Sound Dampening Your Generator

This is my first Instructable and so I would appreciate any comments on areas to improve. That being said, lets jump straight in.

As a modern human being, I find that just about everything that I use at home on a daily basis is electrically powered. Since most of these things make my life a happier place to be, when the power goes out it really is quite annoying. Thanks to the complete and utter uselessness on behalf of our government funded power utility, Eskom, we as a South African public find ourselves ever more frequently on the receiving end of 2:30hr power outages. These planned outages, which have affectionately been given the name "Load Shedding", are due to a mismanagement of funds and a severe lack of maintenance. Occasionally, depending on the strain on the national grid, we can have these outages up to 3 times a day. All of this has resulted in the generator industry posting sales of epic proportions over the last few months.

So I have added to the statistics and bought a 7.5Kva Genset.

The one problem with having a generator at home is noise, and plenty of it. It is disturbing to myself and the neighbours around me. So the only solution, apart from sitting in the dark, is to try and dampen the noise it creates and so I thought about how to do this as cheaply as possible.

Very important before you continue... This is NOT SOUNDPROOFING... there is too much noise created by the motor to completely silence it, and besides, you can't do it anyway because there is always a requirement for places for air to get in and out for ventilation... holes = noise... So this is only to make the sound a bit less ear injuring.

Step 1: The Isolating Layer

Taking into account that if I put the generator in front of my house, there is a strong possibility that it could be stolen, I decided to put it in my back yard. Not ideal, because the confined area would probably amplify the sound, but the logical choice. It also meant that my cable did not have to be so long when I connected it to the distribution board, keeping costs down again.

Because sound is simply a result of air pressure changes, which are caused by vibrations, I wanted to keep these down to an absolute minimum. I figured that the more solid the structure, the less likely it would be to vibrate. With this in mind I settled on a very simple but sturdy design that would structurally be isolated from the main building and floor. I had some old 10mm thick rubber sheeting lying around that had been used as the under layer to an artificial sports field. I decided to use this as the isolating layer between the old floor and the new floor for my enclosure. I cut 3 lengths of the stuff from my roll and made sure it was a bit bigger than I thought I needed. I placed these on top of each other in the position of the new slab.

Step 2: Setting Out the Structure

I measured my generator and decided on the best way of putting it into the box I was building. This gave me the minimum internal dimensions. I added some room around the unit so that there could be a reasonable amount of airflow to the engine. Also important was that the structure I built was not touching the walls of the house, so I left a 40mm gap between it and the house. I marked a line down with chalk and then packed some 190mm thick cement blocks down on the rubber to get the size of the base I needed to cast. I made sure everything was 90degrees with a carpenters square. As you can see from the bricks on the house wall, the existing ground level was also uneven and this would need to be corrected with the slab.

Step 3: Casting the Base Slab

Because the floor was uneven, I had to adjust the form in order to ensure it was level. This was not a difficult job and I made sure that the tops of the form were the same height so I could use them to level the new surface. I used some 50x76mm (2x3") timber for the side forms and another piece of timber for the front. I then used some lintels to hold them in place for the concrete pour. In order to keep this concrete separate from the house still I added in a piece of rubber at the back to act as a spacer.

No awards for mix design here, just a very simple cement stone and sand mix. I am not building a house on it, so it does not have to be the greatest strength in the world. Mix it so it is not to wet or you will wait hours to have it set enough to float nicely. Make sure it is not to dry or it will crumble and not bond properly. One thing I forgot to photograph was the layer of plastic, or old refuse bag, that I put down on top of the rubber. As my material was porous i did not want all the slush to filter through it and clog it up making it hard.

Cast the concrete, level it off and wait till tomorrow.

Step 4: Building the Walls

With the slab now hardened, I once again dry packed the blocks. This time making sure they were all level and square. I did not want to build the wall with mortar as I was trying to do this as quickly as possible. My thought was that it would waste valuable time. Anyway, it is important that the blocks are level and plumb as there is not chance for making changes later. You can't see it in the pictures, but that small gap is still between the house wall and the blocks.

At this stage I measured the heights of the air intake and the exhaust outlet. I marked these on the walls with chalk. As the air intake was at ground level, I filled up one half of the block with sand. I placed a piece of plastic on top of this to separate the sand from the concrete fill. I did the same for the exhaust outlet, but because it was one block higher, I needed to put some concrete in first before making the sand layer. These sand layers will make it easier to break through those sections of block later.

Mix a nice wet concrete and fill up all the blocks from bottom to top. Be sure to tamp it in with a stick so it comes out the joints as this will ensure full adhesion. (Don't forget your exhaust outlet). Once the concrete was in, I cleaned up the outside by bagging it.

