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Leslie Pavilion: creating audio effects using architectural shapes

Architecture can be used to produce acoustic environments that can enhance the activities taking place with the space, including music, speech and even quietude.

New computational tools allow specific acoustic features to be designed. The Leslie Pavilion is an installation that explores the effect of directional sound through use of shape and material as architectural sound filters. The installation allows the listener to experience the spatial sound-focusing effect of the installation and the use of architecture as an explicit audio filter.

The Arup team designed and built the Leslie Pavilion at a fabrication shop in Amsterdam with an innovative technique using a 2.5D CNC milling machine.

In the Leslie Pavilion, a rotating directional loudspeaker plays into two distinct shapes that mirror each other with opposing curvatures. The shapes are hyperbolic paraboloids (‘hypars’) configured so that sound is specifically guided through a path with a specific reflection time, which is chosen to provoke a psychoacoustic effect called the ‘precedence effect’ or ‘Haas effect’. The reflected sound is heard from the opposite direction of the direct sound.

Sound from a keyboard is amplified and played through the rotating loudspeaker that moves in and out of focus with the hypars’ shapes. A listener between the two hypars hears a spatialized rotational effect that is similar to a Leslie loudspeaker of the kind used with Hammond organs and similar; hence the name of the pavilion.  The sound from the loudspeaker comes in and out of focus, which causes listeners to hear spatial audio effects from the combination of direct sound and delayed indirect sound. The slow rotational movement of the loudspeaker causes a swirling sound quality to the music that accentuates the three-dimensionality and physical spatiality of sound.

When the loudspeaker is rotated off axis, the listener hears less focused sound and more direct or diffuse reflected sound. This is selectable by a pedal beneath the keyboard with slow and fast speeds, like the original Leslie loudspeakers. The periodic difference in amplitude from the left and the right creates dynamic precedence effect (a shift in the perceived location of the sound source by changing reflected amplitudes from the left and right). This is perceived by the listener as a spin or wobble effect.

The installation serves as an experimentation platform to hear and measure sound focusing and diffusing surfaces that can be used in acoustic modelling tools, including for calibration purposes. Current analytically-based modelling tools do not have these capabilities.

Information gathered from the Leslie Pavilion installation can be used for calibration and validation of the form and accuracy of curved surfaces used in architectural projects for acoustical design purposes.

Currently, comparison of real data from sound focusing with modelled ray-based sound focusing lacks accuracy. The Leslie Pavilion manifests concepts and effects that relate to the fundamentals of acoustics. It can be used as a tool for both verification and calibration of analytical models and can act as a platform for further development including modelling the acoustic surfaces more accurately. This can help address the existing knowledge gap between office-based modelling analysis and the issues encountered when actually fabricating and constructing a physical installation.