Following the Second World War, the Croatian city of Zadar underwent a large, rapid reconstructive transformation. The city's seafront became nothing more than a concrete wall until 2005, when architect Nikola Bašić proposed to redesign parts of the seawall to interact with the ocean waves. Concealed beneath marble blocks, the 'Sea Organ' (morske orgulje in Croatian) is comprised of a network of polyethylene tubes and resonating cavities which sing as the waves and wind lap the shore. With thirty five individual pipes spanning a total length of seventy metres, it is the largest aerophone in the world. According to reports, the sound is specifically directed out to sea and is impossible to hear from within the city of Zadar itself. In 2006, the intervention was jointly awarded the European Prize for Urban Space.
The steps are made up of seven parallel flights, each one ten metres wide. The seven flights are juxtaposed in such a way that at each change of flight there is a difference of one step; that means that the steps both at the junction with the parade and at the water's edge the flights present a staggered silhouette. The first three are the longest; they consist of six steps and descend about two metres, which is the highest level of the cruise ship arrival platform. From the fourth flight, the height of the parade gently approaches the water level, so that each new flight loses one step. The last flight, which has reached the definitive level of the parade, has only two steps above the water.
But the proper adaptation to the topography of the parade is not the only explanation for the variations in the dimensions of the flights of steps. There is another which establishes a clear formal analogy with the variations in dimension and arrangement of the parts of a musical instrument. A series of polyethylene tubes of different diameters run along the inside surface of each flight of steps, connecting the submerged part with a gallery that runs along beneath the parade. With the variable force of the waves, the water penetrates the lower end of the tubes and is carried into the subterranean gallery, which collects it and returns it to the sea. In this process the air of the interior of the conduits is pushed to orifices that connect the gallery with the surface of the parade, generating sound vibrations which, given the variations in the diameter and length of the tubes, cover a broad range of musical tones.
You can read more about the engineering behind the 'Sea Organ' here. The description of the intervention can be read in full here.
