(Book). Eargle's Microphone Book is the only guide you will ever need for the latest in microphone technology, application and technique. This new edition features more on microphone arrays and wireless microphones, new material on digital models; the latest developments in surround; expanded advice on studio set up, recording and mic selection. Ray A. Rayburn provides detailed analysis of the different types of microphones available and addresses their application through practical examples of actual recording sessions and studio operations. The book takes you into the studio or concert hall to see how performers are positioned and how the best microphone array is determined. Problem areas such as reflections, studio leakage and isolation are analyzed from practical viewpoints. Creative solutions to stereo sound staging, perspective, and balance are covered in detail. Eargle's Microphone Book is an invaluable resource for learning the "why" as well as the "how" of choosing and placing a microphone for any situation.
About the Author
Ray A. Rayburn, Senior Consultant with K2 Audio LLC. Member of the AES Standards Working Group on Microphones, and Chair of the Standards sub-committee on Interconnections. Recording engineer. Lifetime interest in microphone use, testing, and design.
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Eargle's Microphone BookFrom Mono to Stereo to Surround A Guide to Microphone Design and Application
By Ray A. Rayburn
Focal PressCopyright © 2012 Elsevier Inc.
All right reserved.
Chapter OneA Short History of the Microphone
The microphone touches our daily lives through the sound we hear in movies and television, recordings, concerts, and of course the telephone. In this chapter we will touch upon some of the highlights of more than 125 years of microphone development, observing in particular how most of the first 50 of these years were without the benefits of electronic amplification. The requirements of telephony, radio broadcast, general communications, and recording are also discussed, leading to some conjecture on future requirements.
The Early Years
Children once were fascinated with strings stretched between the ends of a pair of tin cans or wax paper cups, and with their ability to convey speech over a limited distance. This was a purely mechano-acoustical arrangement in which vibrations generated at one end were transmitted along the string to create sound at the other end.
In 1876, Alexander Graham Bell received US Patent 174,465 on the scheme shown in Figure 1.1. Here, the mechanical string was, in a sense, replaced by a wire that conducted electrical current, with audio signals generated and received via a moving armature transmitter and associated receiver. Like the mechanical version, the system was reciprocal. Transmission was possible in either direction. The patent also illustrates the acoustical advantage of a horn to increase the driving pressure at the sending end and a complementary inverted horn to reinforce output pressure at the ear at the receiving end. Bell's further experiments with the transmitting device resulted in the liquid transmitter, shown in Figure 1.2, which was demonstrated at the Philadelphia Centennial Exposition of 1876. Here, the variable contact principle provided a more effective method of electrical signal modulation than that afforded by the moving armature.
The variable contact principle was extended by Emile Berliner in a patent application in 1877, in which a steel ball was placed against a stretched metal diaphragm, as shown in Figure 1.3. Further work in this area was done by Francis Blake (US Patents 250,126 through 250,129, issued in 1881), who used a platinum bead impressed against a hard carbon disc as the variable resistance element, as shown in Figure 1.4. The measured response of the Blake device spanned some 50 decibels over the frequency range from 380 Hz to 2,000 Hz, and thus fell far short of the desired response. However, it provided a more efficient method of modulating telephone signals than earlier designs and became a standard in the Bell system for some years.
Another interim step in the development of loose contact modulation of direct current was developed in 1878 by David Edward Hughes and is shown in Figure 1.5. In this embodiment, very slight changes in the curvature of the thin wood plate diaphragm, caused by impinging sound waves, gave rise to a fairly large fluctuation in contact resistance between the carbon rod and the two mounting points. This microphone was used by Clement Ader (Scientific American, 1881) in his pioneering two-channel transmissions from the stage of the Paris Opera to a neighboring space. It was Hughes, incidentally, who first used the term microphone, as applied to electro-acoustical devices.
The ultimate solution to telephone transmitters came with the development of loose carbon granule elements as typified by Blake's transmitter of 1888, shown in Figure 1.6. Along with the moving armature receiver, the loose carbon granule transmitter, or microphone, has dominated telephony almost to the present—quite a testimony to the inventiveness and resourcefulness of engineers working nearly 130 years ago.
The Rise of Broadcasting
The carbon granule transmitter and moving armature receiver complemented each other nicely. The limitations in bandwidth and dynamic range have never been a problem in telephony, and the rather high distortion generated in the combined systems actually improved speech intelligibility by emphasizing higher frequencies. Even after the invention of electronic amplification (the de Forest audion vacuum tube, 1907), these earlier devices remained in favor.
When commercial broadcasting began in the early 1920s, there was a clear requirement for better microphones as well as loudspeakers. Western Electric, the manufacturing branch of the Bell Telephone system, was quick to respond to these needs, developing both electrostatic (condenser) microphones as well as electrodynamic (moving conductor) microphones. The condenser, or capacitor, microphone used a fixed electrical charge on the plates of a capacitor, one of which was a moving diaphragm and the other a fixed back plate. Sound waves caused a slight variation in capacitance, which in turn was translated into a variation in the voltage across the plates. An early Western Electric condenser microphone, developed by Edward Christopher Wente in 1917, is shown in Figure 1.7; it led to the 640AA microphone first produced by Western Electric for Leo Beranek in the fall of 1941 and is still considered a laboratory standard microphone.
