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Introduction To Speech Reinforcement With Conferencing
Insight into acoustic proprieties of speech reinforcement and applications.
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This article is provided by Rane Corporation.

 
Large conference rooms require speech reinforcement so people at all locations can adequately hear each other.

To perform speech reinforcement without acoustic feedback is difficult, add conferencing and it becomes complex.

Controlling Acoustic Levels
The following operations neglect any effect of room echo or room acoustics.

Terms used for calculating properties:

D0: Distance from talker to the farthest listener.
D1: Distance from the source mic to the nearest loudspeaker.
D2: Distance from the listener to the nearest loudspeaker.
Dn: Distance from the talker to the nearest listener.
Ds: Distance from the talker to the microphone.
NOM: Number of Open Mics

When audio travels from a source, its Sound Pressure Level (SPL) attenuates by half for every doubling of the distance.

The formula for calculating the SPL attenuation is known as the inverse square law and is stated as:

Without speech reinforcement.

Inverse Square Law:

SPL Attenuation = Dn SPL - 20Log(D0/Dn)

When applying sound reinforcement to a large conference room, first you need to know the room’s Potential Acoustic Gain (PAG ). This allows you to determine the maximum amount of sound reinforced, in decibels, achievable before feedback occurs.

The PAG formula:

PAG = 20Log((D0 * D1)/(Ds * D2))

If NOM is greater than 1 then:

PAG = 20Log((D0 * D1)/(D2 * Ds)) - 10LogNOM

When using PAG to setup system gain, it is customary to add 6 dB of Feedback Stability Margin (FSM). Systems that operate at 6 dB below their PAG are usually free of feedback problems.

PAG = 20Log((D0 * D1)/(D2 * Ds)) - 10LogNOM - 6 dB

How much sound reinforcement is needed to achieve an average SPL at a distant listener’s position relative to the non-reinforced SPL at a near listener’s position?

This Needed Acoustical Gain or NAG is the gain in decibels required by sound reinforcement to achieve an equivalent acoustic level at the farthest listener equal to what the nearest listener would hear without sound reinforcement.

The NAG formula:

NAG = 20Log(D0/Dn)

NAG must be less than or equal to PAG to avoid feedback.

Example:

D0: 20 feet
D1: 10 feet
D2: 6 feet
Dn: 4 feet
Ds: 2 feet


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