A Shure Educational Publication AUDIO SYSTEMS GUIDE MEETING FACILITIES By Chris Lyons, Tim Vear, and Michael Pettersen
Above: Phelan, Pope and John “Moot” Courtroom Right: House of Tynwald Below: Minnesota State House Cover Photo: Bank of Tokyo Boardroom
Audio Systems Guide for MEETING FACILITIES Ta b l e o f C o n t e n t s Introduction............................................................................................... 4 Chapter 1 Sound ................................................................................................ 5 Chapter 2 The Sound Source ............................................................................ 7 Chapter 3 The Sound System ............................................................................
Audio Systems Guide for MEETING FACILITIES Introduction Audio for meeting facilities is a topic that equipment requires knowledge of both the encompasses a wide range of applications, from intended purpose of the sound system and the a simple public address system in a courtroom characteristics of individual components. to a multi-channel legislative system with recording and broadcast capabilities.
Audio Systems Guide for MEETING FACILITIES C H A P TE R ONE SOUND Because “good” sound quality is the goal of any meeting facility sound system, it is helpful to be familiar with some general aspects of sound: how it is produced, transmitted, and received. In addition, it is also useful to describe or classify sound according to its acoustic behavior. Sound is produced by vibrating objects. These include musical instruments, loudspeakers, and, of course, human “vocal cords”.
Audio Systems Guide for MEETING FACILITIES Sound is transmitted through some “medium”. Air is the typical medium, but sound can also be transmitted through solid or liquid materials. Generally, a sound wave will move in a straight line unless it is absorbed or reflected by physical surfaces or objects in its path. However, the sound wave will be affected only if the surface is large compared to the wavelength of the sound.
Audio Systems Guide for MEETING FACILITIES C H A P TE R TW O THE SOUND SOURCE The sound source most often found in meeting facility applications is the speaking voice. Voices may be male or female, loud or soft, single or multiple, close or distant. Pre-recorded audio from video or audio tape is also very common in a meeting facility.
Audio Systems Guide for MEETING FACILITIES C H A P TE R THREE THE SOUND SYSTEM A basic sound reinforcement system consists of an input device (microphone), a control device (mixer), an amplification device (power amplifier), and an output device (loudspeaker). This arrangement of components is sometimes referred to as the audio chain: each device is linked to the next in a specific order.
Audio Systems Guide for MEETING FACILITIES C H A P TE R FOUR The operating principle describes the kind of transducer inside the microphone. A transducer is a device that changes energy from one form into another, in this case, acoustic energy into electrical energy. It is the part of the microphone that actually senses air movement (sound) and converts it into an electrical signal. The operating principle determines some of the basic capabilities of the microphone.
Audio Systems Guide for MEETING FACILITIES The construction of a condenser microphone must include some provision for maintaining the electrical charge. An electret condenser microphone has a permanent charge, maintained by a special material deposited on the backplate or on the diaphragm. Other types are charged by means of an external power source. All condensers contain additional circuitry to match the electrical output of the element to typical microphone inputs.
Audio Systems Guide for MEETING FACILITIES The selection of a flat or shaped response microphone involves consideration of both the sound source and the sound destination. The frequency range of the microphone must be wide enough to pick up the desired range of the sound source. This range must also be appropriate to the intended destination of the sound: that is, wider range for high quality sound systems or recording/broadcast systems, narrower range for speech-only public address systems.
Audio Systems Guide for MEETING FACILITIES A cardioid type is most sensitive to sound coming from in front of the microphone (the bottom of the heart shape). On the polar graph this is at 0 degrees, or on axis. A cardioid microphone is less sensitive to sound coming from the sides (off axis), and least sensitive to sound from the rear (the notch at the top of the heart shape). For any microphone, the direction of least sensitivity (minimum output) is called the null angle.
Audio Systems Guide for MEETING FACILITIES It should be noted that this discussion of directionality assumes that the polar pattern for a microphone is uniform, that is, the same shape at all frequencies. In practice, this is not always achieved. Most microphones maintain their nominal polar pattern over only a limited range of frequencies. This is the reason that published polar patterns include curves measured at several frequencies.
Audio Systems Guide for MEETING FACILITIES The output wiring scheme of a microphone can be either balanced or unbalanced. A balanced output carries the signal on two conductors. A shield surrounds these the two conductors to keep out unwanted signals. The signals on each conductor are the same level but they are of opposite polarity. A balanced (or differential) input is sensitive only to the difference between the two signals and ignores any part of the signal which is the same in each conductor.
