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

[Ken Danko]

   Marek:
   Subjective assessments of sound (and many many other things) are
   inherently flawed. That is why we have scientific methods (and
   equipment). My ears did not deceive me but obviously my brain did.
   After one spends money and time on a task like this it is natural to
   want to believe that positive results were attained. In a sense, we
   hear what we want to hear. We are fallible. I'm not a sound engineer
   but I am willing to bet that if I isolated the iPhone from the table
   surface I would get the same reading. But I like your idea and will try
   to do more to make sure that vibration does not enter into it when I
   add sound proofing to the rear cover.
   Ken

   On 6/24/14, 8:13 PM, Marek Fluder wrote:

Ken,

Great info - thanks for taking time to document it.
The disappointing result of the decibel read-out after sound proofing I
would attribute to the fact that you have placed your phone on a hard
surface of the table. I think that to the decibel level you were measuring
vibrations contributed more than the actual sound wave. I would place the
phone on a piece of foam, or better yet suspend it in the air on a piece of
line.
I do not think your ears have deceived you.

Marek
#1028


On Tue, Jun 24, 2014 at 8:30 PM, Ken Danko [1]<320kend at att.net> wrote:


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

References

   1. mailto:320kend at att.net