The FAA outlines acceptable noise levels for commercial aircraft. General guidelines for noise identify sound levels between 55 and 65 dBA at 25 feet as acceptable for residential use. To put this in perspective, normal conversational speech registers 55 to 65 dBA at three feet, a motorcycle will average between 68 and 75 dBA at 25 feet, and a large truck will register at approximately 82 dBA at 25 feet. The FAA has exclusive control over aircraft operations. The ordinance does apply to aircraft on the ground.
Safety is the primary concern in determining appropriate altitude.
Helicopters may fly lower if they pose no hazard to persons or property on the ground. The FAA sets minimum safe altitudes. The published traffic pattern for aircraft landing at the airport is 800 feet. Yes, if they are in compliance with the FAA regulations. Many aircraft flying over and through Boulder are not based at the airport.