FROM BILBAO’S CITY CENTRE TO THE AIRPORT. Bilbao (Spain). Each of the three tunnels that communicate the city of Bilbao with its airport is equipped with a central PA linked via data network to the system control. More than 150 exponential horn speakers (30 W) have been used and also acoustic projectors (20 W), in total, more than 150 loudspeakers deliver sound to more than 3 km, the total length of the tunnels.