Sonic Sentinel System

The sonic sentinel system (3S) is a unique autonet-enabled surveillance system - implemented exclusively as a part of the infrastructure of Bunker Primus - to allow for the detection of people and objects in the cyclopean dark of the city's depths.

Complexity

The acoustics of many parts of Bunker Primus are well-documented; the ruling class of the Manifold Conservation Society tend to be scholars and aesthetes in equal measure. As a result, changes in the acoustics of a space can be used to detect changes in the shape and composition of that space, allowing those either very familiar with that space or provided with the right equipment to echolocate after a fashion within it. The 3S network capitalizes on this fact to provide real-time surveillance data to the Administrators involved with civil defense, law enforcement, and infrastructure maintenance.
The technology behind the 3S system is built upon pre-existing dieseltech computing, pneumatics, and audio recording technologies. A system of pressurized tubes running through the superstructure provides parcel service and a degree of ventilation to the depths of Bunker Primus already, but few people realize they are also inset with resonators to produce a continuous tone at a precisely-calibrated infrabase frequency. Recievers in set locations within the network use pairs polymer gramaphone-like discs to sample the sound of the space; these discs are refreshed at regular intervals using integrated heating and smoothing devices. One disc is refreshed daily to establish a baseline, while the other is refreshed almost continuously as new audio is intercepted. Both waveforms are sent to manned monitoring stations via the municipal autonet for comparison.
At the monitoring stations, the waveforms from the recievers are compared and may be converted back to sound for the interpretation of the monitoring staff. Through a combination of triangulation, engine-enabled Fourier analysis, and intuition, monitoring personnel can determine the location, motion, shape, and composition of things in the monitored spaces even when kept in complete darkness. This is all invaluable information to security and maintenance teams in particular, the former using it to catch groundling contraband activities and the latter using it to keep municipal machinery in the monitored space operating at peak efficiency.
For all its amazing technology, the sonic sentinel system is not perfect. Even talented urban adventurers have difficulty evading the all-seeing eyes of the 3S, but those truly in the know can 'spoof' or otherwise blind the system through various jamming techniques. Infrared equipment (i.e. the DEL-1 "Concertina" Support Auto-Armor's sensor package) can't easily detect the recievers to exploit them, but keen-eared Rostran locals can pinpoint the resonators. The temporal resolution of the system is not as great as it could be. Future developments perhaps involving the implementation of continuous tape or programming gasket media rather than polymer discs to help ameliorate this last problem.