|Now Hear This|
2: Structure-borne and airborne noise
By Tim Clark — September 2001
The low-frequency resonance that FEA addresses is primarily caused by vibration traveling through the boat's structures. The higher-frequency noise that SEA examines often travels through the air, so once engineers quiet the sources as best as they can, specific treatments are for the most part divided into two strategies: isolating vibration (structure-borne noise) and containing loudness (airborne noise).
The vibration from main engines can be significantly cut off from the rest of the vessel with the use of flexible mounts. Special engine installations allowing for exceptionally soft mounts--such as the Aquadrive antivibration system, the Centaflex AGM, and Rubber Design systems, all of which include bearings that keep propeller thrust from the engine--can reduce the transfer of engine vibration to the hull by as much as 95 percent while also diminishing prop-shaft vibration. If you also mount gensets, exhaust systems, and even pumps on isolation mounts, you go a long way toward confining structure-borne noise.
Only now do you turn to sound control methods that most people think of first: insulation and absorption. While isolating vibration impedes the flow of most noise on a boat, it doesn't do a thing to diminish the airborne noise the isolated systems create. Acoustic insulation materials (along with complex baffles on air intakes, highly insulated doors, and airtight gaskets) are used to keep such noise confined to the engine room. Materials range from the conventional and specialized fire-retardant composites that J & A's affiliate Soundown manufactures to lightweight Polydamp Melamine Foam (PMF) often used by Silent Line in as many as 15 layers. There is a broader range of such materials than ever, with increased fire resistance and combined acoustic absorption properties.
They are frequently used not only on engine room bulkheads and ceilings, but also throughout hulls and superstructures, for in some hulls, once sound and vibration from propulsion systems are tamed, noise generated by the hull passing over the water can be significant. When a structure-borne sound source is, to use J & A president Joe Smullin's words, "so largely distributed," it's best to treat individual locations such as staterooms and saloons. While one option is to heavily insulate a cabin, another is to "float" it--isolate it on ranks of mounts or elastic strips in a box-in-box construction with sidelinings and bulkheads stabilized with other flexible attachments. While whole floating decks are common on megayachts, space and expense considerations make floating cabins rare on smaller boats. However, Silent Line has floated cabins on boats as small as 55 feet LOA, and floating a bulkhead between an engine room and stateroom is not uncommon.
A majority of both Silent Line's and J & A's comprehensive, early-design-stage projects are at the behest of custom and semicustom yards, but they are increasingly consulted by production builders. "Some manufacturers are afraid that these solutions are expensive," says van Cappellen, "but we approach every design revision from a practical point of view. On existing boats we go aboard to take noise and vibration measurements, do some computer modeling as verification, then ask the client, `What levels do you want, and how much do you want to spend?' Then we tweak the model according to that criteria."
"The standard of onboard quiet is advancing in the United States," says Smullin. "When production builders come to me with an after-the-fact design, it's not to put out a fire; it's for analysis toward general product improvement."
If trends continue we may turn around one day to find whisper-quiet wheelhouses as industry-wide as bow thrusters. And the loudest noise source on your boat will probably be your brother-in-law, dressed in a Hawaiian shirt.
Silent Line Noise and Vibration Control Phone: (305) 651-7786. Fax: (305) 653-0583.
This article originally appeared in the June 2003 issue of Power & Motoryacht magazine.