The Rolls PM50sOB is designed for performers who need to monitor their own signal, such as their vocal, with the main mix. The unit has become very popular with drummers who need to mix a click with a monitor signal. Church choir singers love the PM50sOB to monitor their voices over the musicians. And many sound technicians use the PM50sOB as a quick line tester to troubleshoot audio systems. All these features and benefits in a battery powered unit.