Polycom HDX series Reference Manual page 326

Room Design and Layout
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.
Perhaps the most difficult issue for any room designer or system planner is
actual microphone placement within the space. Given the fact that many
people view conference table space as sacred (to be used for papers, laptops,
coffee cups and other end-user items), there often is a great deal of pressure to
place the local microphones on the ceiling instead of on the table surface. But
this approach must be taken with great caution. We have already seen the
dramatic impact of changes in the distance between people (their mouths) and
the microphone. Ceiling systems generally place microphones farther away
from the participants' mouths, not closer; critical distance calculations may
eliminate ceiling placement from consideration for this reason alone. In
addition, the ceiling surface generally is one of the noisiest areas of the room.
Proximity to HVAC ducts and vents, attachment of tiles and runners to
building members that are prone to vibration and shaking, and proximity to
noise from other spaces migrating through the plenum make this area one of
the least desirable for placement of microphones. This doesn't, however, keep
people from looking at this broad open surface as the best place for
microphones, to "get them off the table."
If ceiling placement is chosen, the system planner must select the components
with great care from a manufacturer that specializes in this type of audio voice
reinforcement. The manufacturer must be skilled in live audio and capable of
installing the components (that is, being both able and willing to locate
microphones at precisely measured distances from speakers, and locating
those speakers at precisely measured intervals from each other and from the
walls) to extremely tight tolerances. The system provider must fully inform the
endusers of the potential downside effects of this approach. In any event,
simply mounting a standard tabletop microphone on the ceiling tiles or
implementing this solution in an ambient noise environment of 45dBA-SPL or
greater will all but guarantee costly failure. No amount of post-microphone
processing will fix the problems.
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