Since you are using KDE, you should check out the built-in Konsole (the terminal emulator) feature that notices when activity stops:
to monitor changes in a log or any other file, check the View menu and its Monitor for Activity/Silence options. Selecting this will allow Konsole to alert you via desktop notifications when something happens (or stops happening) in the tab for which you enabled the option
As with any other KDE application, you can choose the type of notifications for Konsole. You'll find the dialog under Settings > Configure Notifications.
See under Working With Files and
(a makeuseof.com article).
You should be able to choose a "visual bell" as it is in the Konsole api.
Alternatively, if using X11, not Wayland, you can play with the display output:
xset dpms force off; sleep 1; xset dpms force on