| | | B. VARIABLES AFFECTING PERFORMANCE The most significant variables for noise control materials are weight of the reflecting barrier layer and thickness of the absorption and decoupling layers. Secondary factors are the protective surface facing materials and materials for the mass layer. The particular type of acoustic foam or fiberglass is of lesser importance among the legitimate and effective acoustic materials available. The universal problem in attenuating noise of marine engines, generators, and propellers is reduction of mid and bass frequency rumbles. High frequency sounds of turbo chargers, gearboxes, engine valve chatter, and engine rattle, etc. are reasonably attenuated with an enclosure lined with a thin lightweight composite. A composite using I lb/ft2 barrier sandwiched between /2" layers of foam added inside a 5/8" wood enclosure will give good reduction of these noises if the engine compartment is reasonably leak-free. However, the mid and bass frequency noises associated with cylinder combustion rate, engine rotation, and propeller noise require more careful attention to weight and thickness of materials used in the composite. The effectiveness of absorptive materials increases directly with thickness, in both the amount of energy absorbed and the range of frequencies over which | | absorption occurs (See Fig. 2A). Bass frequencies require thicker absorptive layers. For engine noise /" is generally a minimum useful thickness while I" is good, and I-I/2" to 2" approaches an optimum with in the practical considerations of effectiveness and space available. For the design of composites, mass layers are typically at weights of I and 2 lb/ft2 and the thickness of decoupling layers varies from %" to 3". The effective frequency range of these materials varies with the square root of the weight of the mass layer times the thickness of the decoupler (See Fig. 2B).The I lb/ft2 material on %" decoupler is effective only above 500 Hz, and is generally not satisfactory for noise reduction in boats, except for special applications.The 2 lb/ft2 material on %" decoupler moves the effectiveness range down to 350 Hz (this is a construction generally recommended when there is minimum space available for the composite treatment). For a high level of effectiveness the decoupler layer should be I" or greater and for the highest effectiveness this should be combined with 2 lb/ft2 mass layer, as demonstrated in Fig. 2B. Greater thick nesses of the decoupler increase the bass frequency effectiveness, while heavier mass layers increase effectiveness throughout the entire frequency range. | | |