Audio Output; Direction; Power - Polycom HDX series Reference Manual

Integrator's Reference Manual for Polycom HDX Systems

Audio Output

Direction

Power

A - 10
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.
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.
Power is a function of loudspeaker efficiency and total available system
power. Most speakers operate in a power range that is broader than the range
in which they operate without distortion. For the purpose of conference
communication, we are interested in sound that has little or no distortion.
Sound that is reproduced accurately (with no distortion) will most accurately
represent the voice of the people from the far-end (our primary goal). Accurate
reproduction also will aid the echo-cancellation circuitry in the system,
minimizing the amount of echo that the system sends back to the people at the
far-end, and thereby increasing perceived ease of intelligibility and
understanding. Remember that any distortions present in the playback audio
system—whether harmonic, amplitude (gain compression) or temporal (time