One of life's great mysteries?

I was walking with Gazza to Dick's to get a burger the other day. It's a couple blocks up the hill from me, and I rather like the stroll up there. We took a different street up for variety- I love all the houses here. I can't help smiling when I find more houses painted bright colors. 

Anyway, Seattle isn't a fan of air conditioning in homes. Stores and gathering places like those have a/c, but most homes don't. Since it has been hot, then, most of the houses had their doors and windows open. When I walked past one of them, I knew the tv was on, but I couldn't hear anything. I looked back through the door as I walked past and it was indeed on.

What is it about walking into a room (and apparently from the sidewalk too!) that you can tell a tv is on, despite being muted?