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Rotational aerophones.
N H Fletcher1, A Z Tarnopolsky, J C S Lai
1Research School of Physical Sciences and Engineering, Australian National University, Canberra. neville.fletcher@anu.edu.au
The Journal of the Acoustical Society of America
|April 5, 2002
Summary
The bullroarer, an ancient aerophone, produces sound via an oscillating-rotating dipole, not primarily from shed vortices. Experimental and theoretical analysis clarifies its aerodynamic behavior and sound production mechanisms.
Area of Science:
- Acoustics
- Aerodynamics
- Physics of Musical Instruments
Background:
- Bullroarers are ancient ceremonial aerophones used globally.
- Their sound production mechanism has not been fully elucidated.
- Understanding their aerodynamics is key to understanding their acoustic properties.
Purpose of the Study:
- To experimentally and theoretically investigate the bullroarer.
- To elucidate the aerodynamics of its rotational behavior.
- To determine the primary mechanism of sound production.
Main Methods:
- Experimental analysis of bullroarer aerodynamics.
- Theoretical modeling of slat rotation and sound generation.
- Comparison with wind tunnel experiments.
Main Results:
- Slat rotation frequency is related to slat width and velocity.
- Sound production is primarily due to an oscillating-rotating dipole.
- Vortices shed by the slat play a minor role in sound generation.
- Discrepancies in wind tunnel tests are attributed to bearing friction.
Conclusions:
- The oscillating-rotating dipole is the dominant sound source in bullroarers.
- Aerodynamic principles explain the instrument's acoustic output.
- Experimental and theoretical findings clarify the physics of this ancient instrument.