What also has to be taken into account of course is the weather. Environmental differences on different days can cause the aircraft to sound louder. Same with the police helicopter.

There is a word / phrase evading me right now, which would make more sense if used, but ....

I would recommend using a website such as http://www.flightradar24.com/ or a phone / tablet app to get the actual frequency, altitudes and speeds of the planes, can actually be quite interesting too. I'm sure CrustyRack would agree.