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Integrator's Reference Manual for Polycom HDX Systems
Room Design and Layout
It is important to remember that the use of traditional meeting room downcans—even those that have
color-corrected light sources—for any lighting in the field of view that may include human faces is to be
avoided at all costs. These will result in extremely uneven fields of light, or pools, and heavy, unnatural
shadows on the faces of the participants.
Room Preparation Conclusion
When we follow the above guidelines we dramatically improve the odds for success in the final deployment
of live bi-directional conference-based human communications. An added benefit is that this approach
dramatically enhances the effectiveness of the room as it operates for more traditional meetings and
presentations. The environment is more comfortable and flexible, and less dependent on specialized
electronics for "fixing" deficiencies in the environment.
Audio Elements
Once the space is prepared, we can focus on integration of the various audiovisual tools within the
environment: audio, video and control.
Audio Input
The primary input device for the audio portion of any conference system is the microphone. Elsewhere in
this book [Basics of Audio and Visual Systems Design] we have discussed how these devices operate within
a given acoustic environment. We turn now to a short discussion of how these elements operate within a
conference environment, where such factors as "three-to-one" rules and "critical distance" often are pushed
to the limit or violated entirely.
When sound travels in a room, it follows "the inverse square law." This means that the sound level heard at
a microphone drops by a factor of four every time the distance doubles. Another important consideration in
room audio design is the concept of "critical distance," or the distance at which the loudness of the room
background noise plus reverberation is less than one tenth of the loudness of voices getting to a particular
microphone. (This definition is the result of research conducted by Don and Carolyn Davis. that is
1
referenced in the chapter "Designing for Intelligibility" in the Handbook for Sound Engineers.
)
As an example, we will work with a room having an ambient noise level of approximately 60dBA-SPL. A
person speaking in a normal voice is 72dBA-SPL at about 2' distance. At 4' the loudness drops to
approximately 66dBA-SPL. This already is farther than the critical distance criteria allow, given the ambient
noise level. At 8' distance, a normal speaking voice is approximately 60dBA-SPL. Now the voice energy and
the room background noise are about equal. For "send" audio systems in a room to work correctly, therefore,
the room noise level would have to be below 40-45dBA-SPL at the microphones at all times. This gives us
some measure by which we can begin to plan the microphone array within a space, including selection
based on pickup pattern, sensitivity, noise rejection and signal-to-noise in relation to the ambient noise floor
or level within the space. The good news is that a room designed and built as described in this section will
provide an acoustic space where almost any properly configured and installed audio system can operate
with very good results.
1. Davis, Don and Carolyn. "Designing for Intelligibility" in Handbook for Sound Engineers: The New
Audio Cyclopedia, ed. Glen Ballou (Indianapolis: Howard Sams & Co., 1991), 1279-1297.
Polycom, Inc.
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