Bull Roarer

A “Bull Roarer” is one of the oldest and most primitive air instruments. It consists of an oblong board made of horn, wood, bone, or metal and sometimes has serrated edges. It has a cord through which it is attached directly to a finger or indirectly to a handle held by the hand. When swung above the head or along the user’s side, it causes air to vibrate and produce a vibrating sound, often very low-pitched. The sound is a consequence of two movements: the angular motion around the user caused by the user’s movement, and a spinning or rotating motion of the bullroarer along its long axis as a result of aerodynamic forces. When sufficiently twisted in one direction, the sound briefly pauses and then resumes when the spinning occurs in the opposite direction. The pitch of the sound is dependent upon many factors, but in general, a large bullroarer creates deeper-pitched sounds than a smaller one (more on this below).

Although generally considered an Australian instrument, the bullroarer is known and used in many other parts of the world. It was used by Native American cultures such as the Athabaskan, Nootka, Yokuts, Pomo, Hopi, and Aztecs. The Navajo call their bull roarers "Tsin di'ni" (groaning stick) and used them to drive away evil spirits. It is called several different names, including "Burliwarni," "Ngurrarngay," and "Muypak." Sometimes a bull roarer was used to send animals into ambush or to alert a tribe to another's presence in their area.

Its history notwithstanding, it is an interesting object and a fun departure from the ordinary!

This is a very easy instrument or toy to make, and as it turns out, while the details of the size and shape impact the acoustic properties, it seems you will get sounds from almost any shape.   Some general principles include the following:

1. Blade width tends to control pitch. Wider blades require more time to complete a full-axis rotation and produce a lower, deeper pitch, and a narrow blade rotates faster and produces a higher-pitched whirr or buzz.
2. Blade length controls volume.  Longer blades displace a much larger volume of air per rotation, producing a louder, fuller sound that carries over longer distances.
3. Thickness and object density impact sound stability.  Thicker bull roarers and ones made of denser wood may be a bit slower to start spinning and creating sounds; once moving, they are much more resistant to "stalling" (losing the rotational movement).  Thinner, lighter bull roarers will spin and create sounds more quickly, but are more erratic when spinning.
4. Edge tapering impacts ease of sound generation and pitch.  Tapering the edges of the bull roarer impacts the speed of rotation.  More symmetric tapering reduces aerodynamic drag and makes it easier to spin for a given arm effort and will cause more rapid spinning and a higher-pitched sound.  Blunt or squared edges increase air resistance and vortex shedding, which slows down the rotation and leads to a lower pitch and adds a rougher, breathier wind noise to the roar.  
5. String length affects the bullroarer's pitch, volume, and pulsing rhythm.  A long string creates a wider circle, higher airspeed, and contributes to a higher-pitched and louder roar.  Longer strings produce longer, slower winding and unwinding cycles, while shorter strings speed up the reversal rate.  From a practical perspective, long string lengths also impact the user's method of spinning the bull roarer.  Shorter lengths allow rotation to be vertical, at the user's side, sometimes allowing cautious demonstration inside, while longer string lengths may only allow use outdoors, overhead, and with a horizontal orientation over the user's head.