AcousticalIsolation

Acoustical Isolation

Acoustical isolation is extremely important in a live-aboard boat to keep noise within a comfortable range. We experience sound in terms of a frequency range measured in Hertz (HzHertz, SI unit of frequency) and air pressure measured in decibels (dBDecibel). The ear has a very wide non-linear response to the intensity of sound, so each 10 dB increase is ten times louder. Thus a 20 dB increase is 100 times more intense. Duration is also important. Constant noise over 85 dB will damage hearing.

For reference, the ambient noise level in most homes is 30-35 dB. Car interiors are around 40 dB. AAmpere (amp), SI unit of electrical current normal conversation is around 60 dB. A diesel engine room is around 125 dB (wear ear protectors!}. A jet plane taking off is around 140 dB. For bedrooms, the World Health Organization recommends 30 dB with no single noise event exceeding 45 dB.

A good strategy is to eliminate sound at its source, especially in the engine room. This reduces the cost of acoustical insulation elsewhere. Noise attenuation of 100 dB is feasible. Some areas for consideration are:

• Engine and exhaust-system mounts to decouple engine noise from structural members
• Engine enclosures for generators with heat-resistant sound-dampening materials
• Exhaust mufflers
• Air intake baffles
• Flanking noise via holes in bulkheads and water, plumbing and HVACHeating, ventilation, air conditioning distribution systems
• Floor, ceiling and walls of the engine room
• Bulkheads and doors in living spaces
• Isolating all wall panels from structural boat walls

In addition to the noise of combustion, there are several considerations in designing acoustical-isolation systems for an engine room. These are drive-train vibration, especially from the propeller, the peak frequencies generated by large noise generators such as the engine, and the range of noise to which the ear is most sensitive.

Complicating matters a bit, every engine and engine room is a unique case. However, noise radiation from the crank case and oil sump is typically 500 to 1,000 Hz. Engine block vibrations run below 1,250 Hz while gear noises run above 2,000 Hz.

The human ear responds to sounds in the range of 20 Hz to 20 kHzKiloHertz 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’sSecond 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 such as 1,000-2,500 Hz (wavelengths of 13.5 to 5.4-in or 34.3 to 13.72 cmCentimetre).