Polycom SoundStructure C16 Design Manual page 732

capacity (40 to 80 Watts typical), sensitivity (86 dB SPL @ 1 m typical), and nominal coverage angle (130
degree typical). The loudspeaker coverage angle is defined by the angle where the loudspeaker levels are
no less than 6 dB below the on-axis level. The following figure shows a typical coverage angle for a ceiling
mounted loudspeaker and illustrates that listeners farther away from the loudspeaker axis will receive less
sound than listeners directly below the loudspeaker. In the following figure, not only are listener 2 and
listener 3 farther away from the loudspeaker than listener 1 and receive less audio due to the inverse square
relationship, but they also receive less audio from the loudspeaker due to the inherent 6 dB difference
between the off-axis response from the on-axis response of the loudspeaker.
While this example shows a single loudspeaker and multiple participants, most rooms will require multiple
loudspeakers to provide enough coverage that the remote audio can be heard at a comfortable level by all
the participants, regardless of where they are sitting.
There are numerous rules of thumb for how many loudspeakers and the amount of power required in a
particular size room and there are also software tools that are freely available to help with the power
requirements and loudspeaker placement for a given sized room.
One rule of thumb is that the typical ceiling-mounted loudspeaker will have a coverage area of
approximately 100 sq. feet at ear level assuming a 9 foot ceiling - shown in the following figure as a radius
of about 5 ½ feet. Depending on how close the loudspeakers are clustered, there will be different levels of
audio that are heard by local participants. As shown in the following figure, clustering the loudspeakers
farther apart (about 11 feet in this example) will cover a larger area but have some significant level variation
depending on where the listeners are. Clustering the loudspeakers closer together will increase the overlap
and reduce the level variation that is heard by listeners based on their position in the room. While tighter
clusters (such as the right hand of the following figure) will minimize the level variation, it will also increase
the cost of the system as more loudspeakers will be required to cover a particular area. This is little benefit
of increasing the number of loudspeakers beyond the tightest pattern shown in following figure where the
loudspeakers are approximately 5½ feet apart.
Another rule of thumb about loudspeaker positioning with listeners is to distribute the loudspeakers no
greater than twice the distance from the ceiling to the listener's ear level. In a conference room with 9 foot
high ceilings and seated listeners' ears about 3 ½ feet above the floor, this rule of thumb corresponds to a
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