Similarly, I think most people are unconsciously aware of technology used over the decades in recording studios. As recording equipment has evolved over time, periods of time have come to have their own sound that I think we easily pick up on. I'm not talking about the instruments, but the electronics that the signals pass through on the way to making an album. A solo guitar record from the 60s *sounds* different from one made in the 80s or 00s. (Maybe that's a weak example-- the more complex the recording, the more obvious the imprint of the gear, probably.) Its the microphones, the mixing board, the compressors, the mastering process, etc. All of it has, and will, change over time. I'm not a scientist but I think alot of people can tell in their gut what decade a song is from, even if they've never heard it before. And I think its really interesting that these differences can come through on even the crappiest of stereo systems.