pseudomusical instrument or device that produces a howling or whirring sound when whirled through the air. The bull-roarer is commonly a flat piece of wood measuring from 4 to 14 inches (10 to 35 cm) in length and fastened at one end to a thong or string. This device, which produces sound waves in unenclosed air (as compared to the sound waves produced within a flute or pipe), is classified as a free aerophone. Its pitch is determined by the speed with which it is whirled.

Known as the rhombos, the device was used in the ancient Greek mystery religions (mystery religion). It has been observed in rituals of mystical or religious significance in Australia, Africa, North and South America, and areas of Oceania, where—with its animal-like howls or whirring voice—it may symbolize the presence of totemic ancestors. It is also used to cause or drive away sickness, warn women and children away from men's sacred ceremonies, control the weather, and promote fertility of game animals and crops. European-based cultures have adopted the bull-roarer as a toy or novelty item.