Typical microphone applications for installed audio systems
August 22, 2011, by Chris Lyons, Tim Vear, & Michael Pettersen
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.
This section presents recommendations for some of the most common meeting facility sound applications.
The desired sound source, for a lectern microphone, is a speaking voice. Undesired sound sources that may be present are nearby loudspeakers (possibly overhead) and ambient sound (possibly ventilation, traffic noise, and reverberation).
The sound system in this and the following examples is assumed to be high quality with balanced low-impedance microphone inputs.
The basic performance requirements for a lectern microphone can be met by either dynamic or condenser types, so the choice of operating principle is often determined by other factors, such as appearance.
In particular, the desire for an unobtrusive microphone is best satisfied by a condenser microphone, which can maintain high performance even in very small sizes. If phantom power is available, a condenser is an excellent choice. If not, dynamic types, though somewhat larger, are available with similar characteristics.
For the microphone to match the desired sound source (the talker’s voice) it must first have a frequency response which covers the speech range, (approximately 100Hz to 10kHz).
Within that range the response can be flat, if the sound system and the room acoustics are very good, but often a shaped response will improve intelligibility.
Above 10kHz and below 100Hz, the response should roll off smoothly, to avoid pickup of noise and other sounds outside of the speech range, and to minimize proximity effect.
The choice of microphone directionality that will maximize pickup of the voice and minimize undesired sounds, is unidirectional. This type will also reduce the potential of feedback since it can be aimed toward the talker and away from loudspeakers.
Depending on how much the person speaking moves about, or on how close the microphone can be placed, a particular type may be chosen: a cardioid for moderately broad, close-up coverage; a supercardioid or a hypercardioid for progressively narrower or more distant coverage.
The electrical characteristics of the microphone are primarily determined by the sound system: in this case, a balanced low – impedance type would match the inputs on the mixer.
Of course, this would be the desired choice in almost all systems due to the inherent benefits of lower noise and longer cable capability.
The sensitivity of the microphone should be in the medium-to-high range since the sound source (speaking voice) is not excessively loud and is picked up from a slight distance.
Again, this is most easily accomplished by a condenser type.
The choice of physical design for a lectern micro-phone must blend performance with actual use.
The most effective approach is a gooseneck-mounted type, which places the microphone close to the sound source and away from both the reflective surface of the lectern and noise from the handling of materials on it.
Another approach is the use of a boundary microphone on the lectern surface, but this method is limited by lectern design and by the potential for noise pickup.
As mentioned above, the desired physical design may also suggest the operating principle. The most effective small gooseneck or boundary styles are condensers.
The ideal placement of a lectern microphone is 6 to 12 inches away from the mouth, and aimed toward the mouth. This will give good pickup of the voice and minimum pickup of other sources.
Also, locating the microphone a few inches off-center will reduce breath noise that might occur directly in front of the mouth. It is not recommended that two microphones be used on a lectern as comb filtering interference is likely to occur.
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. It is strongly recommended that a pop filter be placed on the microphone to control explosive breath sounds, especially when using miniature condenser types.
Good technique for lectern microphone use includes:
• Do adjust the microphone position for proper placement.
• Do maintain a fairly constant distance of 6 to 12 inches.
• Don’t blow on microphone, or touch microphone or mount when in use.
• Don’t make excess noise with materials on lectern.
• Do speak in a clear and well-modulated voice.
The desired sound source at a meeting table, is a speaking voice.
Undesired sounds may include direct sound, such as an audience or loudspeakers, and ambient noise sources such as building noise or the meeting participants.
A boundary microphone is the physical design best suited to this application.
This will minimize interference effects due to reflections from the table surface and will also result in increased microphone sensitivity.
A condenser type is the most effective for this configuration, due to its high performance and small size.
The frequency response should be slightly shaped for the vocal range and will usually benefit from a slight presence rise.
A unidirectional (typically, a cardioid) pattern will give the broadest coverage with good rejection of feedback and noise.
Finally, the microphone should have a balanced low-impedance output, and moderate-to-high sensitivity.
Placement of the microphone should be flat on the table, at a distance of two to three feet from, and aimed towards the normal position of the talker.
If possible, it should be located or aimed away from other objects and from any local noise such as page turning.
If there is more than one distinct position to be covered, position additional microphones according to the 3-to-1 rule.
The microphone should be connected and powered (if necessary) in the proper fashion. If the table itself is a source of noise or vibration, isolate the microphone from it with a thin foam pad.
A low-frequency filter may be a desirable or even necessary feature.
A pop filter is not normally required. Make certain the microphones are never covered with papers.
Good technique for meeting table microphone use includes:
• Do observe proper microphone placement.
• Do speak within coverage area of the microphone.
• Don’t make excess noise with materials on table.
• Do project the voice, due to greater microphone distance.