Reducing Engine Noise

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
Brianswhaler
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Reducing Engine Noise

Postby Brianswhaler » Mon Oct 19, 2015 8:20 pm

Has anyone experimented with sound deadening to reduce the noise of an outboard engine? I have a 1999 Mercury 115-HP two-cycle outboard engine on my Montauk. The engine noise at half-throttle to full-throttle is annoying--to say the least. Is [adding sound deadening] a possibility? Or, am I just amusing myself?

jimh
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Re: Reducing Engine Noise

Postby jimh » Thu Oct 22, 2015 10:08 am

I have not experimented with adding sound deadening material to an outboard engine. I do have some experience in trying to control transmission of sound from my work in designing and constructing professional audio mixing control rooms.

The sound of an outboard engine at half-throttle to full-throttle settings is going to be loud, even from the latest generation of very quiet engines. The sounds being made are characterized in three categories

--sound of high volume of air being drawn into the engine

--sound of the mechanical components of the engine moving at high speed

--exhaust sound from the combustion chamber

The sound of the high volume of air being drawn into the engine is generally a higher frequency sound. It may be possible to reduce transmission of this sound by using some foam sound defusion material. One must be careful not to block any air inlet passages, or you will choke off the flow of air into the engine.

The sound of the mechanical components of the engine moving at high speed is a lower frequency sound. For example, an engine operating at 4,000-RPM is creating a fundamental mechanical noise at a frequency of 4000-cycles/1-minute x 1-minute/60-seconds = 66-Hz. Use of foam material to attenuate sound at 60-Hz is ineffective. Typically a sound absorber for very low frequencies like this will employ a layer of lead metal foil.

The sound of the combustion chamber exhaust is usually conducted away and emitted under water through the hub of the propeller. There will also be an idle speed exhaust bypass path. At higher engine throttle operation the idle bypass path should not have much exhaust flowing through it. The degree to which the exhaust avoids escaping by the idle exhaust bypass path probably varies with the engine design and the design of the idle exhaust bypass. The sound of the exhaust escaping underwater can vary with the depth of the immersion of the outboard engine gear case and the boat speed. If the exhaust through the propeller hub is near the water surface, and if the boat wake contains much aerated water, the engine exhaust sound will be greater than if the propeller were more deeply immersed and the hull wake characteristic created less aerated water. These elements are probably out of your control for optimizing minimum exhaust, since usually they are optimized for performance.

Really excellent noise attenuation and resulting low-noise operation of an outboard engine is probably not possible to retro-fit to an existing design. Such properties must be engineered into the original design, and require a great deal of attention and engineering to accomplish. However, this should not discourage you from experimenting. Perhaps adding a layer of foam with a lead foil backing to the cowling of your outboard engine will reduce transmission of noise.

Regarding sound levels, it is very difficult for the human ear to know or remember a particular sound level, and comparison of two sounds for loudness generally cannot be done with accuracy by a human ear unless the listener can be exposed to the two sound levels for comparison with not more than a few seconds separation. If you want to know if your efforts to reduce sound transmission are effective, you will have to invest in some form of sound level measurement instrument, and develop a method for measuring the sound of your outboard engine at various operating levels. Only by taking careful measurements with a sound level instrument can you judge the outcome of any effort to reduce noise transmission from your engine. If you try to judge the sound by ear, you can easily fool yourself.

jimh
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Re: Reducing Engine Noise

Postby jimh » Thu Oct 22, 2015 10:17 am

Regarding modern outboard engines and their sound signatures, the increasing use of four-cycle engines has resulted in an interesting change in the sound signature. The four-cycle-power-stroke engine only has a combustion cycle on every other revolution. A two-cycle-power-stroke engine has a combustion cycle on every revolution. This affects the primary sound frequency. For example, if an engine runs at 1,000-RPM, its fundamental frequency from the combustion process will be as follows:

TWO-CYCLE
1,000-cycle/1-minute x 1-minute/60-seconds = 16.6-Hz

FOUR-CYCLE
500-cycle/1-minute x 1-minute/60-seconds = 8.3-Hz

By a four-cycle engine having a fundamental frequency that is half that of a two-cycle engine, the four-cycle engine is heard as being quieter by a human ear due to the general decrease in hearing sensitivity with reduction in frequency below 100-Hz. The loudness appears to decrease by a factor of ten to the human ear for each halving of the frequency. (See the famous Fletcher-Munson curve.) As a result, humans generally immediate perceive a four-cycle engine's sound to be as much as ten times lower than the same sound level from a two-cycle engine.

