A Soundbeam Tower is a defensive structure.
It can attack both air and ground units (except for henchmen), but is only effective against ground, unlike the Anti-Air Tower.
After researching Defensive Tower Upgrade in Research Clinic, it can attack two units at one time. It can of course attack just one unit too after the upgrade, but it's uncertain if the attacks become stronger.
It's more effective if covered with Bramble Fences.
Creatures with Barrier Destroy and a high attack can quickly destroy the structure. If grouped together in a nest; some large ranged units can stay out of the range of the majority of Soundbeam Towers, destroying them one at a time. Another option is to use artillery, which can be devastating for being able to hit multiple targets at once.
In the Campaign, it is available from Mission 3: Fathers and Sons, after having Lucy gathering the technology.