Introduction
Every day, around the world, millions of people use loudspeakers as some sort of reference in their place of work. These people include those who work in film studios, radio stations, live sound events, discothèques, clubs, music recording studios, television studios, theatres, cinemas and many other associated professions. For many musicians who play electrified or electronic instruments, their loudspeakers are an integral part of their instruments, and as such can significantly affect the interpretation of a performance, as well as its sound. Amongst all of the professionals there are only a very few who actually know much about how loudspeakers work, other than a little knowledge gained in a cursory way that can be as deceiving as it is revealing. They rely on loudspeakers as ‘black-boxes’ which somehow transform electrical signals into sound. Upon opening a box, they see very little inside, except a metal or plastic chassis, cardboard of plastic diaphragms, a magnet, some fluffy material, and perhaps a few simple electronic components and wires that may make up a crossover circuit.
It is often a source of frustration to these people – and to countless millions more of domestic hi-fi enthusiasts – that these simple little boxes fail to deliver an accurate and repeatable sound in a variety of circumstances. It is a further source of frustration that they all seem to sound different, because it seriously complicates the compatibility problems when their work travels from system to system, or room to room. They feel frustrated because it seems that surely the knowledge exists to sort out such uncomplicated devices. This situation is often exacerbated by the powers of marketing, when so many advertisements from so many manufacturers all claim that their loudspeakers have the ability to tell the truth; but obviously the reality cannot be quite so simple. In fact, loudspeakers are electro-mechanico-acoustic systems whose behaviour is complex to a degree that seems totally disproportionate to the simplicity of their appearance.
Drivers of racing cars, helicopter pilots or submarine captains are people who we cannot imagine as not having a through knowledge of the vehicles that they command. It almost goes without saying that when they return to base they would be able to communicate to the technicians, mechanics and engineers in a way that was clear and concise about any technical failings or handling difficulties that had occurred. Conversely, many people who use loudspeakers, professionally, have remarkably little insight into what is going on inside the equipment, and when problems arise with the sound, they are at a total loss to explain either what the symptoms are or what the problem might be. They work by trial and error, and often try to restrict themselves to working in familiar environments where any problems, even if not understood, are at least known.
In all fairness, some professional users of loudspeakers do try to learn something more about the devices which are so important to their daily work, but they are often faced with a choice of two equally blind alleys. The first is to look at some text books on the subject, and the other is to search through popular magazines which publish articles about loudspeakers, such as the hi-fi and home recording publications. The text book approach often grinds to a halt somewhere before the end of page 1 as they become overwhelmed by the complexity of the electro-acoustic theory which, even for specialised loudspeaker engineers, is not always entirely straightforward. [In fact, one of the most authoritative books on loudspeaker theory and application was written by fourteen different people, because such a work would almost certainly be beyond the ability of any, one person1.] The popular press, on the other hand, is largely concerned with filling pages with text and selling advertising space, which is perfectly understandable because their primary raison d’être is to entertain the readers. However, conjecture and opinions are often passed off as authoritative fact, and contradictions are commonplace. It can thus become very difficult for the non-specialist to separate the fact from the fiction, so the avid reading of such publications is liable to result in an information overload, but with no clear facts being apparent in any unequivocal manner.
The object of this book therefore is to try to fill the gap which currently exists between the text books and the popular press. It will try to describe the theory behind, and application of, the loudspeakers which are used for music recording and reproduction in a way that is accessible to those who would benefit from a greater understanding of the concepts, but who do not have anything more than a basic understanding of general science. Nevertheless it is intended that the facts and descriptions will be both accurate and thorough, and where subjective aspects of loudspeaker performance is discussed, it will be backed up with objective and perceptual justification.
It is inevitable that subjective perceptions must be discussed, because ultimately it is the ear of each individual listener which acts as judge and jury, despite what the measurement may say, but recent research has shown that subjective assessments can be reliable and quite precise as long as the variable peripheral factors are minimised and understood. The authors are very aware of the pitfalls, and have had a great deal of experience in dealing with them. One of the authors is a designer of recording studios, film dubbing theatres, television studios and concert halls, who for many years was a recording engineer, live sound engineer, record producer and monitor system designer. The other is a Doctor of Acoustics, and a university lecturer in electro-acoustics who has had much experience in live sound as a front of house engineer. Both are members of the UK's Institute of Acoustics and the Audio Engineering Society, and both are experienced at teaching audio technology from very basic levels. Hopefully, therefore, ways will be found to explain complex ideas via understandable but nevertheless accurate analogies, which should enable the readers to grasp an intuitive feel for the subject, especially where mathematical explanations would elude them.
