Guide to Sound Reproduction--Introduction

Reproduced sound is everywhere. In the author's life, a tiny portable audio device supplies podcasts and music while he walks his dog. Premium car audio systems provide superb surround sound on the road. A whole-house system lets us have the music of choice in most rooms and in the garden. A seven-channel entertainment room system provides music and audio accompaniment to television and movies on large direct-view and front-projection screens.

It’s all very high-tech, all very convenient, and all very enjoyable. It’s also all very good.

Audio-sound reproduction-engages both the emotions and the intellect.

Understanding the process is challenging because it embraces domains with enormous contrasts: human perceptions in their manifold dimensions and technology with its own system of devices, functions, and performance descriptors. The subjective side is notable for its complexity, flexibility, adaptability, and occasional capriciousness. The technical side is characterized by the near absolute reproducibility of the devices, the stability of their performance over time, and the reliability of their measured parameters. The interface of these two cultures has met with mixed success over the years. Both sides seek excellence in the final subjective experience, but there are fundamental differences in philosophy, metrics, languages, and the economic and emotional attachments to the results.

In the midst of all this are the recording and broadcast industries that generate program material for our sound reproducing systems: all manner of music, verbal discourse, and the audio accompaniment to movies, television, and games. These programs are artistic creations, both in their informational content and in the timbral and spatial aspects of the sound. As consumers of these pro grams, we cannot know what was intended for the sound of any of these programs. We were not there when they were created. We may have been at performances by similar, or even the same, musicians, but they were likely to have been in different venues and possibly amplified. None of us ever placed our ears where the microphones were located to capture the sounds, nor would we want to; we were almost certainly at a distance, in an audience. A simple reproduction of the microphone signals cannot duplicate the experience. That is where the professional recording industry steps in.

Microphones are selected from the many that are available, and they are carefully positioned relative to the performers; this is the first level of signal processing. The signals are stored in multiple tracks that are then mixed together in proportions that create a combined sound that is pleasing to musicians and recording engineers listening through monitor loudspeakers in a small room: a control room in a dedicated facility or a home studio. This is the second level of signal processing. While doing this, various knobs are turned, electronically or physically, to add or subtract energy at certain frequencies and to add multiple delayed versions of sounds in the tracks, all designed to enhance the sound: the third level of signal processing. Finally, a mastering engineer transfers the completed "master" recording to the delivery medium, CD, DVD, and so on, and almost always will make further changes to the program: the fourth level of signal processing. In LP mastering, the changes are substantial: mono bass, dynamic compression, rolled-off highs near the center of the disc, and so forth to cope with the limitations of the medium. Digital media need no such dramatic manipulations, but mastering engineers may choose to tailor the sound based on what they hear through their own monitor loudspeakers in their own rooms, and possibly to adjust the dynamic range and bandwidth to be suitable for the intended audience. That is four clearly defined opportunities for signal processing of some kind; everyone in this chain of events is authorized to fine tune the result. None of this is a problem. This is the creation of the art. It only becomes a problem when somebody, somewhere, presses a "play" button and what they hear is not the same as any of the preceding circumstances. Sadly, this is the norm.

The point of this story is that the program is a huge variable in the process of sound reproduction and that sound reproduction (in the control room, home studio, or mastering lab) is a factor in the creation of the program. This situation is well described as a conundrum; it certainly is not a linear, stable, and predictable process. So for the vast majority of recordings, the sound delivered to our ears is a new experience. It’s nothing we have heard before, allowing us to establish even a faulty memory for reference purposes. Knowing that the playback devices are different from those used in the creative process, we cannot be certain what is responsible for what is heard; if we hear something we don't like, is it because it’s in the program itself? Is it something in the playback system that has been revealed by the program? Or is it an unfortunate destructive interaction that is unique to these two factors and may not happen in other circumstances? In normal listening situations, we cannot know and are there fore left in a position of forming opinions on the basis of whether we like the combination of content (tune, musicianship, etc.) and the sound (timbre, directional and spatial impressions, etc.) and whether it moves us emotionally (how it "feels").

The origin of emotion in a listener is the art itself-the music or movie--and not the audio hardware. It’s inconceivable that a consumer could feel an emotional attachment to a midrange loudspeaker driver, yet without good ones, listening experiences will be diminished. Since the true nature of the original sound cannot be known to listeners, one cannot say "it sounds as it should." But listeners routinely volunteer opinions on scales that are variations of like-dislike, which frequently have a component of emotion.

Descriptors like pleasantness and preference must therefore be considered as ranking in importance with accuracy and fidelity. This may seem like a dangerous path to take, risking the corruption of all that is revered in the purity of an original live performance. Fortunately, it turns out that when given the opportunity to judge without bias, human listeners are excellent detectors of artifacts and distortions; they are remarkably trustworthy guardians of what is good. Having only a vague concept of what might be correct, listeners recognize what is wrong. An absence of problems becomes a measure of excellence. By the end of this guide, we will see that technical excellence turns out to be a high correlate of both perceived accuracy and emotional gratification, and most of us can recognize it when we hear it.

The following concepts form the basis for this guide:

¦ Identifying the perceptual dimensions underlying listener responses

¦ Understanding the psychoacoustic rules governing them

¦ Developing record/reproduction methods and devices to maximize those dimensions that listeners respond to in a positive sense and to minimize those that are detractions

¦ Encouraging the music and film production and the consumer reproduction portions of the industry to share the same performance standards and system architectures so the art has a higher probability of being heard as intended

It’s hoped that readers from many backgrounds will find the content accessible. Audiophiles and music lovers, film buffs, reviewers, and journalists may find that they better understand the psychoacoustic underpinnings of what they hear. Recording, mixing, and mastering engineers may find some thought provoking perspectives on how their activities and practices fit into the grand scheme of sound reproduction. Loudspeaker designers will find no help whatsoever in the design of magnets, voice coils, and enclosures, but they may find inspiration in how to better integrate collections of transducers into systems that provide more favorable interfaces with room boundaries and the listeners within them. Finally, acoustical consultants may find some explanations for old practices, suggestions for new ones, and justification for those all-important "billable hours" starts with a historical perspective on sound reproduction. It then addresses the scientific background, examining relevant acoustics and psycho acoustics. The treatment is deliberately nonmathematical. Illustrations and graphs are used to explain the relationships between what is heard and what is measured. The intent is to convey ideas and an intuitive understanding of why things happen and result in certain kinds of perceptions.

