Large Area Assistive Listening Systems: Review and Recommendations

3. MICROPHONE INPUTS

The quality of the signal received by users of an ALS system depends not only on the transmission characteristics to the receiver, but on the nature of the source. If the source is "live", then the quality of the signal at the microphone will affect the quality of the signal at all later stages in the transmission process. Questions and problems related to the appropriate type and placement of microphones in live performances was a recurring theme in both the consumer and manufacturer's focus groups.

3.1 Selecting the Appropriate Microphone

The selection of appropriate microphone types is crucial when they are to be the primary source of input into an ALS. In order to ensure adequate signal strength and maximum speech intelligibility, microphone specifications should meet or exceed the specifications of the rest of the system components. Poor quality microphones will limit the effectiveness of an ALS. To minimize background noise, it is desirable to utilize noise-suppressing directional microphones (such as those with a cardioid or hypercardioid pattern).

The most desirable situation is where all talkers have available a microphone in close proximity to their lips. If a situation, such as theatrical performances that require multiple microphones, then it's desirable to either utilize an automatic mixer (one that switches on only the microphone being spoken into) or for the microphone gain and activation to be controlled by a sound technician. In this latter instance, the sound technician can also control the relative level of the various sound signals in order to favor speech or lyrics and de-emphasize other competing stimuli such as background music. Currently, the provision of a separate wireless microphone for each performer is evidently practiced in only major theaters and then unpredictably. (Theaters do not publicize the details of their sound reproduction system, other than to indicate that one is available as required by the ADA).

3.2 Microphone Location

There are no standards for microphone placement in live performances. The actual location appears to be a pragmatic mix composed of production requirements, financial resources, physical limitations, and a sophisticated appreciation (or lack of it) of sound transmission and perception. In some theaters, there may be one or more microphones near the footlights or dangling from the ceiling. There may or may not be a sound technician controlling the signals. It is easier to control microphone positioning when only one talker is involved, but even so optimal microphone technique cannot be taken for granted (Ross 1991). If the person is using a podium microphone (as opposed to lapel or lavaliere microphones worn on the body), often he or she will move away from it as the lecture proceeds. In any large area listening situation, where only one talker is involved (a lecture, sermon, etc.) it would be desirable to provide speakers with the alternative of using a lapel or lavaliere microphone; this will give them freedom of movement around the podium without impacting upon the quality of sound reaching the microphone.

In situations other than theatrical performances where more than one microphone is required (i.e. panel discussions), then each participant should have a microphone close by. Optimally, these should be normal "off", and either be sound or switch activated. This is to prevent inadvertent vocalizations, or verbal "asides" made by one panel member to another, from being picked up and transmitted to the entire audience. In programs in which audience participat ion is expected (as in a large lecture), then a wireless FM microphone should be available for audience use.

3.3 Need for "Overriding" Principle

Because there are so many permutations regarding types and conditions of microphone usage, it would be helpful to have some overriding principle that can be applied generally, to all situations in which microphones are picking up "live" signals.

The principle used by the Lexington Rehabilitation Engineering Research Center (RERC) was to emphasize the integrity of the sound signal as it is delivered through earphones by focusing on defined electroacoustic products. This concept can be employed for both live and recorded transmissions. It gives installers and manufacturers wide latitude in how they reach these electroacoustic goals. The concept and the research project through which we arrived at our performance recommendations will be expanded upon at length in the last section of this paper.