Restrictors, often called Thumpers by Resistance members, are tall, monolithic structures used by the Combine to deter antlions. Using a heavy piston, they send strong vibrations into the ground which seem to frighten and ward off antlions. They are used prolifically along coastal areas near to Nova Prospekt.
Restrictors are highly effective at deterring antlions. When straying near them, whenever the piston hits the ground, they will jump back and stay out of the effective radius of it. The heavy central piston will lift up above the ground, before dropping back down to the earth, delivering a powerful vibration.
They are deployed by the Combine, however are often utilized by Resistance members for protection in dangerous areas.
The most common restrictor type, found along the Coast near Combine-controlled areas as well as in some Resistance bases, is relatively small, but still effective. The ones seen around Nova Prospekt are larger and presumably more powerful, with their activation switches located at the top of a ladder. What appear to be gigantic restrictors are also attached to the base of the Citadel, where they hammer the ground repeatedly in the same fashion as the smaller restrictors. If these are indeed restrictors they could be used to keep the antlions out of the city entirely, since the failure and fall of the Citadel allowed antlions to enter the city.
The Combine use restrictors extensively in antlion-infested locations, placing them in strategic positions to prevent them over-running important areas.
The Restrictors however, cannot fully deter the Antlion Worker due to ability of launching acid from distance. Thus, the player should always kill these Antlions when encountering one.
Through the course of the chapter Highway 17, several restrictors can be seen and used along the coast to keep the antlions at bay. Some, like the one at New Little Odessa, are used to keep antlions out of more populated or mission-critical areas.
While crossing the antlion-infested beach in the chapter Sandtraps, one restrictor can be found, but it is not operational. In order to get it working, Gordon is required to find and activate a generator trailer some distance away. It is notable that this restrictor is among the few items that are powered indirectly; along the coast, a Force Field generator can be seen powered by an APC.
Nova Prospekt is surrounded by a network of restrictors to keep the hordes at bay. When Gordon Freeman attacks Nova Prospekt with the antlions in his control, he is required to disable a number of the devices to allow the antlions to assist him.
Half-Life 2: Episode Two
When emerging from the Victory Mine, Gordon finds a large open area containing a lift up out of the valley. Three switch-less restrictors can be seen in the area, and their purposes are revealed when two Antlion Guards rush out from the mines and begin attacking. Gordon must use the area protected by the closest restrictor for protection from the following antlions, however the makeshift metal building also protects from objects flung by the guards. In addition, one of the (overrun) Resistance outposts inside the Victory Mine contains a switch-less restrictor as well.
Behind the scenes
- In the early stages of Half-Life 2's development, the thumper was also to have a portable version. It can be found and used in the playable Half-Life 2 Beta, where a yellow fire hydrant is used as a placeholder model. It was simply named "Portable Thumper". The player was to carry only one at a time and put it on the ground to achieve to same effects as the larger, stationary version. A second model, not appearing in-game and named "heavy", was probably a more powerful version.
- In early Wasteland maps, early brush thumpers can be found. They also appear as models in the playable Half-Life 2 Beta files, the hammer being a separate model.
- In older versions of Half-Life, similar devices appear on Xen. While their structural appearance is different, both emit vibrations through the ground.