A flood-warning siren plays from one of Venice’s 15 designated bell-towers.

We should have realised something was afoot when speakers around the city broadcast a relatively soothing tone. Afterwards locals walked down the alleyways in gum-boots and other water-proof footwear. Hours later water had crept into the streets and buildings. It was the Acqua Alta, an exceptional tidal peak lasting a few hours, inundating the city with flood water.

The following day the sirens sounded again, this time we understood their relevance. There is something slightly ethereal about hearing these two notes floating through the narrow alleyways. To a visitor it doesn’t suggest anything dire, and when listening to the field recording the locals seem to take it all in good stead.

The sound of floodwater lapping at doorways in this narrow alleyway. In the canals motorboats continued their daily routine.

As thousands of barges, gondolas and water buses use Venice’s waterways it was vital that a system was installed to warn commuters and businesses about tidal conditions. With extreme high tides pedestrian areas may disappear and access under bridges for boats can be reduced. Ferries may need to be re-routed and raised walkways erected.

In 2007 a massive audio-system was installed throughout Venice. Loudspeakers located in 15 bell-towers throughout the city broadcast warning signals so that residents can plan their movements ahead of the rising water. This use of sound is extremely effective in a city without traffic noise. It has proved to be successful in maintaining the flow of commercial and private life. From a cultural perspective the broadcast of this siren is a definite sound-mark that could only belong to Venice.