Understanding the Principles--Sound Reproduction

PART ONE -- Understanding the Principles

The purpose of this guide is to understand how to design what I call "listening experiences," also known as home theaters, home cinemas, stereo systems, or just entertainment rooms. Video may or may not be a part of the experience, but audio is always there. But, what kind of audio? What sounds need to be delivered to listeners' ears in order to create the appropriate perceptions? What properties do loudspeakers need to possess in order to generate the right sounds, and what acoustical characteristics of rooms provide optimal propagation conditions for those sounds? What is it that we mean by "good sound," and is there agreement on what it is?

In this part of the guide we review the history of sound reproduction because it will help explain how we got to where we are now. Some of the patterns set years ago continue to be daily influences in what we hear, for better and worse.

We will examine the physical sound fields in rooms, and show that from concert halls to homes and cars there is a continuum of gradual change in how they are structured and behave. Part of this change is reflected in what we hear, and in what we need to measure to describe these sound fields in different situations.

A constant factor throughout all of these situations is the perceptual process.

We carry the same two ears and brain with us to classrooms and concerts, to cinemas, circuses, and jazz clubs. They are there while watching a televised football game with family and friends, and while listening to satellite radio in the car. If we understand the basic dimensions of perception, we should be in a good position to design sound reproduction systems that provide maximum gratification for listeners whatever the program and listening circumstances may be. Let us start with some definitions.

SECTION 1: Sound Reproduction

Sound reproduction. When we use these words, we assume that everyone knows what they mean. And it’s very likely a safe assumption, but to be sure, let us begin with the perspective provided by the traditional source of meanings: dictionaries. The following definitions are based on those found in Merriam Webster's Collegiate Dictionary, 11th ed.; The Shorter Oxford English Dictionary, 3rd ed.; and The American Heritage Dictionary of the English Language.

Reproduction

1. Imitating something closely, making a representation, an image or copy, of something

2. To translate a recording into sound

3. To cause something to exist again

The word has another meaning that has to do with the perpetuation of species. Although there is no known science, there is abundant anecdotal evidence suggesting that, for humans, moods cultivated by reproduced sound are often significant factors in this process. One would like to think that settings enhanced by the highest-quality reproduced sound might be the most conducive, even in these distracting situations. One could hardly imagine a better reason to pursue research in this topic than the continuation of humankind.

In definition 1, imitation requires that there is an original, a reference, and the task of the reproducing system is to create a close copy of the original. If we think of concert hall performances as the reference, convincing imitations might be challenging in small living spaces using conventional audio hardware.

However, most of the music and movies we hear are created in recording control rooms and film-dubbing stages. They are abstract artistic impressions of any live performance or sound event. This form of imitation requires a similarity of loudspeakers and room acoustics used for professional playback and in home audio systems. Anyone involved in audio knows that we are far from this ideal situation, there being significant departures from any reasonable sound quality standard on both professional and consumer sides of the audio industry.

The word closely is part of this definition. How similar does the imitation need to be? How much beyond recognizing a melody and rhythm does it go? Because the true nature of the original is almost always unknown, it’s clear that much of what is being endeavored in sound reproduction is affective-on an emotional level.

Definition 2 demands nothing in the way of sound quality, only creating sound from a recording. Cynics have suggested that this is what we all experience in much of contemporary audio. Definition 3 is even easier, requiring only that when a "play" button is pressed, definition 2 is satisfied.

The word sound has many meanings, several having nothing at all to do with music or movies, but if we isolate the meanings that do have a link, we find that sound embraces all of the following:

1. The act of making or emitting a sound

2. The physical event of sound waves propagating in a medium, air being but one of many media through which sound can pass

3. The perceptions arising from sound waves that cause the eardrums to vibrate, which can be (a) meaningless noise, (b) recorded material, or (c) the particular musical style characteristic of an individual, a group, or an area-- for example, the "Nashville" sound.

The inclusion of sound as both a physical event and a perceptual event is notable. It answers the popular riddle "If a tree falls in a forest and nobody is there to hear it, does it make sound?" The answer is both yes and no. When the tree falls, it creates sound-the physical energy-propagating away in all directions. However, with no ears in the vicinity, there can be no perception of the physical event.

