A speaker is a device that converts electrical signal to audible sound, two or more speakers are commonly used in a PA system to project the sound into the venue or auditorium.
Creating Audible Sound through Speakers
In most cases the speakers are placed at each side of the stage so that the sound source comes from both the left and right speaker enclosures and in this way creates a spatial effect which is often referred to as stereo imagery.
On some of the bigger PA systems the frequencies are separated and each frequency is fed through a specific speaker enclosure that has been designed to enhance that particular frequency range. This is commonly know as sound re-enforcement and uses bass speaker enclosures, mid-range enclosures and HF (High Frequency) horns to represent each audible frequency.
On-Stage Monitor Speakers for Musicians
Monitor or foldback speakers are wedge shaped speaker enclosures that are usually placed at the front of the stage directly in front of the performers in order to help them hear themselves and the other signals that are being outputted through the FOH (Front of House) speaker system. This helps a vocalist with pitch accuracy and helps the other band members to hear their own instruments even when they are far from their backline amplification.
The monitor or foldback system usually has its own separate power amplifiers and mixer so that the on-stage mix can be different to the sound coming out of the main house system.