Sound reflecting from the mountains

Another thing that struck me as the planes went past was that mountains reflect sound quite well. I suppose that should come as no surprise because you can hear echoes but they are usually in tight corries with well defined craggy backwalls. What I noticed was that out on the open fellside, I watched the plane go past, "saw" the sound follow it and then yet there was a distant rumble from behind - the sound of the plane reflecting back from the mountain slopes behind. It's just surprising because it isn't a good reflecting surface. I now teach about diffuse and specular reflection of light. A sheet of paper reflects light but you can't see your face in it because the light is sent in all directions producing a diffuse reflection. That's probably what the mountain is doing to the sound.