0018-9162/04/$20.00 © 2004 IEEE September 2004 55 RESEARCH FEATURE Published by the IEEE Computer Society Designing an Urban-Scale Auditory Alert System T he high tides that periodically flood Venice, locally known as acqua alta, are becoming more serious due to recent changes in the surrounding lagoon as well as atmospheric conditions. Several days each year, tidal waters cover many city streets and squares, disrupt- ing the inhabitants’ lives and snarling traffic. Major floods, like the ones that struck Venice in 1966 and 2000, can inundate the city and paralyze activity. Given the severity and increasing frequency of tidal flooding, alerting the population promptly has become imperative so that public officials, mer- chants, and citizens can take appropriate measures. A special office of the Municipality of Venice, the Center for Tide Prediction and Warning (Centro Previsioni e Segnalazioni Maree—CPSM), provides a continuous tide forecast based on computational models as well as astronomical and meteorological data. When a significant high tide is expected, city authorities activate a network of electromechani- cal sirens for a few minutes, usually anticipating the tide peak by a few hours. The sirens, however, emit threatening wails rem- iniscent of air attack warnings, do not convey the gravity of the threat, and may not reach isolated or distant areas. For these reasons, the CPSM, in cooperation with the Venetian Research Consortium and the University of Verona, is investigating the possibility of replacing the sirens with a loudspeaker system that would provide more uniform coverage as well as information about the tide level. Although loudspeakers are more noticeable than sirens—a nontrivial concern in a city celebrated for its visual charms—they make it possible to broadcast any kind of sound. As part of this research effort, our project team first analyzed the current alert system using off-the- shelf acoustic simulation software and a specially designed visualization tool. We then used a form of constraint logic programming to determine the opti- mal placement of loudspeakers in Venice, a complex task with many physical, economic, and social con- straints. Next, we created the alert sounds for our demanding listening environment. The final phase of the project involved iteratively validating and redesigning the alert signals using human testing. ACOUSTIC SIMULATION AND VISUALIZATION The initial phase of our study consisted of a detailed analysis of the existing alert system. We first developed a technique that semiautomatically extracts building and terrain data from digital city maps in ArcView format with reasonable confi- dence. Our extraction technique structures this data as polygons representing land, water, and buildings with their associated height. We then imported this data into SoundPLAN (www.soundplan.com), an integrated software package for noise and air pol- lution simulations, and used it to generate a map of the sirens’ sound-pressure levels throughout Venice. We modeled the city’s eight electromechanical sirens as point sources with an omnidirectional propagation pattern and used Fourier analysis of a steady-state portion of a siren sound to determine Drawing on a wide range of computing technologies and methodologies, the authors present a new auditory alert system for high tides in Venice designed to replace the existing network of electromechanical sirens. Federico Avanzini University of Padova Davide Rocchesso Alberto Belussi University of Verona Alessandro Dal Palù Agostino Dovier University of Udine