| | | For this reason it is very important to seal all of the acoustic leaks in the box and composite skins. The most critical leaks are those penetrating both the box and mass layer surfaces, as occur at an ungasketed access door. The necessary openings for ventilation should communicate to the deck or topsides through ducts or channels having linings of acoustically absorptive materials. Damping - The primary consideration in determining the possible effectiveness of vibration damping treatment is the amount of resonance of the surface, prior to damping. If the surface has resonant booming, buzzing, or ringing, damping treatment may be useful. If the surface is completely rigid or already damped to " thud quality," due to its construction, addition of damping will give doubtful benefit. The primary factor, which limits the achieving of the full damping capability from installed damping, is ineffective bonding of the damping layer to the structure. Great care must be taken in selecting adhesives, preparing surfaces, and applying the materials. | | |
| | | B. Mass Layers - Mass layers must be non-porous and limp for maximum effectiveness. Plywood and other stiff lightweight materials do not block sound as well because their stiffness properties allow them to transmit noise through sympathetic vibrations, i.e. the panel becomes a sounding boat board. Plastic sheets loaded with mineral compounds are the most common type of mass layer. These plastics have their best application in places where the mass layer must withstand high physical abuse. Because of its high density and low stiffness, vinyl is one of the best noise barrier materials and is the most commonly used mass layer in composite insulations. Lead sheet is an excellent sound barrier, which is non-combustible. Because it is classified as hazardous material, its use is generally limited to application mandating non-combustible materials. C. Damping - Damping materials are typically in sheet and tile format, as well as sprayable and bushable compounds whether for extension or constrained treatments. Marine applications require attention to water and oil resistance as well as fire safety. For attachment of tile and sheet damping adhesive systems must be matched to the stiffness characteristic of the treated surface. Easily applied peel and stick (PSA) is appropriate for thinner surfaces. Thicker, stiffer materials require bonding with high stiffness structural adhesives. | | |
| | | NOISE TREATMENTS INSIDE CABIN Treatment can be installed within the cabin to further reduce the noise. An absorptive overhead material such as perforated vinyl over '//" to I" of foam reduces cabin noise caused by the engine, in the same fashion as acoustic ceilings work in a kitchen. These foam-backed materials also provide useful thermal insulation. Noise may radiate from cabin flooring, as often happens when the engine room is located below the salon or poor engine mounts are carrying vibration into the cabin sole. In this situation an underlayment consisting of a plastic mass layer over a foam pad of '//" or /2" thickness can be placed below the carpet, giving a reasonable reduction of noise coming from this surface. Carpet alone will not give this effect, as carpet alone adds only absorption to the space. | | |
| | | PRACTICAL CONSIDERATIONS Absorption - The acoustic absorptive materials act as sponges to soak up sound waves bouncing around in the engine compartment. Absorption materials may be omitted from small sections of the compartment wall without causing a significant increase of noise. Barriers - The reflection barriers, including both the engine box and the mass layer within the composite, act as containers for the noise. Any leakage in either surface can cause a severe increase of noise transmitted to the cabin. | | |