Part III: Can You “Hear” This? Re-Examining the Limits of Auditory Perception
To better understand why the designs of Rupert Neve have had such a profound impact on the world of recorded music, it will be helpful to look more closely at some of the ideas which he, and a handful of others, took as the basis for their work in the 1990s.
These ideas were not exactly new. Audio enthusiasts had recognized their validity as far back as the late 1950s but widespread acceptance and application was slow in coming. Mr. Neve began incorporating them into his designs in the 1970s and by the 1990s he was able to combine his own experiences with newer scientific research, becoming a leading participant in a movement that demanded a critical re-examination of the limits of human perception.
By the turn of the century a chorus of leading audio engineers and equipment designers had taken up the catchy slogan that “life beyond 20 kHz” was indeed a musical and perceptual reality.
This re-examination of auditory perception met with considerable opposition in both consumer and professional publications. Those who insisted on the need for bandwidth out to 40 kHz and beyond were all too often characterized as misguided purveyors of audio mythology. One writer exclaimed after reading Mr. Neve’s statement that there is evidence that signals as high as 100 kHz are used by the brain to add fullness and enjoyment to music:
Download the full essay (PDF)
“Perhaps AMEK should explain whose brain can be so addled with fullness from signals that are at least an octave higher than the average ultrasonic garage door transmitter! Come off it. Who are you kidding! It’s this sort of codswallop that keeps alive the myths and legends of the hi-fi brigade!”
A much more technical columnist wrote in a 2000 issue of TV Technology regarding a discussion at an Audio Engineering Society meeting on psychoacoustics, concerning a proposal for a new ultra-wide bandwidth digital format:
“Does anyone here know of any studies that convincingly show that humans hear above 20 kHz.? And we all just sat there. Not one of us knew of anything like that. So much for what the people who are really interested in what, and how, we can hear tell us. Above 20 kHz? No evidence, zero, zippo, zilch.”
Battle lines were clearly drawn. One of the most respected columnists in the Pro Audio press, with an impeccable background (then and now) in academic recording engineering programs carefully reviewed Mr. Neve’s experiments and the articles that were being widely cited by proponents of life beyond 20 kHz. He concluded in a May 2002 column:
“So, it seems like the audio world above 20 kHz… will probably not turn out to be very important. As I said, there may be other reasons why high sampling rates are helpful… but not because your old technology and mine, is missing anything.”
David Blackmer, the developer of the DBX voltage controlled amplifier, began his own research into the capabilities of human hearing in the late 1980’s. In 1998 he published a groundbreaking article “Life Beyond 20 kHz” which summed up the obstacles that he, Rupert Neve and others faced when presenting the case for a an expanded concept of human auditory perception:
“Many engineers have been trained to believe that human hearing receives no meaningful input from frequency components above 20 kHz. I have read many irate letters from such engineers who insist that information above 20 kHz is clearly useless, and any attempt to include such information in studio signals is deceptive, wasteful and foolish. They assert further that any right-minded engineer should know that 20 kHz has been acknowledged as an absolute limitation for decades. Those of us who are convinced that there is critically important audio information to at least 40 kHz are viewed as misguided.”
Practical evidence for an expanded understanding of human auditory perception had been accumulating for 40 years, as Mr. Neve knew all too well. His designs in the 21st Century would prove the validity of this approach beyond question
Yet even today, hundreds of reference books and articles will inform readers that the commonly stated range of human hearing is 20 to 20,000 Hz. Most of them refer back to the Fletcher and Munson graphs of human hearing experimentally developed in the late 1920s and early 1930s. More recent research allows that under the best laboratory conditions some laboratory conditions, some humans can hear sound as low as 12 Hz and as high as 28 kHz. But as far back as the 1860’s the renowned physicist Helmholtz defined the musical spectrum as extending up to 40,000 cycles.
He acknowledged that people don’t “hear” that high but insisted that they do respond to sensations involving those frequencies. Designers of the circuitry for audio equipment have always grappled with the fact that in listening tests amplifiers with similar specifications for noise, harmonic distortion and frequency response within that 20-20K range of human hearing often sounded very different from one another. As high fidelity equipment became increasingly sophisticated in the 1950s, some designers realized that extending linear response above and beyond the 20-20K absolutes seemed to elicit a positive response from listeners. Along with this came the realization that there were subtle distortions factors, not easily measured at that time, which impacted the true and pleasing reproduction of music
Shortly after stereo LPs were introduced in 1958, the pioneering record label, Audio Fidelity, claimed a total frequency range of 16 to 25,000 cps on their vinyl records stating that while this “may not be within the range of ordinary human hearing… it is the opinion of the manufacturer that if these frequencies were omitted from this record a certain warmth of tone that is felt and sensed rather than heard would be lost.” Likely they were influenced by published interviews with the American designer of amplifiers and loudspeakers, Stewart Hegeman. who in the late 1950s developed a radical theory of amplifier design based on ultra-wide frequency response.