To be rewarding in our context, we need to focus on sound waves propagating in air within the audible amplitude and frequency range so they can result in perceptions. This is physics, pure and simple. The specific informational content of the sound waves is irrelevant to how they propagate. Viewed as a combination of both of these generous definitions, sound reproduction is a very imprecise statement of our objectives. It seems as though almost anything can pass as "reproduced" sound, and, actually, history bears this out.

But the system works. Look around. We live in a global marketplace that is replete with sound-reproducing equipment for public venues, cinemas, homes, cars, and portable use, at prices reflecting "down and dirty" basic usage up to status-symbol level. There are vast libraries of musical recordings and movies.

Stylish periodicals in many languages feed the interest and curiosity of enthusiasts. All of it adds up to a multibillion-dollar industry. Multitudes enjoy these products. Not all reproduced sound is very refined, and it’s clear that much of the time we are willing to suspend criticism of the sound itself to just enjoy the music, movie, or whatever program instigated the sound. All of us at one time or another have felt that chill running down our spine-that tingling sensation that tells us we are experiencing something special and emotionally moving.

Was it "real"? Was it "reproduction"? Good sound or bad? Does it matter? The fact that these feelings happen confirms that the system works.

But-and this is the motivation for this guide-if any sound is rewarding, better and more spatially complex sound may be more pleasurable. This is part of the ever-evolving entertainment industry. With the application of science and good engineering, it’s reasonable to assume that we can enjoy better reproduced sound more often in more places. By understanding the perceptual dimensions and the technical parameters that give control over them, it may be possible to give the artists tools that allow them to move into new creative areas by expanding the artistic palette. If there can be some assurance that customers will hear a facsimile of the art that was created, the greater are the rewards for innovative musicians and recording engineers. Let's see where we are and how far we can go.

1. A PHILOSOPHICAL PERSPECTIVE

Before sound reproduction was possible, sound was a temporary phenomenon.

When the last audible reflection of it flew past an ear, sound was lost. Now, recorded sounds can be made to last forever. There is a complex cultural inter play between live sounds and reproduced sounds. Sterne (2003) perceptively notes that "the possibility of sound reproduction reorients the practice of sound production; insofar as it’s a possibility at all, reproduction precedes originality". Making recordings requires paraphernalia: microphones, stands, wires, electronic apparatus, recording devices, and people. It’s not a spontaneous event. It may require that performers be physically arranged in unfamiliar ways in unfamiliar rooms to optimize the pickup of the sound. The contortions required of musicians to record in the days of Edison were enormous, but even today they are significant.

We have not yet reached the stage of invisible microphones that can be introduced into a live performance so subtly as to not alter the mood or break the spell of musicians who have found a good "groove" in a familiar habitat.

Nevertheless, what can be done is impressive and hugely satisfying. Obviously, many musicians adapted to the context of performing in studio environments without an audience. Pianist and famous Bach interpreter Glenn Gould much preferred the control he could exercise in a recording studio to the pressures of performing live. He would rather be remembered for "perfect" massively edited recordings than, in his mind at least, imperfect, evanescent performances before audiences. He went so far as to predict that "the public concert as we know it today [will] no longer exist in a century hence, that its functions [will] have been entirely taken over by electronic media" (Gould, 1966).

Stravinsky offered the opinion, "How can we continue to prefer an inferior reality (the concert hall) to ideal stereophony?" (a 1962 remark, quoted in Dougharty, 1973). Milton Babbitt was similarly provocative:

I can't believe that people really prefer to go to the concert hall under intellectually trying, socially trying, physically trying conditions, unable to repeat something they have missed, when they can sit home under the most comfortable and stimulating circumstances and hear it as they want to hear it. I can't imagine what would happen to literature today if one were obliged to congregate in an unpleasant hall and read novels projected on a screen. (Gould, 1966) The point here is that "reproduction does not really separate copies from originals but instead results in the creation of a distinctive form of originality: the possibility of reproduction transforms the practice of production" (Sterne, 2003, p. 220). Knowing that the production process will lead to a reproduction liberates a new level of artistic creativity. Capturing the total essence of a "live" event is no longer the only, or even the best, objective. Movies have taken this idea to very high levels of development. It’s more than "high realism"; it includes aspects of extreme fantasy. If something can be done, someone will do it. A harpsichord, a feeble instrument, can be made to sound competitive with a 75-piece orchestra.

