A case history of low frequency air-borne sound produced by a weir in the river Reno

Marzabotto is a small village in the north of Italy, near the city of Bologna. The town lies at the bottom of a broad valley where the river Reno flows. In 1985 strange vibrations of glass windows and doors started to disturb the rest and sleep of the population. Local newspapers wrote about the awakening of the ghosts of ancient Etruscans, a population that lived in this area in olden times. In order to discover the source of such vibrations, the authorities of the village first called researchers of the National Institute of Geophysics who made measurements of vibration in different parts of the village and identified the waterfall of a weir, recently built to control the flow of the river Reno, as the source of the phenomena. The spectrum of the vibrations was characterized by a frequency of 8 Hz. The presence of such at distances of about 4 - respect to the valley floor, of the phenomena could be vibrations 5 Km and brought up by air as far in the well away from the weir, raised positions with idea that transmission as by ground. Local authorities decided to start new investigations and entrusted the work to the Institute of Technical Physics and the Institute of Hydraulics of the University of Bologna. It was decided to begin a campaign of systematic measurements of noise and vibration in order to understand the origin of the phenomena.