Evaluation methods of external airborne noise emissions of moored cruise ships: an overview

The evaluation of noise emissions of a ship is a problem particularly complex. The difficulties are related to the characteristics and to the ways of relating to potential receptors of the source itself. The analysis of noise emission and propagation from a vessel can be divided in several parts, essentially based on the medium of sound energy propagation and on the environmental reference: internal (airborne and structural propagation) and external (airborne and underwater propagation, vibration effects on harbour structures inducted by wave motion). While internal and underwater sound propagation can be effectively evaluated by standardized test methods for many types of vessels, the assessment of external noise propagation requires more attention, because measurement methods do not always fit to the type of vessel and to operating procedures of navigation and mooring. From this point of view, a very interesting case is that of the large cruise ships. In recent years, the gross tonnage rose sharply and the form factor has been modified to allow the housing of an increasing number of passengers. Therefore, several aspects related to acoustic emissions are involved, such as the increase of funnels height and number of sources on the ship sides, mainly ventilation outlets. The consequences are particularly noticeable when the ship is moored, often because its height overlooks the port facilities and the effect of noise produced by power supply and ventilation systems can be significant on the surrounding urban areas. The paper addresses the actual framework regarding ship noise, comparing different measurement techniques and evaluation methods, with reference to airborne noise emission in the external environment of moored large cruise ships.