Perhaps an order of magnitude more? You'd expect an airliner at about 4000 ft at 15 km from the airport, assuming it's trying to land. It's also illegal to fly lower than 1000 ft above inhabited areas in most of the world.
Typically sound follows the inverse square rule, so the difference between the same plane at 1000 feet should be a 1/169th (1/13^2) of the noise at 75 feet. So two orders of magnitude.
The airport is sea level, I live on a little hill in a 3-floor building and the airplanes cruise over my head at what feels like 15th-20th floor max.