[C320-list] Engine compartment sound proofing - Some preliminary results

[Ken Danko]

Hi all:

I have wanted to install engine soundproofing for years and the recent 
discussion here motivated me to finally make the move. I took Jeff's 
suggestion and this morning I went to West Marine and bought their kit: 
$155 including our very high sales tax. I was disappointed to find that 
there wasn't enough material to fit both of the covers. There was enough 
for the front cover and probably the top of the rear cover.

Before I started the job I decided to measure the sound levels with my 
iPhone. So here are the details on that. I have an iPhone 4s running the 
latest operating system. The free app I used is called dB Volume; I 
didn't change any of the default settings. I set the iPhone on the outer 
curved edge of the dinning table, face up , with a 1/2 inch overhang, 
and with the microphone facing the engine. I first measured the 
background noise without the engine running: 42 dB. I then started the 
engine and ran it at 1500 RPM (admittedly an arbitrary speed).  With the 
existing engine cover on and the steps down and the companionway open I 
got a reading of 71 dB. Then I took the front cover off just to see what 
the sound level would be: 80dB.

I spent a couple of hours meticulously cutting and installing the 
soundproofing material on the inside of the front cover. I made sure 
that there were no gaps and I taped all the ends. I had to cut out a 
half-moon shape (1/4 inch deep) in the middle to accommodate the 
flywheel. I taped over this area with the metal tape provided in the 
kit. I have to say, it looks great.

I know that I need to do the back cover to experience the full effect 
but I wanted to see how much sound reduction I would get from my day's 
work so I started up the engine and set it at 1500 RPM again. I was 
delighted to hear the engine, which, to my untrained ears, sounded 
quieter both in the cockpit and below deck. But then I set about 
measuring the resulting sound level with my iPhone. Guess what? 71 dB!!! 
So much for my own judgement about sound! I turned the engine off and 
re-checked the background noise level, it was still 42 dB.

I realize that I am going to have to buy some more material to do the 
back cover and I am exploring alternatives to West Marine. I will report 
back to the group when I finish the project. But I have to say, I'm 
pretty disappointed so far. It was a gorgeous day here and as I drove 
home I looked out over the bay and thought about how I could have spend 
the day sailing instead of installing sound proofing that did nothing to 
reduce the sound levels.

Ken Danko
"Grace"
San Francisco