Sennheiser System 2013 PLL - Manual (Page 31)

R
2.13
Setting microphone sensitivity on transmitter SK 2013 PLL
The microphone sensitivity should be set high enough on the regulator (R) that, on the
one hand, the green LED on the Receiver EK 2013 PLL does not go out during normal
speaking and, on the other hand, low enough that normal surrounding noise does not
activate the LED display.
The optimum setting is usually between 5 and 7.