Generator quieting

[Gdetrailer]

A while back someone mentioned looking for a quieter muffler to reduce the noise of their gen.

That question brought to mind that a video needs to be made which should clearly show that much of the generator noise does not come from the exhaust.

So, finally after a couple of months of super cold weather and lately several weeks worth of daily "monsoon" heavy rain we actually had a nice 50F day with no rain.. I have made a quick under 2 minute video.

Let me preface a few things..

1, this IS NOT A DEBATE as to why I and other folks do not spend the money for the super quiet gens.

The video of this gen IS FOR MY WHOLE HOME BACKUP SYSTEM so if you wish to debate, go somewhere else, I don't care about the noise and I can't hear it in my home and my closest neighbors are across the road and over 500ft away.

2, This is a demonstration that shows that most of the noise comes from internal vibration of the engine. No muffler will fix that.

3, this is a 8750W HF gen with 13 HP Chinese Honda clone engine, it IS louder than say a 6.5 HP Honda clone 4K gen but the SAME PRINCIPALS of where the noise comes from IS THE SAME.

4, While I can play the video on my PC using VLC player, playing it from Google drive you may need to download the file first because Google drive video player does not seem to like a MP4 video file?

5, Video starts out loud, turn down your volume, you have been warned.

6, Video is a bit shaky, newer digital camcorders do not have as nice of shake reduction of my old Sony Hi8 camcorder and I was holding camcorder and sound meter at the same time.

7, Distance for sound measurements was about 10FT, near the gen about 85 DB, exhaust was about 10FT and 70 DB, I don't have a lot of room between the out building and my house so 10ft was a compromise distance. Doubling the distance will reduce the noise level by 3 DB which is considered halving the noise..

That means at 20FT 70DB becomes 67DB and the 85DB becomes 82DB.

8, Building does not have any insulation, just wood siding covered in vinyl siding but does make a decent noise block.

9, Sound reduction via a box or shelter will be more effective than changing the muffler.

OK, now that we got that all squared away..

You can get the video HERE

[pianotuna]

Since a 1 square inch hole transmits as much sound as 24 square feet of gyprock, sound control for a generator is more about reflecting than containing.

[pnichols]

I think most of the sound from our RV's built-in Onan generator is NOT from it's exhaust. For example, when we're parked on soft ground running the generator it's quieter inside and outside the RV - since less sound from the generator's body is reflected upwards off the soft ground surface.

[road-runner]

SoundGuy wrote:

Having built my own inverter genset quiet box I can confidently say that just running cooling air exhaust alone through a muffler would be an exercise in futility. Rather, the key is to design a box that not only features appropriate sound absorbing lining but also has a multi chambered exhaust output compartment that exhausts hot exhaust gases AND hot engine cooling air, along with a method of bringing in cool intake air while keeping it entirely separate from these hot exhaust gases................ Designed properly, using the correct materials, an inverter genset like the Honda EU2000i can easily be quieted to the point where it can barely be heard at all, and without the use of any muffler. :B

These forum communications are so difficult. In my mind, a sound dampening box IS a muffler for the exhaust and cooling air, in the spirit of the definition "muffler: a device to deaden noise".

[MEXICOWANDERER]

I believe a person will find COMBUSTION DETONATION is a major culprit of radiated noise. Example? Diesel. That knocking and clatter is an example and reduction of the noise because of water jacketing is a perfect example.

Because of "chain link association", my work with generators led me howling and fuming to problems with prime movers and it is impossible to isolate noise issues from customer complaints.

Take a 36-turn Wisconsin 37.5 horsepower 15Kw generator PM and compare it to an Onan 4cyl 36-turn 15-Kw generator.

The Wisconsin is deafening. The Onan is merely loud. The Wisconsin is air-cooled.

It's like a speaker. Only the voice coil actually is the origin of the noise -- the fabric is a sympathetic resonator. In an engine, cylinder combustion is the origin of BOTH exhaust AND resonating noise. The surface of the engine acts like the sympathetic resonator.

