Sorry it took me so long to reply, my internet service has been down. With my first design, I think the 18" pit actually amplified the noise. I had originally thought I would have to run the 4" pipe out into the woods about 150' away to counter the noise. I only had a shovel so I quit at 3 feet and it worked. I made a drawing that isn't exactly to scale of what I did, but maybe is representative of my design. It has been working for 8 years now.
