Yes, it is entirely possible to hear a plane cruising at 30,000 feet (approximately 9 km high), but it depends on your environment and the aircraft type. On a quiet night in a rural area with low "ambient noise," the low-frequency hum of jet engines can be quite distinct, though it sounds like a distant, steady "rushing" wind rather than the roar heard at an airport. Because sound travels at roughly 343 m/s, there is a 30-second delay between where you see the plane and where the sound seems to come from. In a busy 2026 city, however, urban noise pollution (traffic, air conditioning, construction) almost always drowns out high-altitude aircraft. Atmospheric conditions also play a huge role: cold, stable air (common in winter) is better at conducting sound waves to the ground. Conversely, if a plane is supersonic (which is rare for commercial flights in 2026), you wouldn't hear it until it had already passed, but for standard subsonic jets, the "sky-hum" is a constant, albeit faint, part of the modern acoustic landscape.