So i've been looking around for a pair of headphones in the price class of the AKG K420 etc., around 30-40 euro. The specs say that they are half-open; what exactly does that mean? I'm going to be using mine during long bus travels and the like, and I prefer not to annoy my fellow passengers too much. If i'm playing music at a low-moderate sound in said headphones, how much would for example the person sitting next to me hear? What is acceptable? Thus far i've been using the standard ipod earplugs, and I haven't realised how much people around you actually hear while using them. I really don't want to spend alot of money on headphones to get some greatly isolating closed ones for example.