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Dutchman
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Re: Reducing Engine Noise

Postby Dutchman » Thu Oct 22, 2015 11:59 am

I have done some with an old 25HP Evinrude 2S I use on one of my dingies. I used a self adhesive type insulating material used in the HVAC industry (for temperature insulating) This has a shiny chrome type aluminum foil (I doubt if you can find lead type) look to it and it did help. I did not use a before and after decibel meter sounding on it but I would say it reduced it a good 10 dBA which is probably an a perceived value due to the higher and lower pitch levels the human ear distinguishes between as explained by Jim in the previous reply.
One thing that you always will run into with OB motors is that there is no room under the cowling/cover to apply the correct thickness of sound deadening material will you need, hence it was designed in already.
If it is really bothersome, all you can do is put up a panel between the engine and yourself absorbing the sound and deflecting it further aft. (not an option I would think)
I'll take a picture tonight to show what I did with the old 2 stroke.
EJO
"Clumsy Cleat"look up what it means
50th edition 2008 Montauk 150, w/60HP Mercury Bigfoot

ALAN G
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Re: Reducing Engine Noise

Postby ALAN G » Thu Oct 22, 2015 5:38 pm

Back in 1985, I bought a new Evinrude 150-HP V6 for my Revenge 19. After using it for a year, it seemed to be louder than my wife or I wanted, especially if we were sitting down and got the reflected noise from the windshield and dash area in addition to the directly radiated noise. We could not converse without yelling at one another at 20-nautical-miles-per-hour or greater. It was annoying. At a boat show, I came across a business called SOUNDOWN, who are still in business selling noise and vibration reducing materials. I bought some one-inch-thick foam composite acoustic insulation material. Starting from the side that faced the engine, this material has layers as follows:

--a layer of vinyl which made it easy to clean and was impervious to water and oil

--a layer of foam

--a thin layer of lead sheet

--another layer of foam.

I was told that this was the best [sound deadening material] they were selling for engine noise, that the foam absorbed some of the sound energy, and the lead contained the noise so it would not transmit to the engine cover and beyond to people in the boat.

The [sound deadening material] came in a roll and was somewhat stiff due to the lead sheet inside. It was a bit like unrolling a sheetmetal flashing roll. [The sound deadening material] was easy to cut using sheetmetal shears. I made some cardboard patterns that fit the inside of the outboard cover and cut out the acoustic insulation in several pieces per the patterns. I stripped away the manufacturer-installed foam that was on the cover, and contact cemented on the SOUNDOWN composite. I also cemented some around the lower cowling and areas I could reach near the air intake below the carburetors. I can't remember if this engine had an upper intake on the back of the cover too, but if it did, I ensured I was not blocking any air flow to the engine. I was also aware that any electronic ignition components that were on the engine (such as rectifier and ignition power pack) would need free air circulation to prevent them from overheating if the foam were to cover them.

It all worked out very well. I did have to cut away some of the foam adjacent to some engine components so the cover would fit. I was lucky that this particular engine had enough room inside the cover to allow the one-inch-thick foam enough clearance. I did not have a sound pressure level meter to take quantitative before-and-after readings, but subjectively [the sound deadening material] substantially lowered the noise level. My wife and I did not have to yell any more to talk while underway. So I concluded it was well worth the try and the cost of the materials.

I also had (and still do have) a 9.9 Evinrude kicker for trolling and emergency use. Since the noise treatment worked so well on the big engine, I used the remnants to do the smaller one. The results on the smaller engine were not as dramatic as the larger engine, but still noticeably quieter. There was less room on the 9.9 and I had to cut the inner layer of foam away from the lead (leaving the lead) so it would fit over the flywheel and around some other components (like the ignition coils and power pack).