He argued that an amplifier’s frequency response should extend far above and below the commonly accepted 20 to 20,000 kHz range. and he proved the validity of this approach with his acclaimed Citation designs for Harman-Kardon. The Citation I preamplifier boasted a frequency response from 5 Hz to 80 kHz. His Citation II stereo amplifier, introduced in 1959, is one of the most highly prized vintage tube amplifiers, with restored models selling for as high as $3500. Among other things the Citation II boasted a full power bandwidth of 18 Hz to 60,000 Hz and output transformers that were capable of high frequency response out to an astonishing 270,000 cycles.
Almost 60 years after its introduction, amplifier designer Bob Carver described the Citation II with the same sort of superlatives that audio engineers use when describing vintage Neve consoles:
“The Citation II was Stu Hegeman’s penultimate masterpiece, inspired by his unwavering desire to build a power amplifier equal to his stunningly great Citation I preamplifier. I consider the Citation II power amplifier to be one of the most original designs in audio history, introducing technology that had never before been seen in an audio product. The Citation II’s high-gain, wide-bandwidth video-pentode design yielded performance that was difficult to believe. Stu Hegeman’s circuits produced a sound that had a life and a breadth and image depth that were stunning. They were the ultimate in sound staging and sense of immersion. The Citation ll’s circuit was most elaborate, the thinking behind it exceptionally advanced, the sound spectacular!”
Sheldon Stokes, a design engineer wrote:
“When the passive components in this amp are updated, this amp sounds just amazing. It produces music with an authority and an ease that is unequaled in the audio world. It tends to have a dry and “correct” sound rather than a “tube amp” sound. …If you want your music served up honestly with a bare minimum of distortion this may be the amp for you.”
Hegeman responded to questions about his wide-bandwidth approach in a 1961 interview:
“A rigorous approach to sound reproduction should include two octaves below and two octaves above those 20 to 20,000 cycles limits, that is about 2 to 80,000 cycles. The extreme bass contributes what you might call the “sound’ of the hall, the aura or the ambience of the environment of live music. The extreme highs contribute to the space and the air of the sounds produced in that environment.”
Hegeman believed that tones which could not be heard in isolation still could influence what was perceived by listeners – that there were factors when limiting bandwidth and excluding these signals which created distortions in the listening experience, much in the way a seawall throws back the surf and creates interference with the oncoming waves. He also believed in the critical importance of minimum phase shift or time displacement within the elements of complex signals. When the relationship between overtones at the frequency extremes and fundamentals was changed the result was a sound perceived by most as less lifelike and pleasing.
Stewart Hegeman recognized that reproduction of complex musical information required amplifiers with extremely wide-band response and phase shift reduced to a bare minimum. He focused on both wide response and eliminating the un-measurable (in the 1950s) forms of transient distortion which imparted an unnatural, brittle quality to sound that he described as canned or electronic-sounding, lacking both clarity and sweetness. As acclaimed as his products were, a NY Times article in 1990 noted:
“It took nearly two decades for Hegeman’s wide band principle to be recognized as essential for low distortion amplifiers… Much later, as suitable measurement methods became available, controlled experiments at the State Technical Research in Oulu, Finland and at the University of Osaka, Japan bore out Hegeman’s findings.”
In Finland and the Netherlands researchers began extensive research in the late 1960s which focused on those transient intermodulation distortions that Hegeman had worked hard to eliminate in his designs. Finnish designer Matti Otala’s research culminated in a landmark technical paper published by the Audio Engineering Society in 1972. His views were highly controversial at the time but within a few years other noted amplifier designers such as John Curl had joined him in the quest to describe and measure what came to be known as slew induced distortion.
The results could be clearly be seen in better sounding amplifiers by the end of the decade. Harman-Kardon, the company whose Hegeman designs had revolutionized tube amplifiers, collaborated with Matti Otala on a solid-state amplifier that would achieve similar fame. 1980 brought the introduction of the Citation XX high current amplifier which boasted stunningly quick transient response and a lifelike, open sound. The Citation XX amplifier was called “the world’s best-sounding power amplifier” by the editors of The Audio Critic magazine.
We cannot state with any certainty the degree to which Mr. Neve was familiar with developments in American high-fidelity equipment during the 1950s. But his work in the 1970s,1980s and 1990s certainly echoed and expanded those earlier concepts. From the earliest days of the Neve company, Mr. Neve had also learned from listening tests that his customers responded well to circuitry with flat frequency response that extended far beyond the 20-20 kHz absolutes. Early Neve consoles were designed with bandwidth and linearity well in excess of the figures considered necessary at the time.
Mr. Neve noted that “the old text books would always tell you never to provide a bandwidth in excess of that which was strictly needed.” But based on listening tests, he started to extend the bandwidth of some of his circuits and by 1970 Neve consoles had response out to 35 or 40 kHz. Some accused Neve of over-engineering, but Neve felt that listening supported the idea that the wider bandwidth circuitry “sounded” better.