Whilst it has to be accepted that the majority of loudspeaker users neither have the time nor the inclination to formally study electro-acoustics, it is still extremely useful for them to understand much more than most of them currently do, in order to help them make more informed decisions about matters which affect their working lives. People beginning to work in the recording world or people with a keen interest in sound reproduction will also probably find this book useful. Inevitably, from time to time, things may need to get a little deep. This will be essential when the discussion requires it, but hopefully it can be done in a series of steps which will not leave the less technical readers too isolated.
The book will begin with a brief history of loudspeaker development in the early days, and will look at the basic concepts of just what a loudspeaker is, and what it must do. Some basic principles of sound radiation will then be introduced, in order to give a better understanding of the principles before looking at the wide range of motor systems technologies that are available, such as moving coil, electrostatic, piezoelectric, ionic, magnaplanar and various other concepts. The pros and cons of different diaphragm technologies will be discussed, as will those of the loading techniques, such as with horns and various cabinets and baffles.
Loudspeakers, of course, require an electrical drive signal, so Chapter 5 will look at the whole concept of crossovers, discussing why they are needed and how they can be realised in practice. Active and passive designs will be investigated, as will various slopes, shapes, phase effects and reconstruction difficulties. The electrical, acoustical and mechanical (physical) factors which affect crossover performance will be dealt with in a thorough, yet understandable way, before the following chapter discusses the amplifiers and cables which are necessary to complete any monitoring system. Different amplifier topologies will be discussed, and their suitability for different specialised uses will be indicated. Without getting into the subjective minefield of loudspeaker cable audibility, an objective presentation will be made of the ways in which it has been shown that cables can, and do, affect system performance. There is no point in paying ten times more than necessary for a special loudspeaker cable if a standard cable will sound exactly the same, but in sensitive circumstances a more esoteric cable may be beneficial. It is the first time that most of this work has been published outside of the proceedings of international electro-acoustic conferences.
The book will then go on to discuss the consequences of the interaction between the loudspeaker systems and the rooms in which they are sited, and how, in so many cases, the rooms can dominate the overall response. Guidance will be given as to where to find out more about the room acoustics, but the consequences of different mounting regimes will be dealt with, here.
Chapter 8 is a wide-ranging analysis of the reasons why different loudspeakers appear to be more appropriate during different phases of the recording/mixing/mastering/listening process. Many concepts of loudspeaker system design will be analysed, and their application to the environments in which they are most likely to be used will be assessed. Motor systems and cabinet options will be brought together in ways that they can be applied to the physical, electro-acoustic and psychoacoustic requirements of each stage of the work. With the introduction of the question of perception, highlighted by the fact that different loudspeakers tend to be chosen for different phases of the work, Chapter 9 analyses the different measurements which can be used to define the performance of a loudspeaker system, and how the objective measurements relate to different aspects of the subjective perception of the music. Chapter 10 will then reverse the concept, and will discuss how the musical arrangements can be the culprits for many system-to-system compatibility problems for which the loudspeakers usually take the blame. This is a crucial subject, but one which one very rarely sees discussed in print.
The final two chapters will deal with subjects which are, in general, very poorly understood by the vast majority of people who work with loudspeakers. Chapter 11 discusses the fundamental requirements of the low frequency radiation from small loudspeakers. It explains why, in accuracy terms, loudspeaker designers compromise the performance in order to reduce box sizes and/or extend the responses. Brand new measurement concepts will be introduced which demonstrate the degree to which these performance enhancements reduce the reproduction accuracy in an exchange of quantity for quality. The chapter then goes on to look at the transient performances, which are so important for the realistic perception of music, but which are consistently ignored by many loudspeaker manufactures, partly because they do not feature in most performance specifications which are used for publicity purposes. They are often compromised for the betterment of some less significant responses benefits which do carry more weight in terms of advertising.
The book concludes with a chapter on surround-sound application; not only in the more physical realms of room interfacing that were discussed in Chapter 7, but also in terms of the application problem that confound the day-to-day use of surround sound, such as format compatibility, the suitability of certain loudspeaker radiation patterns to specific mounting conditions, and the appropriateness of loudspeaker choices for different musical programme.
Hopefully, therefore, this book will fill the gap left between the textbooks and the magazines. It deals with the application and use of the technology and science, justifying ideas with hard facts rather than conjecture, and in a way that should be accessible to anybody with a general level of experience in the use of loudspeakers, whether for work or leisure purposes.
Reference
1 Borwick, J., ‘Loudspeaker and Headphone Handbook’, Third Edition, Focal Press, Oxford, UK (2001)