During a recording, microphones can sample only a tiny portion of the complex three-dimensional sound field surrounding musical instruments in a performance space. What is captured is an incomplete characterization of the source. During playback, a multichannel reproduction system can reproduce only a portion of the complex three-dimensional sound field that surrounds a listener at a live performance. What is reproduced will be different from what is heard at a live event.

Audiophile fans of "high culture" music have repeatedly expressed disappointment that what they hear in their living rooms is not like a live concert, implying that there is a crucial aspect of amplifier or loudspeaker performance that prevents it from happening. The truth is that no amount of refinement in audio devices can solve the problem; there is no missing ingredient or tweak that can, outside of the imagination, make these experiences seem real. The process is itself fundamentally flawed in its extreme simplicity. The miracle is that it works as well as it does. The "copy" is sufficiently similar to the "original" that our perceptual processes are gratified, up to a point, but the "copy" is not the same as the "original." Sterne (2003) explains that "at a very basic, functional level, sound-reproduction technologies need a great deal of human assistance if they are to work, that is, to 'reproduce' sound".

Sound reproduction is therefore significantly about working with the natural human ability to "fill in the blanks," providing the right clues to trigger the perception of a more complete illusion. It’s absolutely not a mechanical "capture, store, and reproduce" process. In addition to the music itself, there is now, and probably always will be, a substantial human artistic, craftsmanship, component to the creation of musical product.

Sterne (2003) goes on to explain that "as many critics of film and photography have shown us, reality is as much about aesthetic creation as it’s about any other effect when we are talking about media". And, in the context of sound recording, "far from being a reproduction of the actual event, the recording was a 're-creation'". The goal is not imitation but the creation of specific listener experiences. This certainly exists dramatically in the directional and spatial experiences in reproduced sounds.

For decades, society has been conditioned to derive pleasure from first single channel sound (mono) and then two channels (stereo). Only recently has music been offered in multichannel formats that permit a somewhat realistic directional and spatial panorama. Impressed by the novelty that music and movies were available on demand, society appeared to lower its expectations and adapted to the inadequate formats. A great deal of enjoyment was had by all. So complete is this form of adaptation that significant new technical developments must go through a "break-in" period before there is acceptance. Having abandoned or forgotten the sound of a three-dimensional sound field, those who grew up with mono often argued that stereo was an unnecessary complication, adding little value. (I remember-I was there!) The same is now happening with respect to multichannel audio schemes. Part of the "break in" applies to the audio professionals, who must learn how to use the new formats with discretion and taste.

In terms of sound quality-fidelity-there have been claims of perfect sound since the very beginning of sound reproduction. In the earliest days, it seems that audiences were simply so amazed to be able to recognize pitch, tunes, and rhythm that they ignored huge insults to sound bandwidth, spectrum, dynamics, and signal-to-noise ratio. Now we do much better, of course, but then, as now, according to Sterne, "sound reproduction required a certain level of faith in the apparatus and a certain familiarity with what was to be reproduced". Expectations are a part of our perceptions-a fact well used by advertisers of audio appliances from the earliest times, and today. A 1908 advertisement for Victor Talking Machines asserted, " You think you can tell the difference between hearing grand-opera artists sing and hearing their beautiful voices on the Victor.

But can you?" (Sterne, 2003, p. 217). The formula must work because, as this is being written 100 years later, boasts of sound quality still abound: "Everything you hear is true"; "Pro sound comes home" (www.jbl.com); "True sound" (www.bowers-wilkins.com); "Pure, natural, true-to-the-original performance" (www.bostonacoustics.com). The suggestion that audio hardware is capable of a kind of acoustical transmigration is clearly attractive.

The introduction of digital audio sent ripples-actually, more like a tsunami- through the audio community. On the one hand were those who claimed that the perfect utterly transparent storage device had arrived. On the other hand, audio critics were hearing flaws. Although irritating swishes, ticks, pops, clicks, wow, and flutter were pleasantly absent, there were other problems. It turned out that early digital hardware did not always perform to claimed specifications.

