This audio was created so that you could have a 30-minute silent meditation without having to watch the clock. It begins with three chimes and ends with three chimes, with silence between. There is also a 15-minute version and a 20-minute version.
If you have an iPhone or Android-based smartphone, a good app is Insight Timer. With it you can set up a meditation period for any length of time and you have a choice of bell sounds. It will even show you the locations of other people around the world who are meditating the same time as you are!