No hard and fast rule. I am on just barely an acre, same with neighbors. The properties are rectangular with homes at front. We have a barn towards the back, and the chicken run is behind the barn. Wind is usually blowing from the west and hits the homes first and blows past them. There is a farm field to the west, so nothing blocks the wind, or causes it to flow differently. My neighbors do not hear our chickens, but can hear the roosters, faintly if windows open. My neighbor had chickens and a rooster. We never heard the girls, sort of heard the rooster. We have no trees, no fences, no brush between the properties. Mostly, I think the way the general air currents flow, helps to mitigate the sound. There is an elevated train track about 1.5 miles away, to the east and we do not hear it 98% of the time. But, when wind/humidity is different enough, we hear it like it is a few hundred feet away. It is loud, and obvious but it is not the norm. Same way at a rental we lived in. Fairly close to the interstate, where it made an inward curve. We could not see the interstate, and there were several properties between us and the highway. We heard the highway 98% of the time. It was notable bc occasionally, it was silent…wind and humidity shifted enough to negate the sound. At that property, we were to the east of the highway, and with the general wind direction, we got the majority of sound coming from the west.
Just for reference I have almost 2 acres and my neighbor is about half an acre away.
