Do I have to connect a horn speaker?

Yes - unless you want to use the PA output terminals to provide an audible alarm via an existing PA system or unless you want only visual indication of incoming calls.

The Universal Alarm's audible output can be connected to the unbalanced auxiliary input (10K ohms) of a PA amplifier via screw terminals 3 & 9. The alarm's tones will then be broadcast through the existing PA system's speakers.

If a visual-only system is required (as in the case study of the inbound call centre), no horn speaker(s) need to be connected.