Water transmits noise, yes? But water in a chamber can also diffuse noise through an intricate physics of echo cancellation. Noise bouncing off a far side wall cancels frequency waves and this action is definitely selective.

I fiddled with sound proofing although I did not want to.

A thin foil of lead absorbs higher frequency emissions. Soft foam with pyramid protrusions is the most effective sound absorbsion for middle and low frequencies.

I had to deal with generator noise while building trawlers. It isn't fun. A diesel genset can turn the hull and bulkheads into giant resonators. Stick a stethoscope against concrete block of a generator shed and hear the effects of this.

Suppress sound. Then stick your stethoscope against the outside wall once again -- amazing eh?

It would cost a moderate fortune for me to clad the inside of my gen shed with 3" suppressor foam. Then there is the issue of flammability. Not feasible with auto extinguishing suppressors like Halon. Beyond my budget.

If you want real life examples of generator sound suppression, movie set tow trailer gen sets are perfect. A 20 Kw unit (diesel) is quieter with an ear a foot from the enclosure than a Honda 2000 watt inverter generator is at 10ft distance. Not bad for a DIESEL versus gasoline comparison. All's it takes is lots of money, lots of space and a significant reduction in engine efficiency due to increased exhaust back pressure. And keep in mind those movie set units have to deal with radiator cooling air flow.

[SaltiDawg]

I think this thread shows the OP has good insight on the complexities of an internal combustion engine's airborne noise sources and the complexity of the issue can seem over-whelming.

In the early 1980's I was The Director of Ship Silencing for the Naval Sea Systems Command (NAVSEA-05H)- responsible for the R&D required to make ships quieter.

One of our efforts was in the area of a large Emergency Generator used sometimes by our Submarines. This was an extremely large internal combustion device in a relatively confined space with its own set of unique features - air intake thru a (Snorkel) pipe and exhaust thru a pipe to the outside world.

Nice presentation above. You've made a convincing argument how to best target your efforts.

If I recall correctly, there is a many page description in the old 1000+ page thread on Chinese Generators. It might be interesting to also dust that thread off. I believe professor95 did the testing.

3000W Chinese Gensets Info.

[wa8yxm]

In my case the building was concrete and steel, likewise the roof, the exhaust was outside and the generator inside.. So I support the Original Poster's findings.

There have been many threads about this.. I'm not sure that "internal vibration" is necessary the issue though Internal vibrartion is transmitted to the outside by the engine block and assorted attached parts.

An interesting thing. I had a 1,000 watt Enclosed portable generator (Genrac 1000) in fact I still own it, I just do not have it any more (If you find one with WA8YXM scratched or painted on it please let me know)

Weight, fuel consumption and noise all abut equal to a Honda EU-2000i..

But a 1,000 OPEN FRAME.. Way, way, way louder.

[SoundGuy]

road-runner wrote:
I came to the same conclusion experimenting with a portable inverter generator. With this, the plastic case already does a fairly good job of muting the engine noise, but then the noise exits the case with the cooling air that is blown out. To achieve good quieting, the cooling air exhaust would have to run through a muffler, which is what happens when the generator is placed inside a nicely built noise dampening enclosure. Adding a big muffler to only the engine exhaust barely reduced the sound level.

Having built my own inverter genset quiet box I can confidently say that just running cooling air exhaust alone through a muffler would be an exercise in futility. Rather, the key is to design a box that not only features appropriate sound absorbing lining but also has a multi chambered exhaust output compartment that exhausts hot exhaust gases AND hot engine cooling air, along with a method of bringing in cool intake air while keeping it entirely separate from these hot exhaust gases. Most DIY designs don't meet this criteria at all but there are some well designed examples that do. Even my own Honda EU2000i Quiet Box, remarkably effective as it was, could have been much better had I followed these guidelines and had I been willing to invest many more $$$ in materials to build the box. Had budget been unlimited I'd have lined a custom box with this West Marine Noise Control Barrier and modified the genset with a Generator-Line Exhaust System and ported it's output to a multi chambered output compartment. Designed properly, using the correct materials, an inverter genset like the Honda EU2000i can easily be quieted to the point where it can barely be heard at all, and without the use of any muffler. :B

