there is, for example, no reference violin against which all other violins can be measured. In addition, no two persons produce the same sounds on the same instrument on any two successive playings of the same piece of music. Add to this dizzying scene the additional complication of the acoustic personality of the space in which a given instrument is being played and you end up with a very complex situation, ignoring for the moment the equally complex results of the inevitable recording pro cess itself if a record is to be made of a performance.
How, then, is the subjective in tuitor/artist portion of our stereophone designer to evaluate musical reproduction of his latest "baby?" Can he simply sit back and say, "Yes, this pair of phones makes this recording sound like Menuhin is truly playing in Boston Symphony Hall on Thursday, March 3?" But is it even possible for the designer to portray the true sonic reality of that Boston performance? What musical criteria are at all relevant? Which ones can be agreed upon by both designers and the buying public? Can any be agreed upon? Victor Campos (then of AR, Inc.) once claimed that in attempting to analyze how sound behaved in Boston's Symphony Hall that it took his computer from 12:00 noon until 4:15 in the afternoon just to plot all the reflections and delays from one quadrant in Symphony Hall.' The generation of most musical sounds is a complexity of constantly changing variables, with no fixed standards of reference and low predictability both as to execution and reception. It is not presently possible, therefore, to electronically duplicate and analyze a reference violin sound, for example, with any reasonable degree of accuracy or meaningful correlation to the real world of violins.
Neither electronic engineers nor stereophone engineers have yet developed any definitive test or set of non-musical signals, such as white noise, pink noise, tone bursts, square waves, sine waves, warble tones, etc., that produces high correlation between measured performance and reproduced musical performance of audio products.
In the absence of such a definitive objective test, therefore, the final arbiter of truth in high accuracy stereophone reproduction is-of necessity-subjective judgment. But is this a fault or a find?
Psychoacoustics and Subjectivity
Let us admit at the outset that the experience of listening to music is one of man's most personal activities and certainly involves communication of the most subtle feelings. Emotion, passion, sentiment, as well as thought, can be projected via that wave motion we call sound. To expect that such complex experiences can be reduced to physical formula is in some ways naive. The difficulty of high accuracy objective test correlation to reproduced musical performance is further enlightened by a cursory examination of some unique aspects of man's hearing characteristics.
To some degree, we hear what we expect to hear. Experts in psychoacoustics have confirmed this by playing the same passage of music coupled with loud white noise over and over to a group of stereophone listeners and gradually reducing the music level to zero until only the white noise remained. Almost all the subjects always continued to "hear" the music long after it had been turned off. The results of this and other similar experiments lead the honest, critical designer to conclude that he must keep a wary mind when performing subjective testing, lest he "hear" last week's concert instead of today's replay.
Many of the most striking phenomena of hearing have to do with the interaction of tones or sounds heard simultaneously. There is a law of acoustics called Ohm's Acoustical Law, which states that when we are exposed to two tones simultaneously we have the distinct sensations characteristic of hearing each tone separately.' Although this is true for superimposed sinusoidal waves of 0' relative phase, it is not true in situations where we are listening to a complex sound made up of sinusoidal components of many frequencies in which the crests of the component sine waves have different relative phases.' These two conditions would produce vastly different sounds',' contrary to Ohm's Law. The design of single driver acoustic products, such as stereophones, is strongly affected by this consideration. Such complex wave forms covering the audio spectrum can be reproduced without measure able waveform deformation only when the driver utilizes either a flat diaphragm (such as the Koss Models ESP-9B and Auditor/ESP-10) or the unique contouring of conventional moving-coil diaphragms (such as that used in the Koss Pro/4AA and Auditor/Dynamic 10). In listening to two sounds close together in pitch, we are strongly aware of the phenomenon of beats. When we listen simultaneously to two sounds of comparable magnitudes but of slightly differing frequencies, the resultant sound appears to be a fluctuating sound of a single frequency. The frequency with which the amplitude fluctuates is the difference between the frequencies of the two beating sounds.
