Sound proofing the engine compartment

One of my winter projects was to see if I could quiet down the engine noise a bit.

The summer months in Seattle are hot and short of wind. We also are destination-focused and run the engine a lot. And since I was spending so much time in my lazarette tidying up my wiring harness and installing a new fuel tank, it seemed prudent to also see what could be done to reduce as much engine noise as possible.

When I installed the Yanmar 3GM30 in 2001, I added Barrier 104 to the to the engine room ceiling, and forward and aft bulkheads. I had left partial gaps aft and on the sides of the companionway. There were also opportunities to double up areas next to the galley.

I had a decibel meter from a previous project so used it to get a baseline reading and measure the improvements.

Roughly stated, a noise source doubles in volume with every 6 - 8 decibel increase, and is halved when reduced by corresponding decibel amount. Given the gaps I had where sound transmission was occurring, I felt there was a pretty good chance for significant improvements.

Common noise sources and their decibel values are listed below:

• 40 dB : quiet residential neighborhood
• 70 dB : normal speech at 12" distance 90 dB - heavy city traffic
• 95 - 100 dB : diesel engine room, not soundproofed
• 120 dB : jet take-off

In general, sounds above 85 are harmful, depending on how long and how often you are exposed to them and whether you wear hearing protection, such as earplugs or earmuffs.

My initial readings were:

	750rpm	2700rpm
Cockpit	61	70
Galley	66	79
Berth	62	80

I first created a PVC scaffolding to block out the areas where I had nowhere to attach the insulation. I used 3-8ft lengths of 1 inch, many elbows and T connections. The idea was simply to:

• Keep the insulation vertical and not allow it to come in contact with any moving parts,
• Ensure there was still plenty of incoming air to allow the engine to breathe
• Save our ears from premature hearing loss

Hamilton Jet supplied the Barrier 104 insulation material. At 2" thick, it is stiff enough not to fold. There is a lead lining, a thick, dense core and a foil backing sheet