| | | A. BASIC CONCEPTS OF BLOCKING AND ABSORBING SOUND Acoustic insulation materials work by two processes: absorption of sound energy, which dissipates sound as heat energy, and reflection, which reflects noise away from a location where quieting is desired. A single composite insulation material will be effective as both an absorber and a reflector. Considering the example of an engine in a boat, if it is sitting open with no enclosure around it, it will create a very loud noise throughout the boat, perhaps as much as 105 dBA . If we install an engine box around the motor using 5/8" thick plywood, we might reduce the noise to 90 dBA in the boat, as the mass of the plywood box reflects the sound back into the enclosure (generally the more mass in a panel, the better). Within the enclosure the sound waves bounce off the plywood walls and a reverberation occurs, similar to that experienced in any hard surface room. The noise near the engine, inside the box, is actually louder after the box is installed due to the reverberation effect. Absorbing a large fraction of the noise each time a sound wave strikes one of the walls can reduce the reverberation within the box. This is done by placing sheets of acoustic absorptive materials on the walls of the box. These are porous materials, such as fiberglass batts or open cell porous foam, with little weight. If we install I" of absorptive material on the top and four sides within the engine box, the noise in and outside the box may drop by 3 to 5 dBA, giving us a range of 85 to 87 dBA in the boat. If, in place of the simple absorptive material, we install a composite material using a mass layer of I lb/ft of barrier sheet sandwiched between two I" layers of foam or fiberglass, we may achieve a l2 to I4 dBA reduction of noise, giving us 76 to 78 dBA in the boat. The composite material achieves this result by combining the effects of absorption and reflection: the free layer of foam material facing into the enclosure acts as absorption against reverberation; the barrier, separated from the enclosure wall by the bottom layer of the composite (often called the decoupling layer) acts as a second reflecting wall in addition to the plywood sides. This type of composite insulation is the most commonly sold acoustic insulation for marine use. | | |