Noise Control Treatment Design

NCE develops optimal noise control designs based on comprehensive sound and vibration analyses, see airborne noise analysis section. Custom designs are provided for each specific ship program. Common treatments include vibration isolators, acoustical insulation, damping, floating floors, HVAC linings, enclosures and silencers. Each treatment is selected for a specific purpose - to control noise contributions from a specific source that are transmitted via a specific path, and each has its own cost benefit tradeoff. Understanding when to apply specific treatments is critical to minimizing impacts to non-acoustic factors such as weight, space, and cost.

The following is a brief description of each of the noise control treatments listed above:

• Vibration Isolation: Structureborne noise emitted from machinery is one of the predominant sources of noise. An effective way to reduce structureborne noise is to resiliently mount the offending machinery, in particular for diesel engines/generators, air compressors and refrigeration compressors. There are numerous types and styles of vibration isolators. NCE can advise on the most cost-effective isolators for your specific application.
  
  In all cases, the vibration isolation system needs to be properly designed and implemented, or vibration can actually get worse. NCE works with clients to properly design these systems for machinery of all sizes and functions. NCE selects vibration isolators, reviews proper equipment mounting points, isolator characteristics, and performs six-degree-of-freedom (6DOF) "natural frequency" calculations. Lastly, during sea trials NCE will measure the transmission loss (TL) or attenuation of the isolation system to check for proper performance.


• Acoustical Insulation: This usually consists of fiberglass or mineral wool for U.S. Coast Guard inspected vessels. For non-inspected vessels foam-based products and other types of materials may be used. Acoustic insulation is most commonly used on the boundaries of the engine room or machinery space. It may also be used on auxiliary machinery spaces or any space with elevated sound levels. Some insulation types have multiple layers with limp mass (or barrier material) sandwiched in the middle of two layers of a base material, usually fiberglass.


• Damping: This treatment has two general forms: tile and spray-on. The tile products are typically installed to the middle of un-stiffened steel plating. The spray-on products are used throughout a treated area. Damping treatments convert vibration within a ship's structure into small amounts of heat and thereby dissipates the structural vibration. This treatment has some technical and regulatory limitations which NCE can help you to understand and work with.


• Floating Floors/Rooms: These treatments have many forms. The common design is to cover the steel floor with a resilient material upon which another non-structural, but load-bearing floor is placed. This floor usually floats on a layer of continuous or discrete resilient material. A finish flooring material (usually carpet or poured floors) is installed over the non-structural floor. Floating floors are good for reducing noise for a selected group of compartments as the treatment must be used in each compartment where noise reduction is required.
  
  A floating room is a compartment where the walls (usually joiner panels) are located on top of a floating floor. In these cases, the entire room's boundaries are all isolated from the ship structure which provides one of the highest levels of shipboard sound reduction.


• HVAC Silencers: Reducing sound from noisy HVAC systems is accomplished with either HVAC silencers usually located near the fan. Another noise reduction approach for HVAC systems is to internally line the ducts for a certain length past the fan. NCE can compute shipboard HVAC system noise and then evaluate the effectiveness of silencers or lined ducts.


• Exhaust Silencers: This treatment is used to reduce the sound emitted to the exterior of the ship. They are needed to reduce noise to the ship's working deck and bridge wings or even to shores-side areas. Exhaust silencers are usually selected by the ship designer or engine vendor. Silencers have various grades such as "Critical Grade" and "Hospital Grade". Usually, the better the grade, the higher the sound attenuation, the higher the cost and larger and heavier the silencer. NCE can determine the best silencer option for each vessel.

Once any of the above treatments have been selected, NCE can provide material specifications, areas of coverage, installation guidelines, potential vendors and noise reduction estimates. Further, NCE will work with the ship designer to minimize the adverse impacts on cost, weight, space and operation of the vessel.

While NCE will recommend treatments by vendor and model, NCE does not sell or represent any noise control material vendors. NCE gains no financial benefit from any noise control treatment recommendations. Thus, NCE can work independently with our clients to identify the best solution from both acoustic and non-acoustic perspectives.