A public address system comprising: a transmission medium including a main transmission path and at least one branch transmission path branching from the main transmission path, the transmission medium being configured to carry a transmission signal; and a plurality of loudspeakers connected to the transmission medium; and a plurality of monitoring devices connected to the transmission medium, each of which includes an input operative to produce and output a selection signal to selectively set up the monitoring device as a first branch monitoring device connected to the transmission medium at a first branch point where the branch transmission path begins, as a second branch monitoring device connected to the transmission path at a second branch point where the branch transmission path ends, or as a non-branch monitoring device not being located at any branch point; wherein the public address system includes: a monitoring device which is arranged at the first branch point and is set up as the first branch monitoring device, and another monitoring device which is arranged at the second branch point and is set up as the second branch monitoring device, the first branch monitoring device set up as the first branch monitoring device according to the selection signal is configured to transmit the transmission signal from the main transmission path into the branch transmission path while transmitting the transmission signal through the main transmission path, and the second branch monitoring device set up as the second branch monitoring device according to the selection signal is configured to: determine that it has detected a transmission of the transmission signal through the branch transmission path, block passage of the transmission signal from the main transmission path into the branch transmission path if the transmission of the transmission signal through the branch transmission path is detected, while transmitting the transmission signal through the main transmission path, and allow passage of the transmission signal from the main transmission path into the branch transmission path if the transmission of the transmission signal through the branch transmission path is not detected. a transmission medium including a main transmission path and at least one branch transmission path branching from the main transmission path, the transmission medium being configured to carry a transmission signal; and a plurality of loudspeakers connected to the transmission medium; and a plurality of monitoring devices connected to the transmission medium, each of which includes an input operative to produce and output a selection signal to selectively set up the monitoring device as a first branch monitoring device connected to the transmission medium at a first branch point where the branch transmission path begins, as a second branch monitoring device connected to the transmission path at a second branch point where the branch transmission path ends, or as a non-branch monitoring device not being located at any branch point; wherein the public address system includes: a monitoring device which is arranged at the first branch point and is set up as the first branch monitoring device, and another monitoring device which is arranged at the second branch point and is set up as the second branch monitoring device, a monitoring device which is arranged at the first branch point and is set up as the first branch monitoring device, and another monitoring device which is arranged at the second branch point and is set up as the second branch monitoring device, the first branch monitoring device set up as the first branch monitoring device according to the selection signal is configured to transmit the transmission signal from the main transmission path into the branch transmission path while transmitting the transmission signal through the main transmission path, and the second branch monitoring device set up as the second branch monitoring device according to the selection signal is configured to: determine that it has detected a transmission of the transmission signal through the branch transmission path, block passage of the transmission signal from the main transmission path into the branch transmission path if the transmission of the transmission signal through the branch transmission path is detected, while transmitting the transmission signal through the main transmission path, and allow passage of the transmission signal from the main transmission path into the branch transmission path if the transmission of the transmission signal through the branch transmission path is not detected. determine that it has detected a transmission of the transmission signal through the branch transmission path, block passage of the transmission signal from the main transmission path into the branch transmission path if the transmission of the transmission signal through the branch transmission path is detected, while transmitting the transmission signal through the main transmission path, and allow passage of the transmission signal from the main transmission path into the branch transmission path if the transmission of the transmission signal through the branch transmission path is not detected.