Fluttering Reverberance: Real Life Examples of Chaotic Billiards with Convex Sections
* Presenting author
Billiards are used to explore dynamical systems. The mathematical literature focusses on idealised physics and simple shapes like the Bunimovich stadium. But real enclosures where sound waves created by such shapes exhibit strong audible artefacts are uncommon. Also, billiard modelling is a high frequency approximation and diffraction is often important. Notwithstanding, some rare real-life acoustic billiards where the effects of closed-orbits are clearly audible have been found and examined with simulations and measurements. The abandoned Thurgoland railway tunnel near Penistone, UK has extraordinary metallic flutter in its long reverberant decay. The booking hall in the National Theatre subway station in Oslo has a very strong warble, and sounds different to the flutter echo heard in a simple cylindrical space. Both spaces feature concave geometries: the Thurgoland tunnel has a horseshoe-shaped cross-section, whereas the Oslo station is a distorted cylinder in plan with a domed roof. Ray-tracing reveals closed non-isolated orbits, which lead to repeated reflections and marked flutter echoes in both spaces. Examining the angles of the rays arriving at the receiver, and the autocorrelation of the impulse responses, provides further evidence of non-diffuseness. FDTD simulations are used to reveal the role that diffraction plays in the spaces.