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

[Jeff Hare]

I'm not a sound engineer either.

As I think I mentioned, sound proofing this engine doesn't make it quiet.  There are many frequencies of sound and vibration through the hull that make up the sound you hear.  Some of these are rather harsh to my ear anyway.  The sound proofing can't change what vibrates through the hull (lower frequencies mostly), but can make a big difference in what frequencies make it to your ear through the engine compartment covers (which are like speaker boxes).  I also mentioned that if you are going to leave gaps, you probably shouldn't spend the money because the offensive frequencies might be reaching the engine covers and will probably transmit right through.  Given that the aft cover isn't done, that's a big gap.  :-)  When I shut the aft cabin door and run the engine with the aft cover off, the noise is really loud!  It also makes a big difference whether the aft bunk cushions are in place.

There may also be something else going on with measuring the sound, like it may be measuring only a specific range frequencies. I believe the reading is correct at the frequencies it's measuring but I doubt the microphone on the iPhone is set up for reading anything outside than the normal conversational voice range.  You should also measure the sound from the cockpit.

So, my take is that if you're going to make decisions on empirical data, you get good empirical data and measure the DB across a range of frequencies.  You may find that it is significantly changing the sound spectrum, but not the hull vibration effect.

I can certainly say that the noise with WM or SoundDown (and from others that the SS solution also) insulation is significantly deadened. The high freq. pinging /rattling of the diesel is almost completely gone.  What you have to ask is whether you're willing to insulate the compartments along the prop shaft and the back wall under the bed that contacts the rudder / fuel tank area.  All these would make a big difference considering that the muffler isn't insulated, etc...

So, before you give up, finish the whole engine compartment just like you started and use your ears.  Can you now have a comfortable conversation in the cockpit?  :)  My wife complained about the noise before and hasn't since, but we're all happy to turn off the engine and sail.

Good luck!
Jeff


-----Original Message-----
From: C320-list [mailto:c320-list-bounces at lists.catalina320.com] On Behalf Of Ken Danko
Sent: Tuesday, June 24, 2014 11:58 PM
To: C320-List at Catalina320.com
Subject: Re: [C320-list] Engine compartment sound proofing - Some preliminary results

   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