It is variable. And the BRIDGE has been supplanted by the BOOST. 

The BRIDGE is an S1 product only, that you connect to your router, so that your system is in ‘wired’ mode, I.e running off of SonosNet, and not using your Wi-Fi signal, although still using your router. You can also wire a speaker to your router, and it will act as a BRIDGE/BOOST. The BRIDGE is often used in cases where people want a wired network for their Sonos system, but don’t have a speaker close enough to their router in order to have the speaker wired to the router. Many people have been reporting issues with the power supply on the BRIDGE, likely due to age, which causes the voltage to drop below 5.0 volts, and creates seemingly random issues that are difficult to put your finger on, since they constantly change. The BRIDGE also uses what is called a version of SonosNet that is slower than the current version. It doesn’t make a huge difference, but it is version 1.0 of SonosNet.

The BOOST is a newer product, S1 and S2 compliant, which has functionally that is the same as the BRIDGE, but with SonosNet 2.0, slightly better interference rejection, and better antennas built in. To date, there has been no indication of any failures in the power supply voltage. 

Because a speaker can do the same thing as a BOOST or BRIDGE, they aren’t required, even for a wired system, but are extremely convenient for people like me, who want to run a wired environment, but don’t have a speaker close enough to the router to wire easily.