Arguments also went metaphysical, with assertions that in converting audio signals to numbers, a crucial link to the original sound was lost. The passage of time has brought immense improvements in all aspects of performance. Now, in a new calm, we discuss just how much digital bandwidth is needed to store "all" the music.

There is cause for some sadness, though, because some of the old analog tapes being converted for delivery through digital formats are not the original masters, which were lost or reused, but the LP master tapes used to drive the cutter heads for creating LP master discs. It seems that important decision makers thought the LP was the final development in the delivery of audio signals. These tapes have been skillfully manipulated--predistorted, in fact-to compensate for some of the significant limitations of LPs and therefore cannot sound their best when played through digital devices. The art has been compromised. This is an example of an old technology executing a strange form of revenge on the new. Perhaps there is something to the metaphysical argument after all.

2. RECORDINGS AND THE MUSIC BEING RECORDED

Finally, it’s worth noting how sound reproduction has influenced music itself, especially jazz. Because they offer perfect repeatability, recordings became learning aids for musicians. According to Katz (2004), the widespread availability of recordings of major artists "gave budding musicians unprecedented access to jazz. Without this feature of recording technology, some jazz artists might never have pursued their careers". This is good, of course, but recordings of the pre-LP era had a severe restriction on playing time: a ten-inch, 78 rpm record had a maximum 3 m 15 s playing time. This forced a change in playing style, and long jazz improvisations were abridged for recordings. After much repetition, these improvisations became ritualized in performances of other artists, so they were no longer improvisations.

Even the vocal content changed because of recordings. Morton (2004) states, "Nearly all the early jazz captured on records was 'cleaned up' for white audiences. Live jazz was improvised and disorderly. Its lyrics and themes often had sexual overtones".

The early recordings that were formative in the development of jazz also suffered from recording difficulties. Spectral and dynamic limitations did not flatter instruments like pianos and drums, so substitute instruments were used.

Live performances came to reflect some of these substitutions and sometimes even playing styles. For example "slap" bass playing was a means of minimizing bothersome low-frequency output but retaining some of the essential sound of the upright bass. Said Katz (2004), "The bass drum was a troublemaker even into the 1950s". (Wrapping it in a blanket was a common studio remedy.) Katz explained as follows:

Whether in France, the United States, or anywhere in the world, most listeners who knew jazz knew it through recordings; the jazz they heard, therefore, was something of a distortion, having been adapted in response to the nature of the medium. The peculiar strengths and limitations of the technology thus not only influenced jazz performance practice, it also shaped how listeners-some of whom were also performers and composers-understood jazz and expected it to sound. Recordings also had significant effects on classical music. In a live performance, we wait intently while a musician pauses, lifts a bow, and leans forward to continue a work. In a recording, lacking the visual input, such a delay is "dead air." Recorded music has, accordingly, better continuity. When sound emerges from the violin, it will likely be played with more vibrato than was customary in earlier times. Katz (2004) makes the case that this is linked to influences of sound recording, and he includes a CD of some examples with his book.

Today, we make recordings from parts of other recordings. LP-era disc jockeys developed the technique of "scratching"-manually moving a disc under a phonograph cartridge, thereby repeating, reversing, and varying the speed of a musical excerpt. Now, in the original or in digital incarnations, it’s used as a component in some kinds of recorded music. Digital "sampling"-excerpted recordings of anything that makes sound-is an enabling technology for some digital keyboard instruments, and, in editing, it’s an ingredient in much popular music. Some compositions are clever compilations of nothing but sampled sounds. In musical terms, this is abstract or surreal art with no "live" equivalent.

It’s totally a studio creation.

And so it will continue in the unending interplay between musical creation, reproduction technology, and listener expectations and preferences. Katz states, "Recording is not a mysterious force that compels the actions of its users. Ultimately, they-that is, we-control recording's influence. Recording has been with us for more than a century; it will no doubt remain an important musical force, and users will continue to respond to its possibilities and limitations". The technologies discussed in the rest of this guide are part of the future of our audio industry, our music, and our movies. We will see that there are several ways to improve the process, ensuring a superior and more reliable delivery of the art we know. We will also examine ways to introduce new ingredients into the art.