Do I have to connect an external horn-speaker?

No. The Tone Ringer has an inbuilt speaker that is adequate for most situations.

However, adding an external horn-speaker will allow the alarm tone to be focused towards a particular area, increasing the apparent loudness of the alarm.

Just remember that the Tone Ringer (even with an external horn-speaker) is designed for operation in a relatively quiet environment and that one of our other alarm products such as the Universal Alarm may be better suited in a medium-to-high ambient noise location.