If you have the clearance and can avoid blocking the airflow, I would say give [adding sound deadening material to your outboard engine] a try. By the way, I never had a problem with the foam coming loose. The small engine is now 30-years-old and still has the foam as installed. I sold the larger engine with the foam still fine. Choose a good grade of contact cement. The only downside: the cover was twice as heavy due to all the added lead.
Al

Brianswhaler
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Re: Reducing Engine Noise

Postby Brianswhaler » Sat Oct 24, 2015 3:41 pm

Thanks to everyone who responded!! I appriciate the technical info provided. I am going to check into the Sounddown products and see if it will fit my application before i consede to buying a etec engine. My current engine runs well and its paid for. I really dont want a 4 stroke because i love the punch of my 2 stroke just to noisy. Again thank you. Brian

PJMSport15MY1984
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Re: Reducing Engine Noise

Postby PJMSport15MY1984 » Sun Oct 25, 2015 12:56 pm

I have heard of some paint like material when restoring classic cars that is used inside the interior before paint that is supposed to curtail some of the engine and road noise. You could do some research and try and find out what it is, and then coat the inside of your outboard cowling. Good Luck!

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Dutchman
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Re: Reducing Engine Noise

Postby Dutchman » Tue Oct 27, 2015 9:16 am

Here are a couple of pictures of the stuff I glued (self adhesive) inside my 1986 Evinrude 25HP commercial I use on our Achilles.
Not much room left as you can see by the edges curling up and the indentation of the flywheel cover.
Bottom line it did help [reduce the noise from the engine], together with the old foam (that started to deteriorate).
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EJO
"Clumsy Cleat"look up what it means
50th edition 2008 Montauk 150, w/60HP Mercury Bigfoot

kwik_wurk
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Re: Reducing Engine Noise

Postby kwik_wurk » Wed Oct 28, 2015 1:43 pm

The main culprit in sound transmission is open airways, providing unhindered pathways to open air (our ears). Now given that an outboard needs intake air, this creates a conundrum. Outboards have baffles on the cowling and carb bodies to manage back-fire, reduce moisture (from rain and spray), and help with noise control. The swoosh of intake air is really not the source of problematic noise on a [two-stroke-power-cycle engine of] 115-HP: there is not enough volume to be excessive unless there is a blocked intake or similar (which would be odd). High frequency noise from air intake volume is a bit different with a larger volume engine blocks. Put it this way, you always here the bass from a car at the stop light, but never the snare drums. The annoying source is mainly firing rate noise coming from the engine, straight out the air intake.

Additionally vibration control (and to a lesser degree sound) is managed by isolation in the mounts, which are typically rubber dampener on various connections and mainly on newer engines.

We are left with the cowling to work with, to which you can add a better sound barrier. It will get a decibel or two of noise reduction at some speeds, but that's it. I use this material, it comes in sheets: http://www.fisheriessupply.com/soundtec-sound-insulation-vinyl-foam-barrier-composite

I re-did sound proofing on an 27-foot sterndrive boat, and I exceeded my expectations; you could not tell the engine was running at idle except for the bubbling of exhaust from the out drive. On plane the loudest noise source was actually wave noise coming from the impact zone on the hull--still a loud sound. The first step was to seal off all air gaps and cracks--obviously the intakes were on the side of the hull and used ducts that pointed to the bilge. Since I have commercial grade sound meters at my job, it was rather easy for me to poke around and find these air gaps, cracks, and sources. A modern smart-phone with appropriate app would be effective. Them I insulated and isolated the entire engine cover, and engine space--meaning all the fish boxes, and voids where engine sound would reflect or refract. I think I spent about $350 on materials, barriers, gaskets, and the like.

AZdave
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Re: Reducing Engine Noise

Postby AZdave » Thu Oct 29, 2015 2:09 am

It may be efficient to provide hearing protection for yourself and your passengers. I think this has been standard procedure for aircraft. You can enjoy the day together until the run out and back.

jimh
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Re: Reducing Engine Noise

Postby jimh » Thu Oct 29, 2015 10:16 am

As related to measurement of sound, many people employ the term decibel without really understanding its meaning. The best definition of what amount of sound change occurs when sound is decreased by 1-deciBel is given in this humorous example:

Q: When you ask your teenager to turn down the volume of his music, how much does he lower it?

A: One decibel

This encapsulates the real meaning of a change in sound of one decibel: it is about the smallest change in volume that the human ear can notice.

I have mentioned this many times: it is very difficult for a human to make quantitative comparisons of sound levels by memory when the two sound levels are compared with a time gap of more than a few seconds. It is unlikely that a human ear can remember a sound level that was heard several days or weeks earlier, then compare it to a sound level they're presently hearing with any sort of accuracy. If you want to perform any sort of meaningful comparison of sound levels by ear, you must compare the two sound levels with only a very short time separation, just a few seconds.