HOW THE BULLROARER WORKS
A simple slat of wood typically measuring 12in x 2in (30cm x 5cm) is tied to a 3ft (90 cm) length of string. The slat is whirled in a circle, either on the horizontal plane above the player’s head or on the vertical plane alongside the player’s body. The edges of the slat are usually filed to a sharp edge resembling an aerofoil section; generally, the sharper the edges, the louder the sound produced. As the slat is whirled, typically at around 100rpm, it also spins on its long axis, producing a roaring sound that pulses with each revolution. It is usual to begin by suspending the slat and manually setting it spinning before beginning to whirl. As the slat spins, so its string twists tighter. After about six seconds, when the string can tighten no more, the slat slows down to a flutter, then stops spinning for half a second or so. It then begins to spin in the opposite direction until the string again tightens to a stop, and the cycle repeats.
The amplitude and frequency plots below show how the sound varies through time. For the longest part of each cycle, the slat produces a fundamental tone f1 of about 80Hz. Frequency plot A shows this as the loudest peak, accompanied by the quieter second harmonic f2 at 160Hz and the even quieter third harmonic f3 at 240Hz. Frequency plot B shows what happens when the string tightens. There is an audible lowering of pitch as the fundamental tone drops to below 50Hz; then as the spinning slows to a halt, the pitch drops accordingly to zero Hz. Because the lower limit of human hearing is 20Hz, this part of the cycle passes briefly through the infrasonic range – frequencies that are inaudible but can still be detected by the human body.