This page explains the relationship between speaker line (SP line) and attenuator (ATT).

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Attenuator (ATT)

ATT is used when you want to adjust the volume for each speaker (SP).
ATT is installed between the main line of the SP line and the branch line to the SP, and is used to reduce the volume of the SP on the branch line. Most of them are mounted on the wall and have a knob to lower the volume. For example, if you do not want to broadcast background music only to the conference room, an ATT can be attached to the wall of the conference room.
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Of course, the user can lower the volume using the volume on the AMP, but the volume of all SPs connected to that AMP will be lowered. And since the AMP is in the machine room, the user has to go to the machine room to turn down the volume. By using ATT, the user can lower the volume only at a specific SP.​

​Type of Speaker line (SP line)

There are 2-wire, 3-wire, and 4-wire types of SP line. Normally, 2-wires would be enough to make the SP sound, but the reason there are 3-wire and 4-wire is to disable the ATT sound reduction setting during emergency broadcast (EMG broadcast). AMP and ATT are divided into types for each method. There are also products that can be used for both methods.

3-wire and 4-wire systems are divided by region. The 3-wire system realizes EMG broadcasts using only the SP line. The 4-wire system consists of 2 SP lines and 2 control lines. This control line disables ATT during EMG broadcast.
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ATT capacity

Multiple SPs can be connected to an ATT. In other words, if there are 4 SPs installed in a room, the volume of all 4 SPs can be reduced with one ATT. The number of SPs that one ATT can handle is indicated as Input Range (W) of ATT specifications. Compare this with SP's lated input (W) and connection is possible within the range below.

​             ATT's input range (W) > Total of SP's lated input (W)