While first employed as a driving element for loudspeakers and headphones, the moving coil and its close relative, the ribbon, eventually found their place in microphone design during the mid-1920s. Moving coil and ribbon microphones operate on the same principle; the electrical conducting element is placed in a transverse magnetic field, and its motion generated by sound vibrations induces a voltage across the conducting element. Under the direction of Harry Olson, Radio Corporation of America (RCA) was responsible for development and beneficial exploitation of the ribbon microphone during the 1930s and 1940s.
The Rise of Mass Communications
Beginning as far back as the 1920s, a number of smaller American companies, such as Shure Brothers and Electro-Voice, began to make significant contributions to microphone engineering and design. General applications, such as paging and sound reinforcement, required ingenious and economical solutions to many problems. Commercial development of condenser microphones was more or less ruled out due to the requirements for a cumbersome polarizing supply, so these companies concentrated primarily on moving coil designs.
The work of Benjamin Bauer (1941) was significant in producing the Shure Unidyne directional (cardioid pattern) design based on a single moving coil element. Wiggins (1954) developed the Electro-Voice "Variable-D" single moving coil element, which provided low handling noise with excellent directional response.
Other companies designed crystal microphones for low-cost, moderate-quality paging applications. These designs were based on the principle of piezoelectricity (from the Greek piezien, meaning pressure), which describes the property of many crystalline structures to develop a voltage across opposite facets when the material is bent or otherwise deformed. The early advantage of the piezos was a relatively high output signal, but eventually the availability of small, high-energy magnet materials made them obsolete.
The Great Condenser Breakthrough: The Electret
For many years the penalty carried by the condenser microphone was its requirement for external polarization voltage. In the early sixties, Sessler and West of Bell Telephone Laboratories described a condenser microphone that used a permanently polarized dielectric material between the movable diaphragm and the backplate of the microphone. Early materials exhibited significant losses in sensitivity over time, but these problems have been overcome. Further improvements have come in miniaturization, enabling electret microphones to be designed for a wide variety of close-in applications, such as tie-tack use, hidden on-stage pickup, and many other uses. Today's small electret microphone requires only a miniature self-contained battery or external phantom power to power its equally miniature preamplifier. It is a testimony to the electret and its long-term stability and excellent technical performance that the Brüel & Kjær series of studio microphones (now sold by DPA) designed in the 1980s used electret technology.
Studio Microphone Technology
The microphone is the first stage in the complex and extended technical chain between live performance and sound reproduction in the home or motion picture theater. Little wonder that so much attention has been paid to the quality and technical performance of these fine instruments.
Condenser microphones have dominated studio recording since the late 1940s, when the first German and Austrian condenser microphones came on the scene. As with any mature technology, progress comes slowly, and the best models available today have a useful dynamic range that exceeds that of a 24-bit digital recorder. With regard to directional performance, many newer microphones exhibit off-axis response integrity that far exceeds the best of earlier models.
At the beginning of the 21st century, it is interesting to observe the great nostalgia that many recording engineers have for the earlier vacuum tube condenser models, especially the Neumann and AKG classic microphones of the 1950s. All of this reminds us that technology is so often tempered with subjective judgment to good effect.
The microphone per se is so highly developed that it is often difficult to see where specific improvements in the basic mechanisms are needed. Certainly in the area of increased directionality, via second- and higher-order designs, there is additional development engineering to be done. Engineers are working on a direct-converting, high-quality digital microphone, but they are not yet on the market. Digital output microphones using conventional condenser microphone elements are available, and digital signal processing is being used to shape the directional characteristics of active microphone arrays.
New usage concepts include microphones in conferencing systems, with their requirements for combining and gating of elements; and microphones in large arrays, where highly directional, steerable pickup patterns can be realized. These are among the many subjects that will be discussed in later chapters.
Chapter TwoBasic Sound Transmission and Operational Forces on Microphones
All modern microphones are used with electrical amplification and thus are designed primarily to sample a sound field rather than take power from it. In order to understand how microphones work from the physical and engineering points of view, we must understand the basics of sound transmission in air. We base our discussion on sinusoidal wave generation, since sine waves can be considered the building blocks of most audible sound phenomena. Sound transmission in both plane and spherical waves will be discussed, both in free and enclosed spaces. Power relationships and the concept of the decibel are developed. Finally, the effects of microphone dimensions on the behavior of sound pickup are discussed.
Basic Wave Generation and Transmission
Figure 2.1 illustrates the generation of a sine wave. The vertical component of a rotating vector is plotted along the time axis, as shown at A. At each 360° of rotation, the wave structure, or waveform, begins anew. The amplitude of the sine wave reaches a crest, or maximum value, above the zero reference baseline, and the period is the time required for the execution of one cycle. The term frequency represents the number of cycles executed in a given period of time. Normally we speak of frequency in hertz (Hz), representing cycles per second.
Excerpted from Eargle's Microphone Book by Ray A. Rayburn Copyright © 2012 by Elsevier Inc.. Excerpted by permission of Focal Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
A Short History of the Microphone
Basic Sound Transmission and Operational Forces on Microphones
The Pressure Microphone
The Pressure Gradient Microphone
First-Order Directional Microphones
Microphone Measurements, Standards, and Specifications
Electrical Considerations and Electronic Interface
Overview of Wireless Microphone Technology; Microphone Accessories
Basic Stereophonic Techniques
Stereo Microphones; Classical Stereo
Recording Techniques and Practice
Studio Recording Techniques
Surround Sound Microphone Technology
Surround Recording Case Studies
A Survey of Microphones in Broadcast and Communications
Fundamentals of Speech and Music Reinforcement
Overview of Microphone Arrays and Adaptive Systems
Care and Maintenance of Microphones
Classic Microphones: The Author's View