The Neve designs garnered great praise in the early 1970s from leading engineers and producers, but there were some recording engineers in the USA who were not overly enthusiastic about the sound of Neve consoles. Initially Rupert believed that response out to 40 kHz would prove “more than adequate.” He was therefore puzzled when he received comments from American engineers that a newly delivered console sounded muffled. Rather than dismissing the criticism, as many would have, Rupert began an intensive round of listening to various circuits, with and without transformers, and he began to further extend the bandwidth of some of the circuits in his designs.
Rupert Neve’s epiphany came in 1977, not long after he had sold the NEVE company. AIR Studios had taken delivery of a new NEVE console and during its installation and testing engineer Geoff Emerick wasn’t at all happy with it although he could not be very specific about that the problem was. A team of NEVE engineers was sent to check it out but they found nothing out of the ordinary. They wrote it off to Emerick’s personality and concluded it would be best to just do nothing and eventually the “problem” would go away. Sir George Martin intervened and pleaded with Rupert, “Please come and make Geoff happy, while he’s unhappy we can’t do any work.” So, Rupert Neve himself sat down with Emerick at the console, set up a tape playback and compared the signal directly from the source with the signal emerging from the console’s signal path. At first Mr. Neve couldn’t hear any problem and Emerick could only say it seemed that the console path sounded “brighter.” With additional listening, Mr. Neve, too, could perceive a difference.
The testing continued on a channel by channel basis and Emerick was able to identify a channel module had that slightly brighter sound. Upon examination it was found that the channel had an improperly terminated transformer which was producing a 3dB rise at 54 kHz. Geoff Emerick was able to correctly identify two more modules that had a similar mis-wiring. Rupert Neve commented in a number of interviews in the 1990s:
“Theoretically that should be outside the range of human hearing, but we both perceived it in the same way, even if we didn’t “hear” it in the traditional sense. Did the brightness exist? Absolutely, although a team of engineers was unable to determine the cause for Emerick’s displeasure and therefore said it didn’t exist… This started me wondering how so many people could hear something that was out of band when conventional wisdom stated that anything above 20 kHz should be inaudible.”
The following year NEVE began work on a new console (A4792) for Sir George Martin’s new AIR Studios on the Caribbean island of Montserrat. The AIR Montserrat studio opened in July 1979. Three “Montserrat” consoles were eventually produced for AIR and Rupert Neve believed that these consoles represented a “substantial step forward” in terms of bandwidth, noise and distortion. The bandwidth was well in excess of 100 kHz. Rupert recalled with great pride Geoff Emerick’s reaction when he first recorded and mixed on the “Montserrat”: “Rupert – It sparkles!” So many famous names would echo these sentiments in years to come and these consoles have achieved an iconic status,
When Rupert Neve returned to designing modules and consoles after 1985, the extended bandwidth of the Montserrat design was the foundation upon which these were built. He was determined to know more about the seemingly un-measurable aspects of human perception and to confirm whether harmonics falling outside the audio band could make a difference to the audible sound. He was not content just to design better sounding equipment for creative artists and their engineers. He was fond of quoting the great physicist, Lord Kelvin, “To Measure is To Know.”
His initial tests were deceptively simple, requiring little in the way of specialized equipment. With an oscillator he would switch between sine waves (the simplest of all waveforms containing a single fundamental frequency with no harmonics or overtones) and square waves (the fundamental frequency and its odd harmonics). Conventional wisdom is that as the frequency rises sine waves and square waves begin to sound more and more identical, as the third harmonics begin to move “outside” the range of human hearing.
So with a 4 kHz tone the third harmonic at 12 kHz is getting hard for many to hear and by the time you get to an 8 kHz tone that third harmonic is at 24 kHz and almost no one should be able to “hear” it. But Rupert found that if you go on raising the frequency, listeners could still tell the difference between a sine wave and a square wave, even though you have exceeded the accepted limit of hearing.
Rupert began to conduct public experiments in 1987 starting with an audience at the Institute of Broadcast Sound. Being broadcasters, most present would have an ingrained skepticism about the ability to hear signals above 15 kHz. Yet 30 to 40 percent of those present could still hear a difference between sine and square waves when the fundamental was around 12-14 kHz. Rupert repeated the public experiments many times in the years that followed, in different venues around the world and often using other peoples’ equipment. The results were consistent from demonstration to demonstration.
These experiments were especially impressive when the audiences consisted of younger professionals who had been trained in critical listening. Some 60% of those present in those younger audiences were able to distinguish a difference between sine and square waves when the fundamental was as high as 10 kHz. Some in that age group could hear fundamentals out to 18-19 kHz. and they could still detect differences between the sine and square waves indicating an ability to perceive harmonics up to almost 60 kHz!
Can You “Hear” This? High Resolution Audio in the 1990’s
Rupert Neve and other leading equipment designers spearhead a movement for High Resolution Audio which would challenge the audio industry to accept an expanded concept of human auditory perception, leading to new standards for audio quality in the 21st century.