Audio Systems Guide for MEETING FACILITIES Free-standing mounted microphones (mounted away from large surfaces) come in a variety of styles suited for different fixed settings. These range from fullsize microphones, on heavy-duty stands, to miniature types on unobtrusive goosenecks or booms, to hanging microphones of any size. Mounted microphones are generally selected for permanent installation although many handheld types may be placed in mounts and removed as needed.
Audio Systems Guide for MEETING FACILITIES CHAPTER FIVE MICROPHONES: USES Once a microphone is selected for a given application, it must be used properly to get the best possible results. Again, there are two key areas: the interface of the microphone with the sound source, and the interface of the microphone with the sound system. The first area involves primarily acoustic considerations for optimum placement of one or more microphones.
Audio Systems Guide for MEETING FACILITIES This leads to the first and most important rule of microphone placement: Place the microphone as close as practical to the desired sound source. It has several corollaries: 1) place the microphone as far as possible from loudspeakers and other undesired sources; 2) use directional microphones to minimize ambient sound pickup; 3) aim directional microphones toward the desired sound and/or away from undesired sound; and 4) keep the system gain to a minimum.
Audio Systems Guide for MEETING FACILITIES The solution to this problem is to use the three-to-one rule: for multiple microphones, the microphone-tomicrophone distance should be at least three times the source-to-microphone distance. For example, when using individual microphones for a city council, if a talker’s microphone is one foot away, then the next nearest microphone should be at least three feet away from the first.
Audio Systems Guide for MEETING FACILITIES Surface-mount or “boundary effect” microphones are designed for this purpose. They effectively reduce interference from the surface to which they are attached. If they are located at the junction of two or more surfaces, such as the corner of a room, they reduce interference from each adjacent surface. In addition, a boundary microphone exhibits increased sensitivity due to addition of the direct and reflected sound.
Audio Systems Guide for MEETING FACILITIES cause input overload. In practice, however, most mixers are capable of handling a very wide range of microphone signal levels and sensitivity values are not critical. Occasionally, for extremely high sound levels, an attenuator may be necessary to lower the output of the microphone. These are built into some microphones and most mixers. Otherwise, accessory attenuators are available that may be inserted in line with the microphone cable.
Audio Systems Guide for MEETING FACILITIES Finally, the use of microphones in particular applications is often facilitated by microphone accessories. These are mechanical and electrical hardware items that are often used in mounting and connecting microphones. Mechanical accessories include various kinds of acoustic devices such as windscreens and directionality modifiers.
Audio Systems Guide for MEETING FACILITIES CHAPTER SIX WIRELESS MICROPHONE SYSTEMS A wireless microphone is actually a system consisting of 1) a microphone 2) a radio transmitter and 3) a radio receiver. The function of the microphone is unchanged and the function of the transmitter and receiver combination is to replace the microphone cable with a radio link. Although this objective is simple, its accomplishment is not.
Audio Systems Guide for MEETING FACILITIES Non-diversity receivers work well in many applications but are subject to a phenomenon known as dropout: a temporary interruption of the radio signal. The audible effect may range from a slight “swishing” noise to a complete loss of sound. Since radio signals become weaker over greater distances, a dropout can occur when the transmitter is very far from the receiver antenna.
Audio Systems Guide for MEETING FACILITIES proportional to their relative strength. When both are strong, the contribution from each signal is equal. If one signal becomes weaker, its contribution is similarly reduced. Finally, if a complete dropout occurs for one signal, the receiver uses only the good signal. Since the combining technique acts as a continuous balance control rather than as a switch, it further reduces any audible effects of diversity action.
Audio Systems Guide for MEETING FACILITIES interfere with each other if those frequencies are not carefully chosen. The rules for frequency selection are complex enough that computer programs are used to calculate compatible systems. Again, the wireless microphone manufacturer should be consulted for any multiple system installation. The selection of a wireless microphone system includes several steps, some of which are similar to wired microphone selection.
Audio Systems Guide for MEETING FACILITIES The last aspect of the use of wireless microphone systems and the least predictable is radio interference. Potential interference from other wireless systems operating on the same or nearby frequencies has been discussed, but what about other possible sources of interference? High-band VHF systems are generally not subject to interference from radio stations, amateur radio, pagers or cellular telephones.
Audio Systems Guide for MEETING FACILITIES SCM810 automatic microphone mixer C H A P TE R SEVEN AUTOMATIC MIXERS The function of an automatic mixer is twofold: 1) to automatically activate microphones as needed and 2) to automatically adjust the system gain in a corresponding manner. With some automatic mixers, ordinary microphones are used and the actual control is provided by the mixer. In others, special microphones are integrated with the mixer to provide enhanced control.
Audio Systems Guide for MEETING FACILITIES Summary of automatic mixer benefits • The primary function of an automatic mixer is to keep unused microphones turned off and to instantaneously activate microphones when needed.
