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Microphonesmicrophones Methodsmethods Ofof Operationoperation Andand Typetype Examplesexamples Microphones for Studio and Home-Recording Applications GerhartGerhart BoréBoré / / Stephan Stephan PeusPeus MicrophonesMicrophones MethodsMethods ofof OperationOperation andand TypeType ExamplesExamples Microphones for Studio and Home-Recording Applications Operation Principles and Type Examples by Dr.-Ing. Gerhart Boré and Dipl.-Ing. Stephan Peus An Engineering Service of For the information contained in this book, no responsibility is undertaken with regard to exemption from industrial rights (patents, registered designs, trade marks). Trade names, commercial names and merchandise appella- tions reproduced in this book may likewise not be regarded as free for gen- eral use in the spirit of the laws for the protection of trademarks and brand names. Any infringement of these rights is an offence under existing laws, with consequent liability for damages. Fourth Edition: 1999 Revised and expanded version of a contribution to the “Taschenbuch der Unterhaltungs-Elektronik 1973” All rights reserved Printed by Druck-Centrum Fürst GmbH, Berlin Printed in Germany 4 Preface to the fourth edition The microphone is the first link in every chain of electroacoustic transmis- sion. Problems caused by poor inherent characteristics or unfavorable mi- crophone placement can seldom be corrected by subsequent processing equipment. Thus it is worth learning about the properties and quality at- tributes of the various types of microphones available. This work was originally written as a contribution to the German book “Taschenbuch der Unterhaltungs-Elektronik” (“Handbook of Entertainment Electronics”). Its purpose is to convey some basic knowledge about micro- phones and the way in which they work, to all who are professionally or privately involved or interested in the recording and transmission of sound, and to help them use microphones effectively. No specialized knowledge is required. Readers will learn about to the various types of microphones in existence and the characteristic features of those most commonly in use. Following a resume of some of the aspects common to all types, condenser and dynamic microphones are described in somewhat more detail. Above all, this fourth edition has been revised to reflect newer develop- ments in microphones and updated standards. On the whole, however, the character and style of the “Boré booklet” has been preserved. This small compendium has served as companion and tutorial guideline for generations of sound engineering students and other persons interested in audio engi- neering. We hope that it will continue to provide guidance and inspiration to microphone users everywhere. The author thanks Mr. M. Schneider for valuable comments and additions. S. Peus Dr.-Ing. Gerhart Boré studied electrical engineering in Munich and Aachen, and specialized in microphone development at Georg Neumann GmbH from 1956 to 1982. Dipl.-Ing. Stephan Peus studied electrical engineering and acoustics at TU Berlin. He has been with Georg Neumann GmbH since 1971 and has been director of microphone development there for many years. 5 6 Contents Chapter Page 1. Terminology of microphone characteristics ......................................... 9 2. Classification ........................................................................................ 9 3. Microphone fundamentals .................................................................. 11 3.1 Pressure transducers ................................................................ 11 3.2 Pressure gradient transducers .................................................. 11 3.2.1 Behavior in a plane sound field ................................................ 12 3.2.2 Behavior in a spherical sound field........................................... 14 3.2.3 Cardioid characteristic ............................................................. 16 3.2.4 Hyper-, super- and subcardioid characteristic ......................... 18 3.3 Influence of microphone dimensions ....................................... 20 3.3.1 Pressure microphones .............................................................. 20 3.3.2 Pressure gradient microphones ................................................ 21 3.3.3 Free-field and diffuse-field frequency response ...................... 22 4. Dynamic microphones........................................................................ 26 4.1 Ribbon microphones with figure-8 characteristic .................... 28 4.2 Ribbon microphones as pressure transducers........................... 29 4.3 Ribbon microphones with cardioid characteristic .................... 29 4.4 Moving coil microphones as pressure transducers ................... 29 4.5 Moving coil microphones with cardioid characteristic ............. 30 5. Condenser microphones ..................................................................... 32 5.1 Pressure condenser microphones ............................................. 32 5.2 Pressure gradient condenser microphones ............................... 33 5.3 Cardioid condenser microphones ............................................ 33 5.4 Condenser microphones with symmetrical capsules ............... 34 5.5 Dc polarizing method .............................................................. 36 5.5.1 The selection of various polar patterns .................................... 37 5.6 Electret microphone capsules .................................................. 38 5.7 The microphone amplifier ....................................................... 39 7 Chapter Page 5.7.1 Transformerless microphones .................................................. 42 5.8 The RF circuit method ............................................................ 43 5.9 Power supplies and connections .............................................. 45 5.9.1 A-B powering ........................................................................... 45 5.9.2 Phantom powering ................................................................... 46 6. Microphone types used in recording practice .................................... 49 6.1 Miniature microphones ............................................................ 49 6.2 Microphones of larger size ....................................................... 49 6.3 Hand-held and soloist’s microphones ...................................... 49 6.4 Noise-suppressing microphones ............................................... 50 6.5 Flexible or fixed capsule extensions, active capsules.......................................................................... 50 6.6 Microphones for room-oriented stereophony .......................... 51 6.7 Microphones for head-oriented stereophony ........................... 54 6.8 Ultra-directional microphones (shotgun microphones) ........... 56 6.9 Lavalier and clip-on microphones ............................................ 60 6.10 Wireless microphones .............................................................. 61 6.11 Boundary-layer microphones ................................................... 62 7. Some criteria for assessing sensitivity and operating characteristics ..................................................................... 65 Appendix ................................................................................................. 68 8 1. Terminology of microphone characteristics Terms defining the characteristics of microphones are laid down in IEC 60268-4. A glossary of these terms, which are essential for understanding microphone specifications, is provided in the Appendix, together with brief explanations of their meanings. 2. Classification Microphones can be classified according to various criteria: Passive transducers*) convert acoustical energy directly into electrical en- ergy (and vice-versa) without the need for any external power feed. This group includes dynamic, magnetic and piezoelectric microphones, as well as condenser microphones using dc polarization. Active transducers*) convert electrical energy from an external source syn- chronously with the sound vibrations they receive. Carbon microphones and RF-condenser microphones make use of this principle. Microphones which operate as displacement-controlled transducers gen- erate a voltage output which is proportional to the displacement of the dia- phragm. This applies to all capacitive and piezoelectric microphones. The ohmic resistance of carbon microphones likewise varies in proportion to the diaphragm displacement. Velocity transducers is a term applied to all magnetic microphones operat- ing on the basis of the law of induction. Their output voltage is proportion- al not to the extent, but to the velocity of the diaphragm displacement. Almost all microphone types are manufactured according to their applica- tion: 1. as ‘pressure transducers’ (predominantly omnidirectional) or 2. as ‘pressure gradient transducers’ with special directional properties. *) These definitions are taken from IEC Publication 50-08, Section 15. They appear to be some- what less logical than the formerly recognized classifications as “pure transducers” and “control voltage transducers”, although based apparently on terms used in studio technology, in which, for example, an “active” equalizing network must have an electrical power source, while a “pas- sive” equalizing network manages without one. – With measured-variable transducers, the dis- tinction between “active” and “passive” is mostly in the exact opposite sense. 9 All microphones will show a special response when their dimensions ap- proach or become greater than the wavelength of the impinging sound. This effect is at first disregarded
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