[road-runner]

I came to the same conclusion experimenting with a portable inverter generator. With this, the plastic case already does a fairly good job of muting the engine noise, but then the noise exits the case with the cooling air that is blown out. To achieve good quieting, the cooling air exhaust would have to run through a muffler, which is what happens when the generator is placed inside a nicely built noise dampening enclosure. Adding a big muffler to only the engine exhaust barely reduced the sound level.

[theoldwizard1]

Gdetrailer wrote:

9, Sound reduction via a box or shelter will be more effective than changing the muffler.

Yep, that sums it up !

[Gdetrailer]

SaltiDawg wrote:
Thanks for the reply. I was not asking about your conclusion re the exhaust.

I again simply ask, "What is the mechanism that you hypothesize for "internal vibration of the engine" to be seen as airborne noise?"

The fact that this happens is not obvious.

I would ask in a little more depth, but I'm afraid that my "credentials" would be again questioned.

Simple, the engine case resonates.

Anything moving inside makes small vibrations.

Those vibrations in turn are coupled mechanically to the case.

The case then vibrates.

Case vibrations move air particles that are against the case.

Air particles then vibrate the adjacent air particles..

Works a lot like a speaker.

Imagine dropping a small drop of water into a bowl of water..

The drop creates waves that radiate out from the original drop.

[Gdetrailer]

MEXICOWANDERER wrote:
My Lombardini sounds like a munitions dump accident...

What can I say, it IS huge, it IS heavy, it IS a low cost $529 HF 8750W gen with electric start and it can run my water well, lights, furnace, fridge, freezer and still have lots of available power left for more..

Even if I had bought a Real Honda 8000W contractors gen, it WOULD be just as loud, these puppies are not designed to be quiet but are still much quieter than a few 50Kw backup gens I have been around..

[SaltiDawg]

Thanks for the reply. I was not asking about your conclusion re the exhaust.

I again simply ask, "What is the mechanism that you hypothesize for "internal vibration of the engine" to be seen as airborne noise?"

The fact that this happens is not obvious.

I would ask in a little more depth, but I'm afraid that my "credentials" would be again questioned.

[Gdetrailer]

SaltiDawg wrote:
What is the mechanism that you hypothesize for "internal vibration of the engine" to be seen as airborne noise?

TIA

Exhaust noise is "isolated" via the building.

When standing OUTSIDE you are hearing mainly the exhaust note with a slight bit of the gen vibration coming through the walls.

The building since it is wood and the floor is also wood does not 100% stop the mechanical noise but in that video the mechanical noise is reduced and it is good enough to show that the exhaust component is the smaller part of the overall sound level.

Concrete block building with concrete floor would most likely isolate the exhaust from mechanical better but I am not putting the effort or money into this demonstration to make a block building.

INSIDE THE BUILDING, the MECHANICAL noise IS what is LEFT OVER WITHOUT THE EXHAUST NOTE.

I connected flex pipe directly to the muffler outlet. The flex pipe makes a turn out the side of the building.

Moving parts inside the gen like valves, pushrods, piston movements, crank shaft and even slight balance issues resonate all through the engine case, the case then radiates the mechanical noise.

Smaller engines like in say a Honda Inverter gen have special attention paid to noise due to mechanical mass and vibrations caused by pushrods and valves. Part of that is a case design that may have reinforcements to specific areas that help to dampen the transmission of mechanical noise.

A pretty simple but yet effective 15 DB of noise reduction by placing the gen in some sort of box or in my case a out building.

Inside my home, I cannot even hear the drone of the gen.

[SaltiDawg]

What is the mechanism that you hypothesize for "internal vibration of the engine" to be seen as airborne noise?

TIA

[MEXICOWANDERER]

My Lombardini sounds like a munitions dump accident...