All,
In previous thread I have struggled with the best plan of action of quieting down an 1800rpm diesel Onan DK 7.5 and relayed all the issues I struggle with: serviceablity/accessability vs lower sound and a busy genset compartment.
My question to the group is this:
Could I apply an accoustic absorbing coating such as:
http://www.quietcoat.com/index.html

DIRECTLY to the gensets diesel engine? The only possible downside I could see is that the engine would maintain internally more radiant heat that would have otherwise bled off into the compartment. However, worst case scenario, I could install an oil cooler and/or larger radiator to compensate for this if it were to become a problem.
I can't see how dampening the mechanical sound as it leaves the block/heads/oil pan/valve cover would in any way adversely affect the diesel engine? I'm no pro here and therefore I'm posting this question up.
If this methodology would work, it sure would meet the tests I've applied: 1) genset remains easily serviceable 2) solution is economical and 3) DB reduction achieved is worth the effort.
I don't expect that this solution would be the same an insulated cabinet enclosure, but I believe I could meet my test of DB reductions worth the effort.
All feedback appreciated.

Did I tell you guys I flew Experimental Aircraft for many years. With an experimental brain, I like to think outside of the box (enclosure)...

I watched with interest your previous thread and decided to let it run for a while before I responded. I'm in Military acoustics, mostly ANR and comms related, but sound mitigation is my business to a great extent.

For the most part, people got it right on attenuating noise. Although in one case there was a comment that sound mostly travels in a straight line, which is a bit of a misconception as sound will wrap around an object, reflect, pass through and refract, making it almost unstoppable unless it is a) absorbed, b) contained c) converted d) canceled or e) reflected away.

Your idea to spray the engine with the absorption material has some merit, but worries me on several accounts. First, it is likely a flammable substance which may be dangerous in an engine bay. Second, it may give off toxic fumes when heated. Third, unless it is completely sealed, the engine noise emanating will still find its way out. Fourth, I believe this material is more attuned to stopping auto body panels from reverberating and thereby allowing sound to pass through or be reintroduced. It is likely ineffective on a heavy iron engine.

What you need to understand is - Where is the noise from my generator emanating from? The exhaust, the internals of the motor, the intake, the generator rotation, a vibration being propagated into sound through acoustic migration? I'm guessing it is mostly exhaust, intake and vibration related. In other words, I doubt much of it is migrating through the engine castings, so save your time and money on coating it.

The idea of surrounding the engine bay with material will work well on high and mid frequency noise, but will not do much for low frequency noise.

A more efficient exhaust will either absorb or deflect noise away that is escaping through that "hole" in the motor. Remember though, a better exhaust may reduce performance or cause other headaches with the way the motor was designed to work.

The biggest gain, without giving anything up, is to isolate the vibrating mass (the entire generator/engine) so it cannot migrate noise into the living compartment. This is often done by rubber insulators such as body mounts or rubber exhaust hangers.


Good luck,
Corey

quote:

Originally posted by noble97monarch:
What you need to understand is - Where is the noise from my generator emanating from? The exhaust, the internals of the motor, the intake, the generator rotation, a vibration being propagated into sound through acoustic migration? I'm guessing it is mostly exhaust, intake and vibration related.

My own measurements on our gas genset indicate that most of the sound is mechanical. I had my wife record readings while I held big loose ball of rock wool in a bag over the engine exhaust and then the intake openings. There was very little sound reduction. Most of the noise was mechanical clatter. I did not attempt to isolate it down to engine clatter or generator mechanical noise.

As for diesels, as previously posted, I did measure the noise from a Barth Regency and a Country Coach with the same diesel genset. The Barth, with a smaller grille opening at the front of the coach, was 2dbA quieter at 21 ft. The CC with its larger grill opening, allowed a straighter path for the sound to escape. In fact, you could look straight in and see the genset. The Barth made the sound turn a corner or two, but had no absorbant material. I looked at the mufflers, and they appeared to be the same. From that, I would assume that for that Kohler diesel genset, mechanical noise was the major source.

I should also add that in all instances, my measurements were taken with charging loads only. My experiments with measuring when air conditioning is operating are clouded by the extra noise added by the air conditioning fans and compressor, making accurate comparisons difficult. An electric heater of a known wattage and setting would be a silent load. At work, we use a load bank for generator testing that was just a big coil of nichrome heating wire.

quote:

I'm in Military acoustics, mostly ANR

Well now you've peaked my interest. I've followed ANR since inception when I was still on active duty. So when will a firm offer something for an engine bay. All my searches have turned up nothing except a bunch of patents that apparently haven't made it to market. ANR is my obvious choice to my issue were it available. I also saw an ANR muffler years ago being tested on an automobile, I think by the people at Walker Muffler. But again it never made it to the market. I suppose all ANR devices for such applications are presently prohibitively expensive.
Until then... I will research other measures and come up with something that meets my humble criterion of serviceablity, economy and results woth the effort.

