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The science of soundproofing: Noise control is a growing expertise at Lyman-Morse

by Lyman-Morse Boatbuilding 2 Mar 2020 07:28 PST
The science of soundproofing © Lyman-Morse Boatbuilding

An unheralded generation of marine acoustic engineers are taming the once punishing din of boat engines and generators. Today's marine "sound proofers" are using complex acoustical physics to reduce, reflect, or control noise on a boat. Acoustic woven matts, high-tech nano reflective wallboards and complex composite insulating mounts are making their way into vessels. Today's quiet marine mufflers are evolving into works of industrial art.

Not surprisingly, leading-edge noise control is a growing expertise here at Lyman-Morse. Major new builds like the Hood 57 power yacht and the sloop Anna featured sophisticated sound mitigation technologies throughout. But also, soundproofing has crept into major refits we have conducted on Sea Chase, Scout and several recently refitted Swan 100s.

"Practically speaking in marine applications, sound mitigation boils down to three systems: Structural insulation, engine mounts, and exhausts," says Chris Murray, director of sales at Soundown, an acoustic engineering firm located in Salem Mass.

"In a custom build, where creature comforts cater to a demanding customer, quiet is critical."

Acoustic science started with the Greeks in the 6th century BC. The basic concepts have since spread into medical imaging, architectural design, and marine sound management. Yet, for all of acoustic's complexities, sound at sea is simple: A noise is made. That noise radiates out either through air, structure, or water, where it is then bent, boosted, suppressed, or bounced.

"The simple solution to most noise problems is to add weight, so it takes more energy to excite materials," says Kevin Houghton, Design Division Manager at Lyman-Morse. "But on a boat, if you simply add mass to calm things down, that's an issue."

Houghton uses his recent acoustic engineering work on the Hood 57 express cruiser, currently in mid-construction on the Thomaston campus, as an example. The process of quieting down a powerful 45-knot power yacht, like the Hood 57, starts by collaborating with Joe Smullin, a leading marine noise scientist, from J&A Associates, based in Salem, Mass.

Smullin and Houghton quickly realized that even though the Hood 57 featured an ultra-stiff custom wood laminate hull, powered by twin quiet Volvo D13-IPS1350 power pods, the aft engine and generator space would still be the dominant noisemaker on-board.

To quell the airborne noise radiating from these engines, the team started out by calling for 3-inch composite sound insulating foam, featuring 2 pounds per-square-foot of dense vinyl plastic. The foam and vinyl acts as a floating membrane that both absorbs and reflects engine sound, with less overall weight than simply specifying heavier walls and floors.

To calm the deep-frequency throb of the twin 1,000 HP motors, radiating through the entire structure of the boat, acoustically resilient engine and generator mounts were also installed. An intricate engine air intake passage that further deflects motor noise, was designed with a moisture proof polypropylene pellet acoustic absorber called Quiet Pro.

Houghton was careful to insulate all cockpit, cabin, pilot house and living spaces with 2-inch composite insulating foam, featuring 1 pound of vinyl mass per square foot. The final touch for this sound-insulating envelope was to treat key panels with acoustic tiles, developed from nano technology used to keep nuclear attack submarines quiet.

Careful attention was also paid to the hatch gaskets, of all things. Similar to the annoying hiss from a leaky car window, high-frequency sound can sneak through the smallest gaps in hatches.

As to the future of sound design? "You can imagine the day when active noise canceling comes to boats," says Houghton. "Particularly at night in a harbor, you could target annoying noise and get some nice peace and quiet."

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