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→Isolating the Engine Room
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= Acoustical Isolation =
The human ear responds to sounds in the range of 20 Hz to 20 kHz but is most sensitive in the 1,000-5,000 Hz band with a peak around 4,000 Hz. The auditory canal alone has a closed-tube resonance of 2,000-5,000 Hz.
For a production boat it’s feasible to do a spectrum analysis and design suitable sound attenuation and test it. For a bespoke boat this is unlikely to be feasible. Thus a good cost-effective approach is to focus on reducing sound transmission , in a target range for engine noise such as 1,000-2,500 Hz (wavelengths of 13.5 to 5.4 in or 34.3 to 13.72 cm).
== Methods of Isolation ==
In the engine room an ideal ceiling would provide attachment points for a drop ceiling. This would require spring hangers (or even rubber engine mounts) capable of withstanding high accelerations of mass to avoid detaching the ceiling in a capsize. The attachment points should be 100-200mm long. Attach two layers of non-combustible panels bound with viscoelastic glue. Several marine-approved panel materials are available.
[[File:DropCeiling.png|thumb|left|220px280px|Schema for a robust drop ceiling]]
Where the composite panel meets the walls, prevent flanking noise leaking through using a flexible acoustic strip such as neoprene.
On the room side, apply to the composite panel composite insulation such as lead-fibreglass with the fibreglass rated at, say, R13. Since we are dealing with wavelengths of, say, 5 to 14 in, the thickness of the insulation should be in the same range. <i>An alternate arrangement is shown in the image.</i>
A ceiling of this type, combining mass, decoupling, resilient mounts and insulation is capable of attenuating noise 60-75 dB over 1000-5000 Hz.<ref>The Green Glue Company, http://www.greengluecompany.com</ref>
Similar techniques could be used on the bulkheads. For example, in the engine room use a viscoelastic glue to stick a non-combustible panel on the bulkhead, then attach the composite insulation. In the living space on the other side attach furring strips using neoprene rubber. If more isolation is needed incorporate rubber mat or more insulation. Always leave an air gap to prevent conduction.
<i>Make sure materials are fire-resistive according to [[MarineFirePreventionManual|marine codes]]. A fire in the engine room can transfer by conduction to other spaces through steel bulkheads.</i>
== Some Cautions ==