My company owned the patent(until it ran out) on a number of ANR breakthroughs 30 years ago. One of them was the "fail safe" ANR that allows comms to work even when ANR power (often battery) goes out. If you wear a David Clark headset with ANR, that, is our ANR module in there - Racal Acoustics.

Our focus is military, and comms must always be fail safe in that environment. David Clark saw the beauty for the commercial market way back when and used us as their supplier. Today, all MRAPs in the field as well as about half the rest of the wheeled tactical fleet use our Raptor ANR headsets.

Back to the issue at hand - why no one has a whole vehicle ARN. The simple answer is physics. Sound travels away from a source in all directions, like dropping a pebble in a still pond and watching the waves emanate out. In order to cancel noise in air, you would need to virtually surround the offensive source with ANR modules!! The further out, the more space to cover. Might as well just encapsulate or insulate the source as it is much more cost effective and energy efficient.

I know what you're thinking, you've heard of ANR in vehicles like aircraft. True but.....it's canceling noise on the skin of the aircraft, not in the air. That technology could be done in a RV, but it is often very expensive and quite frequency dependent (I'll get to that).

Another huge issue revolves around the power it takes to electronically cancel noise. One must literally create anti-noise or a noise wave that is 180 degrees out-of-phase to cancel the noise. Back to the pebble in the pond analogy, you can cancel the wave in the water if you exactly duplicate the wave form and have it intercept the wave coming at you. Make any sense?

Now for frequencies; ANR is good for low frequencies; highs and mids, not so much. We use absorbing/blocking materials for that. This is why when a car goes by with the stereo cranked, you hear only the bass (lows)! The skin of the car attenuated the mids and highs, but the lows powered through.

So...to me....the best way to cancel noise is in the sealed earcup of a headset where you have already canceled the mids and highs by the earcup shell and foam.

If you're interested in seeing what is being done on the soldier with today's military technology, I wrote a feature article that will appear in next months Armor and Mobility magazine.

Hope this is of some interest.

Corey

quote:

Originally posted by noble97monarch:
If you wear a David Clark headset with ANR, that, is our ANR module in there

Yeah, at the airline, we had them for high power runups and trim checks. Worked really well. Could've used them a coupla decades earlier when we had to manually adjust a jet fuel control at take off power while communicating with the cockpit. Interestingly enough, we never found any better mic for that purpose than the old WWII surplus throat mics.

Contact mics, including throat mics,have the advantage of eliminating sound unless it is picked up at the point of contact. They are impervious to wind and are typically waterproof. The disadvantages are comfort and comparatively low intelligibility if in a normal noise environment.

One reason throat mics work well in the "contact mic" arena, is that the flesh on your throat is a fair attenuator to bone conduction noise. Bone contact mics can be easily overwhelmed in a high noise environment.

Studies are being done to see if noise in the military may be harming war fighters by damaging their skeletal structure.

Corey

Here is how I did it. I installed an Onan 7.5 kW DKD in my 1991 Travel Supreme 'fiver. The mechanical noise of the machine was absorbed & blocked by adding a heavy vinyl acoustic barrier with a 1/4" isolation damper (foam) which faced the wall of the compartment, mechanically fastened to the wall. I covered that with a 2" thick, melamine-based acoustic foam on every available surface.

Duct all air in and out of the comapartment through ducts lined with the same material used in the comapartment. Discard the original muffler if it is the Onan one, It's outlet is only about 1 1/4". Replace it with a full-sized 2" in/out automotive style muffler. It allows the low frequencies room to expand and improves engine breathing i.e., better fuel economy. The intake is the trick though. build and connect hoses & PVC plumbing as required, to the air filter inlet. Route that to & through another the-larger-the-better 2" automotive style muffler, then route that through more 2" plumbing. The noise redution is usually 75%, about 20 dB. In 1993, Trailer Life magazine wrote a three-page article on how I installed generators and shut them up. During the test Jeff Johnston, the author made a comment that the generator was quieter than the roof-top air conditioner while standing about twenty feet from the RV, just enough to see the roof-top AC. Bottom line when silencing, if you can see the machine, you will hear the noise. The DKD, sitting on good original mounts doesn't emit intolerable vibration, not like the Kohler that rattled our bedroom cabinets.