“Noise loggers”, noise recorders that “listen” pipes to understand where there are leaks

In Denmark the two largest water suppliers in the country, Hofor and Novafos, are transforming parts of their water distribution networks with noise loggers, noise recorders that “listen” to the pipes to understand where there are leaks and to avoid flooding.
But how do they work? The noise loggers are compact devices composed of an accelerometer sensitive to the sound frequencies that propagate in a water pipe, from a data acquisition and recording unit, from a radio transceiver system for data exchange with a separate central unit, from the electronic part that manages the automatic operation, and from a battery for the power supply.
The water escaping from a pressurized pipeline generates a noise that propagates along the water network: the noise loggers can record these noises and analyze them, distinguishing them from other background noises that propagate in the pipes. In this way it is possible to establish whether there is a loss in the vicinity, based on an evaluation carried out automatically by the internal software.
The noise loggers are used to analyze the networks on which it intends to control water losses, occasionally for a control or even permanent if you intend to keep the network under constant monitoring. Operationally the noise loggers are connected a few hundred meters from each other along the water network to be analyzed, using a magnet that makes them adhere to the pipeline; the device records the sound frequencies present in the network for a predefined time, preferably at night, defining through the internal algorithm the presence or absence of nearby leaks. Through an external data acquisition unit, the results of the listening of each unit are downloaded, the results thus acquired allow to intervene in a timely manner where there are losses and to avoid flooding.