In the case of very faint sounds, this beating phenomenon disappears if the sounds are more than 20 to 30 Hertz apart in frequency. In loud sounds, however, we hear the beat as a separate, fainter sound of the difference frequency. (Now that would tend to confuse your listening session!) Closely related to the phenomenon of beats is another hearing curiosity called masking, or the obscuring of one sound by another of a different frequency. The curious thing here is that if the masking sound is loud, it produces beats with other tones having its harmonic frequencies as well as with other tones of the same frequency. In addition, the relative ability of one sound to mask either a higher pitched or lower pitched sound changes with level. For example, in listening to a complex sound containing frequencies of 400, 300 and 2000 Hertz at respective levels of 50, 10, and 10 dB above threshold, the loudest sound would so mask the 300 Hz sound that the ear would hear only the 400 Hz and 2000 Hz sounds. If all the sounds are raised by 30 dB, however, then only the 400 Hz and 300 Hz sounds would be heard. Thus the quality of this complex sound would change markedly with loudness. (Now you hear it, now you don't.) It should be apparent that masking is extremely important in considering what we hear on stereophones. Because of masking, we may be completely unable to detect some of the fainter frequency components of a complex sound, under certain conditions of playback level, and can also be influenced by outside sounds when using hear-through stereophones, more so than sealing stereophones. Knowledgeable audio buffs are correct in maintaining that high accuracy sound quality can be reproduced faithfully only if the original loudness for that particular recorded perspective of the musicians is accurately re-created. Surely the subjective appraiser of high quality musical reproduction must consider all the above and many more hearing peculiarities in arriving at honest judgments, lest he fault the reproduction equipment for user indiscretion.
Let me emphasize that the phenomena of beats, masking, and loudness perception do not exist outside of the listener's head. In other words, these hearing peculiarities reflect non-linearities in the ear's response mechanisms to auditory stimulation and constitute built-in subjective distortion.
A further serious difficulty in developing objective and subjective standards of reference for musical reproduction lies in the fact that like his wealth, man's level of auditory expectation rises with his level of auditory achievement. In other words, the human "ear" never stops learning how to hear better, and even persists in hearing in some notables (e.g. Beethoven) long after the "microphones" have been unplugged.
The ear, apparently, like the other cerebral functions, operates on the basis of pattern recognition, that is patterns of sound. This may help to explain why most people find simple sounds such as sinusoidal waves to be very uninteresting, since the most interesting sounds to us are complex sounds and complex sequences of sounds.
Joining The Halves
With regular exposure, the human ear is capable of consistent and seemingly endless improvement in its ability to analyze complex sounds. At the same time, becoming ever more intolerant of minor aberrations in performance which would have been perfectly acceptable only weeks before. This characteristic makes it exceedingly difficult to establish any meaningful long term standards for analyzing state-of-the-art developments in musical reproduction.
The uncommonly complex demands placed both on recording technology and product design by the unrelenting pursuit of higher accuracy reproduction on the part of the audio enthusiast over the last ten years has spurred several major advancements in audio quality. As more and more information is generated concerning the intricacies of human hearing, the nature of three-dimensional musical space, the causes and effects of linear and non-linear distortion, etc., the need for meaning and consideration of long-term consumer benefit in the application of our expanding knowledge into worthwhile products will require a careful balance of our scientific and artistic best efforts. How else can we adequately serve an industry that demands excellence in both areas? This author argues not for an anarchy of individual audio perceptions, nor for the tyranny of technical specifications, but rather the judicious and intelligent use of both in designing and evaluating high accuracy musical reproduction equipment. The final goal, of course, must be to provide the highest level of musical satisfaction possible under the circumstances. That is our business.
References
1, 2 Lincoln Barnett, The Universe and Dr. Einstein (Bantam Books, Inc., New York, 1972).
3 "Great Quad Almighty!" The Boston Phoenix, Oct. 7, 1975, p. 30.
4 William A. Van Bergeijk, Waves and the Ear (Doubleday, 1960).
5 D. Preis, "Linear Distortion," J. Audio Eng. Soc., vol. 24, pp. 346-367 (June, 1976).
6 V. Madsen and E.R. Madsen, "On Aural Phase Detection," J. Audio Eng.Soc., vol. 22, pp. 10-14 (Jan. -Feb., 1974) .
7 R.J. Matthys, "Telstar-Shaped Electrostatic Speaker-In Two Parts-Part 2," Audio, vol. 48, no. 6, p. 28, June, 1964.
(Source: Audio magazine, May 1977; )
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