Step 5: The Roof and Ventilation

With the walls setting nicely it is time for the roof slab.

I was lazy, and I wanted this project to be done in 3 days. I used lintels to cover the roof area and filled them in with mortar. I placed a layer of mortar down underneath them before setting and used a rubber mallet to make sure they were properly settled. Try and measure this accurately, as I did not and ended up having to redo it because my front lintel did not line up nicely with my front blocks. I left this to set for the night and then pulled out the heavy hammer.

I broke through the areas that I had marked for the inlet and exhaust. The blocks break easily and although I still need to neaten things off, time was precious as another spell of "load shedding" loomed. I am not going to get into the wiring of the generator to the house as every country and every board is different, but you can see the cable coming out of the enclosure next to the mesh I put in the inlet hole. This mesh is just to stop small animals making my enclosure a home... and then dying of exhaust fumes.

The inlet hole is directly opposite the air intake and the same can be said for the exhaust hole.

Step 6: Add a Fan and Seal the Opening

As an air cooled unit, having it inside a box with no air movement would kill the generator very quickly. The air would get to hot and the thermal kill switch would be as forgiving as the power company. The other problem is that it requires oxygen to run and if there is no airflow, then the exhaust fumes fill the box and it will kill the engine as well. I used a decent sized fan to suck fresh air into the enclosure and directly past the air intake. Because the fan is quite powerful it circulates the air very well and the extra fan attached to the generator itself also helps with directing cool air onto the engine. It is connected directly to the generator so that once it is started, the fan will start blowing. No need to remember = less likely to give a problem.

The next step was the front door. I wanted to make it as "airtight" as possible and as heavy as possible without being a metal or concrete safe door. I used an old commercial grade solid door that I had over from a job. It is super heavy (for a door), but also able to be cut and worked on easily being timber. I added some gate hinges to the concrete base and attached these to the door. I then added rubber seals all the way round so that when it closed it would have a really good fit. This was mostly self adhesive, but the "D-Shaped" rubber was glued on. As you can see, the rubber seals tightly against the blocks and the lintels.

Step 7: Closing the Door and Getting Rid of the Fumes

The next task was to close the door and keep it closed. I wanted it to be easy to open and quick to close, but the problem with latches and locks it that you have to force the rubber seals first before you can get the locks or latches to work. I settled (for now) on a ratchet strap as it is easy to use and can pull the door very tightly. For now it is fine but I will put a better solution together at a later stage.

Something else to improve on is the fumes. Currently the fumes exit the exhaust and come out the same hole as the hot air from the box. This works, but it would like to put a pipe on the outlet to direct the fumes above my gutter line. This should hopefully cut down on the smell (and maybe a bit of noise). I have some pipe and a bend that I need to weld up but it fits snugly on the exhaust and I am sure it will work. Everything has to able to be removed from the generator as it is slid out of the enclosure for refueling, so simple is the best. The extended exhaust will be held with a bracket and be able to pivot slightly to come away from the generator exhaust. The rest of the opening still needs to be there for the hot air in the box to escape.

Step 8: The Results

So after all this work, 3 days later it is ready for testing. This is my own design and if it fails it will become a very robust wood store. I had no idea what the results would be but as you can see, there is a significant drop in the DB readings with the door open and closed. The most important aspect for me was that my neighbours were not disturbed too much.

I don't have all the fancy equipment to do the tests, but the apps that I used on my Samsung Galaxy S4 Mini are all that I have to work off. The first two pictures are of the sound reading right at the enclosure with door open (73db - you can see the generator wiring in the background) and door closed (64db - you can see the front closed in the background). The next is inside the house with the door leading to the back yard open (58db) and then closed (48db - the door to the generator being closed both times). Lastly are the readings from against the wall at my neighbours property. This is about 10m away and for us is the most important as we really don't want to disturb them too much. The first is 57db and the second with the door closed is 55db. Although there is not much of a decrease for some reason (perhaps it was due to other noises from around my pool pump) there is a significant difference in perceived sound level. After chatting to the neighbour, he is very happy with the difference and I guess that is all that matters. Perhaps and offer of power will have an even greater effect :) but who knows. There may still be some room for improvement, but for now I am happy and he is happy... AND I will not be left in the dark due to our beloved "Load Shedding".

Remember that a decrease of 10db is the equivalent of 10x less perceived volume

As mentioned before I am not going to go into the wiring of the generator into the board as there are different regulations for different countries.

I hope you found this useful.