Healing Circle

“Healing Circle”   June – July , 2013

Site and time specific project: Art Gallery of New South Wales. Sidney, Australia, 2013.

Please find beneath an stereo adaptation excerpt of part of the soundscape reproduced during the installation.

This project, result of the collaboration between Carlos Vaquero and Christian Thompson, was commissioned as an invitation to the Anne Landa Award 2013 Festival by its curator Charlotte Landman. The installation was showcased between May, 15th and July 15th, 2013, at the Art Gallery of New South Wales (Sidney).

Context

The bullroarer is an instrument belonging to the family of aerophones. Made by attaching a rectangular thin slat of wood to a long cord, the instrument can vary in shapes and sizes. The slat length can be between 20-50 cm and may be 4-7 cm wide. The wooden slat is normally trimmed down around the edges but it can also have a rectangular shape.

The term bullroarer is a western vulgarization to this sort of instruments common not only in Australia but also in countries belonging to continents such as America (Inuit culture) or Africa (since ancient egypt). In the Bidjara language, the actual translation to English should be something similar to “secret-sacred”. The australian ‘bull-roarer’ is often used either to communicate over long distance or in initiation ceremonies and in burials by some Aboriginal tribes men. The instrument is forbidden for women, children, non-initiated men, or outsiders. The sound it produces is considered to represent the rainbow serpent and is also often related as a wife-caller.

Acoustics

The sound being produced by the bullroarer is the result on the combination of the air friction of the slat being rotated around the axis of the person that holds it through a cord. The rotations in the axis of the arm (being considered a torque in this case)

An excellent description of the acoustics model of its sound can be found on Fletcher, Tarnopolsky, & Lai (2002) which have inspired the latter sound design of the installation. 

The angular rotation rate f of a rectangular slat of width W swung through the air on a string of length L with rotation frequency F can be shown to be around:

f = 1.1 LF/ W – 5 ,

Being f and F rotations per second, on each rotation the rectangular slat oscillates as a flow dipole having a sound frequency radiation of 2f.  The radiated acoustic power P is approximately:

P = 3 (p/c3) H2V6

where H is the length of the slat, p is the density of air, c is the speed of sound in air, and V=2πLF is the speed of the slat through the air (“Australian Aboriginal Musical Instruments – The Didjeridu, The Bullroarer And The Gumleaf,” n.d.).

Therefore, the sound frequency emitted from the swung of the bullroarer is proportional to airspeed and rotation rate and may depend as well on the width of the slate being used. Sound radiation is nearly omnidirectional.

Having these elements an algorithm was adapted to an octophonic setup in which the rotation frequency would be re-created by the sounds being triggered in circular motion on each of the speakers of the octophonic setup. Having then an alternation on each of the speakers of the room , the visitor would be perceiving the sound displacement as an analogy to the perception of the rotation of the bullroarer around the perpendicular angle from the players head.

Sound design

After establishing a maximum speed the frequency proportional to the rotation increases proportionally by using a constant of pitch shifting and pitch dissonance, achieving like this the effect produced by the angular rotation F.

The radiated acoustic power was not measured since in-situ specific amplitude measurements were missing. However, assuming that the power radiated would increased by a constant of the fourth root of the rotation rate, the amplitude of the general sound was increased by this factor.

The effect achieved is a transformation of the voice of Christian Thompson into a rotating bullroarer and displacing around the visitor of the room in the museum.

The algorithm was implemented in SC3 while the mixing and mastering of the different outputs were manipulated in Ableton Live.