Loudspeakers - How Much Power Is Required; Spatial Directionality; Microphone And Loudspeaker Placement Considerations - Polycom SoundStructure C16 Design Manual

Loudspeakers - How Much Power Is Required

Once the locations of the loudspeakers have been determined, the next step is to determine how much
power is required to drive each loudspeaker to achieve the required level at the listeners. Loudspeakers
have a power rating that is expressed in dB SPL @ 1 meter. This specifies how much sound is created at
1 m from the loudspeaker assuming 1 W of power is applied to the loudspeaker. For instance a loudspeaker
rated at 86 dB SPL 1 Watt/meter requires 1 W to generate 86 dB SPL at a distance of 1 m from the
loudspeaker. In a normal room, the loudspeakers are approximately 2 m above the participant that is seated
directly underneath a loudspeaker, resulting in 80 dB SPL at the listener's location with 1 W of power. As
normal speech conversations are in the 70 dB to 77 dB SPL range, this level of sound would be more than
sufficient for playback for a listener located directly underneath a loudspeaker assuming there is not a
significant amount of background noise in the room (such as in an NC 30 style room). As mentioned earlier,
having a sound system that is at least 25 dB above the ambient background will yield excellent intelligibility.
To compute the power requirements for each of the distributed loudspeakers, it is convenient to take the
desired SPL at the listeners' ears (for example 85 dB SPL), add additional level to compensate for the
distance from the loudspeakers (for example listeners 2 m away from the loudspeaker will require an
additional 6 dB to compensate for the 6 dB loss 2 m away from the loudspeaker), and subtract the
loudspeakers sensitivity (86 dB SPL). In this example this results in 85 dB + 6 dB - 86 dB = 5 dBW. This is
the power in dBW (referenced to 1 W). To convert this back to Watts, it is necessary to take the inverse log
(recall that a power in dBW is 10 log (Power in watts/1 watt)). The result is that 3.16 W is required for each
loudspeaker. As most constant voltage systems have a switch to select the appropriate power setting,
selecting the tap closest to 3 W would generate the proper level for the loudspeakers.
Once the power requirements have been calculated for the loudspeakers, add them up and use this as the
baseline for the size of the amplifier required to drive audio into the room. Keep in mind that there will be
transformer losses on the order of a 1 dB or so for each loudspeaker, so choose an amplifier at 25% to 50%
larger than the sum of the required loudspeaker power. This will allow some headroom for additional
loudspeakers or louder transient signals.

Spatial Directionality

In video conferencing applications, it is common to have the remote audio come from a location close to the
display screen to give the perception that the remote audio is coming from the remote participants shown
on the video screen. In this situation, when the room is larger than about 20 ft. in length, reinforcement of
the front speakers with additional ceiling loudspeakers distributed through the room will better fill the room
with sound (keeping all listeners well within the critical distance of the loudspeakers) without losing the
perceptual directionality associated with the front of room display device. To maintain the perception of the
audio coming from the front of room display device, the reinforcement of the ceiling loudspeakers can be
delayed slightly (1 msec for each foot of separation) from the front loudspeakers and can be attenuated by
approximately 6 - 10 dB from the level sent to the front of room loudspeakers.

Microphone And Loudspeaker Placement Considerations

Once loudspeakers have been placed through the room to achieve good sound coverage, and microphones
have been placed through the room to provide good sound pickup, it is still necessary to double check the
placement of the microphones relative to the loudspeakers to ensure the ERL of the conferencing system
hasn't been compromised in the process. If loudspeakers are placed in the direct pick up pattern of the
microphone, the ERL will be reduced and there is a higher chance of residual echo being sent to the remote
participants. Typically the configuration software with the audio conferencing product will provide a tool
where the ERL can be checked to ensure it is within the recommended operating range for that particular
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