Microphones: Microphone Placement In The Classroom

By Chris Lyons, Shure Brothers Microphones

Audio for distance learning or instructional television can be frustrating. Instructors, students, and installers usually complain of hollow sound, feedback, or echo. At best, these problems discourage student participation and increased instructor fatigue; at worst, the learning process may be completely disrupted. Here "in orders of importance" are some guidelines for getting good audio in a classroom environment.

1. Choose a room that is very quiet, with soft coverings on walls and floor. This is the most critical factor in determining audio quality in a classroom. The distance at which microphones can be placed from talkers is determined primarily by the acoustic characteristics of the room itself. The nosier or more reflective the classroom is, the closer the microphones must be to the students for acceptable results. Noise from typical air handling systems can be clearly audible to participants at other sites, even though it may not be noticeable to people sitting in the classroom. Hard, acoustically reflective room surfaces cause talkers to sound hollow or "like they’re in a barrel", and increase problems with feedback (a howling or whistling sound) and echo. Acoustically absorbent wall panels absorb over 80% of the sound waves that strike them, while indoor-outdoor carpeting only absorbs about 20%.

2. Place microphones within arm’s reach of every talker. Close microphones – preferably on the table or desk in front of the student – sound fuller and more natural, pick up less background noise, and reduce feedback and echo problems. With 4 or 6 students seated at each table, every two students can share one microphone. These can be either flat surface-style microphones or flexible gooseneck microphones. If the microphones must be put away at the end of the day, the gooseneck microphones can be mounted on a moveable base. For classrooms in which desktop microphones are impossible due to flexible seating arrangements, small directional microphones hanging one or two feet down from the ceiling can work, if the room is very quiet and non-reflective. The Shure MX202 is designed for this purpose. In classrooms with noisy air handling systems or large areas of hard surfaces (tile floors, plaster or concrete block walls, large windows), this is not a viable option. Do not put microphones-even surface-style models – directly on the ceiling. Ceilings usually vibrate or rumble and the noise from the air ducts and light fixtures will probably be as loud or louder than the weak remnants of speech that reach the microphones.

3. Choose the right microphones. Unidirectional types, usually with a cardioid or super cardioid pickup pattern, pick up less ambient and reflected sound. When positioned so that the side or rears of the microphones are oriented toward the loudspeakers, they also reduce feedback and echo problems. Condenser microphones tend to be smaller and more sensitive than dynamic types, but usually require external dc power (called "phantom power") from the mixer. Flat surface mount or "boundary effect" microphones are almost invisible on camera, but may be covered by books or pick up the sound of papers shuffling directly in front of the mic. Slim gooseneck microphones rise above the potentially noisy table surface, reducing pickup of unwanted sounds.

4. Keep microphones away from loudspeakers. Eight feet is usually adequate separation. If the sound from the loud speakers is picked up by the microphones either directly or after being reflected, it may be transmitted back to the originating site. This can cause either feedback or a distinct echo of the talker’s words, depending on the type of transmission network. The more reflective the room is, the lower the loudspeaker volume must be to avoid feedback or echo.

5. Turn off unneeded microphones. Multiple open microphones ‘blur’ the sound of the talker’s voice, pick up additional background noise, and (surprise!) increase feedback and echo problems. Students can operate their own microphones if equipped with push-to-talk switches, or an automatic mic mixer can do the work for them, turning the appropriate microphone on the instant that sound arrives at the microphone. Some push-to-talk microphones or automatic camera controller, directing the camera to a preset shot.

Even the most elaborate, expensive, cleverly installed, and carefully adjusted audio equipment cannot compensate for having noisy, reverberant sound fed into it. In other words, :garbage in = garbage out." And if just one site on a network breaks the "Audio Rules", every other site on the network will pay the price in the form of poor sound quality.

-Chris Lyons is a Senior Applications Engineer specializing in Distance Learning at Shure Brothers Incorporated.