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en-US en-US en-USJuly 2012 en-USwww.sname.org/sname/mt appropriate masses for water loading. Adjustments must be made for higher fre -quencies though, as the mass loading eect decreases with higher frequencies. Damping loss factors Damping is one of the most dicult param -eters to predict before the construction and testing of a ship, as the factors that influ -ence damping are varied. e total damping can include effects from structural losses (including the increase in damping from welding), water loading, cargo, and other fac -tors. e applicable damping for any single mode is dependent not only on these factors but also on its location within the structure and how the structure moves for that mode. Damping loss factors of a welded steel metal are not constant with frequency, but typically decrease as frequency increases. In general, damping must be estimated, and is often estimated conservatively, that is, with values that would lead to elevated vibration levels. For forced frequency response analyses (where some damping loss factor must be used), errors in damping are most signicant at frequencies where strong resonances exist, and therefore identication of such resonances becomes important in understanding areas of a prediction that may require further study. Damping is less important at off-reso -nance frequencies, though the magnitude of response can be expected to be lower at these frequencies, for obvious reasons. Accurate modeling Computer aided acoustic design tools that incorporate SEA are known to provide accurate habitability noise modeling, typically within 3 dB of the measured A-weighted noise collected by the authors on more than 70 different vessels. With this level of accuracy, treatments can be optimally selected while minimizing impacts on cost, weight, space, opera -tion, and performance of the vessel. With the development of this type of toolset, crew and passengers will be subject to an acceptable acoustic environment, the vessel will operate more efficiently, and the vessel?s impact on its environment will be minimized. When properly modeled in FEA, vibra -tion in the forms of hull natural frequencies and forced vibration levels can be accu -rately predicted. A number of factors not commonly seen in other FEA applications including frequency dependent damping loss factors, water loading, and force inputs are required to accurately model the vibra -tion levels experienced on a ship. A number of other common factors including element size, formulation, and boundary conditions also contribute to making the vibration modeling a unique application. MTRaymond Fischer is founder and president of Noise Control Engineering (NCE) in Billerica, Massachusetts. Jesse Spence is senior engineer at NCE. Ronald Dempsey is engineer at NCE. Figure 4: Forced analysis results showing local reso nances. It is generally recommended that the model contain structures from the area of interest out to the next major structural connection (deck, bulkhead, or shell) in each direction. Approach ASound