Integrator's Reference Manual for Polycom HDX ... - 1 PC Network Inc

Room Design and LayoutAs an example, we will work with a room having an ambient noise level ofapproximately 60dBA-SPL. A person speaking in a normal voice is 72dBA-SPLat about 2’ distance. At 4’ the loudness drops to approximately 66dBA-SPL.This already is farther than the critical distance criteria allow, given theambient noise level. At 8’ distance, a normal speaking voice is approximately60dBA-SPL. Now the voice energy and the room background noise are aboutequal. For “send” audio systems in a room to work correctly, therefore, theroom noise level would have to be below 40-45dBA-SPL at the microphones atall times. This gives us some measure by which we can begin to plan themicrophone array within a space, including selection based on pickup pattern,sensitivity, noise rejection and signal-to-noise in relation to the ambient noisefloor or level within the space. The good news is that a room designed andbuilt as described in this section will provide an acoustic space where almostany properly configured and installed audio system can operate with verygood results.Perhaps the most difficult issue for any room designer or system planner isactual microphone placement within the space. Given the fact that manypeople 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 toplace the local microphones on the ceiling instead of on the table surface. Butthis approach must be taken with great caution. We have already seen thedramatic impact of changes in the distance between people (their mouths) andthe microphone. Ceiling systems generally place microphones farther awayfrom the participants’ mouths, not closer; critical distance calculations mayeliminate ceiling placement from consideration for this reason alone. Inaddition, the ceiling surface generally is one of the noisiest areas of the room.Proximity to HVAC ducts and vents, attachment of tiles and runners tobuilding members that are prone to vibration and shaking, and proximity tonoise from other spaces migrating through the plenum make this area one ofthe least desirable for placement of microphones. This doesn’t, however, keeppeople from looking at this broad open surface as the best place formicrophones, to “get them off the table.”If ceiling placement is chosen, the system planner must select the componentswith great care from a manufacturer that specializes in this type of audio voicereinforcement. The manufacturer must be skilled in live audio and capable ofinstalling the components (that is, being both able and willing to locatemicrophones at precisely measured distances from speakers, and locatingthose speakers at precisely measured intervals from each other and from thewalls) to extremely tight tolerances. The system provider must fully inform theendusers of the potential downside effects of this approach. In any event,simply mounting a standard tabletop microphone on the ceiling tiles orimplementing this solution in an ambient noise environment of 45dBA-SPL orgreater will all but guarantee costly failure. No amount of post-microphoneprocessing will fix the problems.A - 9