Audio Systems Guide for MEETING FACILITIES CHAPTER EIGHT TYPICAL APPLICATIONS In order to select a microphone for a specific application, and to apply it properly, it is first necessary to know the important characteristics of the sound source(s) and of the sound system. Once these are defined, a look at the five areas of microphone specifications will lead to an appropriate match. Finally, proper use of the microphone, by correct placement and operation, will insure best performance.
Audio Systems Guide for MEETING FACILITIES Proper operation of the microphone requires correct connection to the sound system with quality cables and connectors, and correct phantom power if a condenser is used. Use a shock mount to control mechanical noise from the lectern itself. Some microphones are equipped with low-cut or low-end roll-off filters, which may further reduce low frequency mechanical or acoustic noise. Goosenecks should be quiet when flexed.
Audio Systems Guide for MEETING FACILITIES Hand-held Speech Microphone The desired sound source, for a hand-held microphone, is a speaking voice. Undesired sounds may include loudspeakers, other talkers, ventilation noise, and other various ambient sounds. Suitable microphone performance for this application can be provided by dynamics or condensers. Due to frequent handling and the potential for rough treatment, dynamic microphones are most often used, though durable condensers are also available.
Audio Systems Guide for MEETING FACILITIES Balanced low-impedance output is preferred as usual. Sensitivity can be moderate, due to the relatively close placement of the microphone. The physical design is optimized for body-worn use. This may be done by means of a clip, a pin, or a neck cord. Small size is very desirable. For a condenser, the necessary electronics are often housed in a separate small pack, also capable of being worn or placed in a pocket.
Audio Systems Guide for MEETING FACILITIES A particular method that is sometimes suggested for overhead placement is a ceiling-mounted microphone, usually a boundary microphone. This position should be used with caution, for two reasons. First, it often places the microphone too far from the desired sound source, especially in the case of a high ceiling.
Audio Systems Guide for MEETING FACILITIES The use of audience microphones is normally exclusively for recording, broadcast, and other isolated destinations. It is almost never intended to be mixed into the sound system for local reinforcement.
Audio Systems Guide for MEETING FACILITIES Conclusion Though it is one of the smallest links in the audio chain, the microphone is perhaps the most important. As it is the connection between sound source and the sound system, it must interact efficiently with each. Choosing this link successfully requires knowledge of sound and sound systems, microphones, and the actual application. This presentation has included the basic principles not only of microphones but also of sound and sound systems.
Audio Systems Guide for Reference Information MEETING FACILITIES Bibliography Here is a reading list for those who would like to learn more about the technical aspects of audio. These titles are available from Mix Bookshelf at: 800-233-9604 in the U.S.; 908-417-9575 International; 908-225-1562 Fax. Davis, Gary D., and Jones, Ralph Yamaha Sound Reinforcement Handbook. Hal Leonard Publishing Co., Milwaukee, WI.
Reference Information Audio Systems Guide for MEETING FACILITIES Appendix A: Design hints for city councils, school boards, and legislative chambers Appendix A Shure automatic mixers have emerged as the product of choice for city councils, school boards, and legislative bodies. The following are helpful hints on designing and implementing a successful legislative system using a Shure automatic mixer. Mayor or Chairman Position • Privacy switches are normally required in legislative chambers.
Audio Systems Guide for Reference Information MEETING FACILITIES Appendix B Shure microphones and automatic mixers have emerged as the product of choice for courtrooms, video arraignment chambers, and other legal facilities. The following are helpful hints on designing and implementing a successful audio system using Shure products. Suggested Microphones and their Placement • A low profile surface mount microphone is often chosen for a courtroom because of its styling.
Audio Systems Guide for About the Authors Chris Lyons MEETING FACILITIES Michael Pettersen Chris Lyons is Manager, Technical and Educational Michael Pettersen is the Director of Applications Communications, with Shure Incorporated. With more than Engineering at Shure Incorporated. He is the author 21 years of experience in training, technical support, and of Microphone Techniques for Music, of which over product management at Shure, he has presented training 100,000 copies have been printed.
Audio Systems Guide for Reference Information MEETING FACILITIES Shure Product Selection Chart MICROPHONES APPLICATION MODEL PHYSICAL DESIGN DIRECTIONALITY OPERATING PRINCIPLE Lectern MX412/MX418* Miniature gooseneck Omni, cardioid or supercardioid Condenser Lectern SM57 Handheld Cardioid Dynamic Lectern MX392/MX393* Boundary Omni, cardioid or supercardioid Condenser Meeting Table MX392/MX393* Boundary Omni, cardioid or supercardioid Condenser Meeting Table MX412/MX418* Miniat
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