This recording is done with two Contact mics, attached to the wire rope which acts as a support for the pylon, used by SEPA, for measuring water levels in the River Forth. There are so many harmonics travelling through the wire, and it sounds like its own symphony. The vibrations from the traffic crossing the nearby motorway bridge are travelling through both the air and ground, which causes the wire to vibrate. Although not instantly recognisable, you hear a rhythm of the vehicles passing over a gap in the bridge structure, which causes fluctuations in the frequencies of the vibrations. You also hear the Doppler effect from the passing vehicles.
The wire is part of the structure for a SEPA/NRFA Water Level Gauging Station, which measures the river level and flow over time. (Thanks to the members of Old Stirling facebook group for that information) Originally opened in 1972 – known as Drip Bridge, it was rebuilt on the same site in 1982, on a 70m wide section of the river bank – part of a large meander just above the tidal limit.
SEPA, Scottish Environmental Protection Agency, underpins hydrological research and water-management activities in the Scotland and delivers data and expertise to UK Government and international organisations. Water level data is collected at gauging stations using a variety of electronic sensors and data loggers.
Data from these telemetry stations is collected automatically at least once a day – sometimes more frequently depending on telemetry linkages or, if required, for operational reasons. It is then updated on their online pages.
The SEPA online level graphs show the change in level over the past two days relative to a local datum, although this level may not directly represent the depth of water between the bed and the surface.
Increases in water levels are normally associated with rainfall in the catchment. However, certain sites are subject to artificial control – for instance, hydropower stations may artificially influence the level of the river. Some stations are tidal sites or river level sites affected by the tide – at these stations, the water level may rise and fall several metres twice each day.
Keep an open mind when listening to the hidden sound categories, and understand that this phenomenon is going on all around you, every where you go, you just don’t hear it.