They say its the worlds loudest bike horn. The AirZound uses compressed air which you can pump in via your bike pump (Schrader valve – the big one). You pump it up to a recommended 80-100 psi. This is done best via a track pump, though with a bit of effort you should be able to get it fully pumped up with a hand pump. When fully pumped it can emit a very loud warning honk and should you decide to hold it down (or use it a lot) you should get about half a minute before it runs out of air. Unless you misuse it (or have an unfortunate ride) you should have enough pressure to last a reasonable amount of time between charges. A couple short sharp bursts normally does the trick and a few light taps can work well as a warning at junctions.