What causes PA speakers to whistle?

PA speakers can whistle due to feedback, which occurs when the sound from the speaker is picked up by a microphone and re-amplified. This creates a loop that amplifies certain frequencies, leading to a high-pitched whistling sound. Other factors, such as improper speaker placement, excessive gain, or reflective surfaces, can also contribute to this phenomenon. To mitigate whistling, adjusting microphone positioning, lowering gain levels, or using equalization can be effective.