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Integrator's Reference Manual for Polycom HDX Systems
Room Design and Layout
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.
Audio Output
For conference communication we do not really care about producing the thundering roar of jet aircraft
engines, or other sounds reproduced on TV or in the movies. We are interested in reproducing the human
voice. The tone, intonation, pitch and level of people speaking from the far-end should sound as much as
possible like the sound they would make if they were speaking in the room. Given what has been covered
in other sections of this book [Basics of Audio and Visual Systems Design], we will touch base here on a
couple of simple, basic elements of the speaker technology we deploy in the conference room. These basics
fall into three subcategories: direction, power and range/frequency response.
Direction
As human beings, we feel most comfortable when the voice we hear appears to come from the same
direction as the image of the person speaking. This means that reliance on ceiling speakers alone is not an
ideal practice when the system is used for videoconferencing. In many small and medium-sized systems,
front-firing speakers alone can provide proper direction and adequate coverage. Larger rooms (greater than
12'x15') probably need both front-firing and side or top-fill speakers in order to maintain proper coverage at
nominal power levels.
In planning systems for larger rooms, we need to take advantage of the HAAS effect. Basically stated, this
is the human brain's interpretation of sound direction when the same sound arrives at the ear from two or
more directions within a certain time period. We attribute the direction of the sound to the direction from
which the sound is first perceived, even if it is mixed with that same sound arriving from a completely
different direction, as long as the two (or more) instances of the sound are within about 30ms of one another.
Since sound travels faster electronically than it travels through the open air we may need to add audio delay
to the side firing or ceiling speaker arrays in order to keep the primary perceived point source as the front
of room/front-firing speakers.
Polycom, Inc.
578
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