Large Room Environments; Microphone Selection And Placement; Microphone Fundamentals - Polycom SoundStructure C16 Design Manual

In small conferencing room spaces, a tabletop conferencing phone is often an adequate solution for audio
conferencing and provides an all-in-one package that is easy to deploy and provides good sound quality in
the many room environments.

Large Room Environments

The typical room environment introduces ambient noise (from HVAC, outside noise, projectors, computers,
etc.), in-room reflections of the audio (multipath audio), and constraints on microphone and loudspeaker
positioning.
Conference rooms should be designed to a NC 30 standard if possible. An NC 30 rating corresponds to
approximately 40 dBA SPL of background noise, leaving about 30 dB of SNR for normal talkers (70 - 77
dBA SPL at 1 meter). A lower NC rating will further improve the quality of the audio conferencing system by
improving intelligibility and reducing listener fatigue but will also increase the overall cost of the room
build-out. For an audio system to have good intelligibility, it is important for the signals that are heard by the
local talker to be at least 25 dB above the background noise level.
The surfaces in the room including walls, ceiling, and furniture will affect the quality of the conferencing
system. Hard surfaces will increase the amount of reflections in the room causing multiple versions of a local
talker's audio signal to be heard by the microphones and for multiple versions of the remote talker's signal
to be heard in the local room. Multiple versions of the signals that are time delayed with respect to each
other will cause noticeable comb filtering effects that will filter out particular frequencies (dependent on the
time separation of the multiple versions of the signals), degrading the quality of the signal. Once the
frequencies are filtered out by the comb filtering, they can not be restored by equalization.
Often the conference room location is selected due to the outside view from the room or because it is a
convenient location. However the location of a potential conference room should be evaluated to make sure
it is not directly underneath a building's HVAC units, nor near other environmental noise sources such as
shipping rooms, loading docks, copy rooms, network operations centers, and other such rooms to ensure
that the outside noise sources are minimized in the conference room.

Microphone Selection And Placement

The type of microphones used and their location will have the largest impact on the audio conferencing
quality. Microphones translate the acoustic signals from the local talkers into electrical signals that can be
processed and sent to the remote participants.

Microphone Fundamentals

Most microphones used in conferencing systems are electret microphones, a version of condenser style
microphones where an acoustic signal on a thin film dynamically varies the capacitance of an electrical
circuit which in turn creates an electrical voltage that represents the microphone signal. Condenser
microphones require a bias voltage, called phantom power, to operate properly. Electret microphones are a
variant of condenser microphone that replaces the thin film with a dielectric material that is permanently
charged and suspended above a metal plate. While electret microphones don't require a bias voltage to
operate due to being permanently charged, they do typically contain an integrated preamplifier that is
powered using the phantom power from the device the microphone is connected to. Due to the design of
electret microphones, these microphones come in a large variety of sizes and shapes and can provide
excellent audio quality.
Polycom, Inc. 717