Scientists often seek mathematical descriptions for relationships, including relationships between what is measured and what is heard. An equation does not add information; it attempts to describe information in a different form. In fact, almost always the equation is a simplification of the raw data that emerge from psychoacoustic examinations of a phenomenon. But such attempts are important in modeling more complex aspects of perception. Several simplified relationships may be combined into an explanation of something complex. The hope is that it can be done well enough that the input of technical data can yield an output that is a good prediction of a human perception. The long-term objective in the context of sound reproduction is to find technical metrics that usefully evaluate the physical world of electronic and transduction devices, operating in rooms.

The author tried to keep the discussions on topic and brief, but it’s a multidimensional subject, and a certain amount of explanation is essential.

There is also some redundancy because it’s assumed that the guide won’t necessarily be read in a linear fashion. It’s fully anticipated, for example, that many readers will start with Part Two, perhaps even the last SECTION, seeking the answers without the explanations. That's fine, but please go back and learn the rest, because only by understanding the principles can you hope to deal with the infinite variations in the real world. In some cases, it may be found that there is no remedy because there is no problem. Human adapt ability is an often ignored factor. We have a remarkable ability to "listen through" rooms to hear the essence of sound sources; that is, after all, the basis of live performances.

In recent years, the audio industry has seen the evolution of the custom installation business, driven by "home theater," but embracing all aspects of home automation, convenience, and entertainment. This business has energized an audio industry that was, after about 50 years with two-channel stereo, facing a lackluster future. Stereo has not gone away, and it will never disappear completely because of the huge collection of valuable legacy music that exists. Interestingly, it was film that provided inspiration for stereo, and it’s film that has paved the way for multichannel sound to enter homes. For that we can be grateful.

The term home theater says it all: movies in the home. Movies are an important component of home entertainment, no doubt, but the facilities we call home theaters should not end there; that would be a sad compromise. Some consultants promote the notion that these spaces should emulate what is heard on a film-dubbing stage and, by inference, a cinema. In the important spectral, dynamic, directional, and spatial aspects, there is some merit in this proposition. These are basic dimensions of sound reproduction, and they apply to music and games as well as to movies. But the small physical size of the domestic installation allows us the freedom to use components with greater finesse and the flexibility to provide for different kinds of entertainment. So go ahead and call it a home theater, but design it for many uses, and, if it’s a good one, be prepared for it to sound better than most (any?) large-venue presentations.

This is the first time in the history of audio that there has been a significantly capable service industry operating in the space between audio manufacturers and customers. These designers and installers are in a position to offer informed advice on the purchase of audio and video equipment and to complete the installation and calibration of it. With the complexity of contemporary A/V equipment and the need for centralized control, that is not a trivial advantage.

Part Two addresses this custom installation audience and anyone else who wants to create a state-of-the-art home theater or sound reproduction system.

It’s hoped that the presentation style and language serve their needs. This guide is not a cookbook. Sadly, such simplifications-and several have been attempted- end up being oversimplifications. In an attempt not to scare off readers, the "keep it simple" principle has been applied to a topic that in reality isn't that simple. Only a very tolerant consuming public could be pleased with many of the home theaters constructed according to these compromised principles. Yet, there is nothing in the design of a home theater that is beyond the capabilities of a determined do-it-yourselfer.

With good design, absolutely superb sound reproduction can be achieved in rooms used for normal living and decorated as such. However, if isolating the dynamics of rock-and-roll or a blockbuster movie from the rest of the dwelling is an issue, then a dedicated home theater is needed, as well as, most likely, the services of a competent acoustical consultant to design effective sound isolation. A dedicated room won’t have normal furnishings, and therefore interior acoustical treatment is necessary. This is an opportunity to design the "listening experience," optimizing it for the programs and playback formats favored by the customer. In this guide we will review the accumulated wisdom of the industry.

We will add to this the results of some scientific investigations that are relatively unknown in the audio business and, out of the combination, arrive at guidelines for the design of entertainment systems and rooms.

"Between believing a thing and thinking you know is only a small step and quickly taken." Mark Twain.

Audio is a mature industry, and as a result of countless repetitions, certain beliefs have come to have a status comparable with scientific facts. Many of these ideas have been altruistically well intended, others commercially motivated. Some of them are wrong. This is a wasteful situation because the casualty is so often the art itself.

Readers will undoubtedly find conflicts between some of the recommendations in this guide and ideas published or promoted elsewhere. If so, try to understand why the recommendations came about by tracing the story through this guide and to the references if necessary. Inevitably, parts of the story are incomplete. If there is a better or more correct way, now or in the future, the scientific literature provides opportunity for a free exchange of information: new evidence and new interpretations of evidence. This is the scientific method.

Science in the service of art is the only sustainable position. In this guide, we "follow the science" to see where it leads us. In the end, it will be found that in some ways it points to where we already are, but in other respects, some course adjustments are necessary.

Understanding the Principles

1. Sound Reproduction
2. Preserving the Art
3. Matters of Perspective
4. Sound Fields in Rooms
5. Effects of Reflections
6. Reflections, Images, and the Precedence Effect