Sound solutions: Harnessing architecture for better speech environments

6 May 2024
Designing acoustically retroreflective surfaces
Researchers at the University of Sydney’s School of Architecture, Design, and Planning have been pioneering the development of acoustically retroreflective surfaces, which redirect sound back to its source. Retroreflectors have familiar applications in optics, such and bicycle reflectors and road signs, but their application in acoustics remains uncommon. Their work aims to explore how to optimise the design of these surfaces for sound and explore their practical applications in improving speech clarity within rooms.
A man surrounded by orange, angular structures

Associate Professor Densil Cabrera

Previous studies indicate that clear and strong self-perception of one's voice promotes relaxed voice projection, potentially reducing voice strain in certain environments. Additionally, in multi-talker settings where speech babble can be distracting, innovative design could help create a more comfortable sound experience.

In architecture, the concave right-angle corner, where three surfaces meet, acts as a basic acoustic retroreflector. However, to effectively concentrate sound, numerous visible corners of considerable size are required simultaneously. This poses a challenge: designing surfaces that effectively concentrate speech frequencies, redirecting each talker's voice towards them.

The world’s first highly retroreflective room 

In a groundbreaking first demonstration, researchers at Sydney have transformed a normal office into the world's first intentionally designed acoustically retroreflective room. Associate Professor Densil Cabrera’s office at the University of Sydney’s Wilkinson Building now features 157 explicit concave right-angle corners, many sourced from repurposed Ikea steel box cupboards (Cabrera et al. 2024).  

The room provides five decibels more reflected sound to a talker compared to a standard room. Unlike focusing rooms, such as domes, the retroreflective room uniformly concentrates sound regardless of the talker's position, resulting in surprisingly clear and loud self-perception of one's voice.

While this room represents the first intentional design of a highly acoustically retroreflective space, incidental examples of retroreflection in architecture exist. Some building facades possess unusually retroreflective properties due to numerous deep corners (Cabrera et al. 2020). The researchers have also explored the acoustic potential of Indian stepwells, characterised by multiple visible concave right-angle corners, with on-site measurements conducted in Bundi (Rajasthan) by Dr. Manuj Yadav in 2023.

Interior viee of the Wilkinson Building entrance, featuring an angular, metallic installation suspended from the ceiling.

Retroreflective installation at the University of Sydney's Wilkinson Building

Further developments also include focusing acoustic retroreflectors by warping the surfaces of retroreflectors to increase the sound concentration on the source.

A simple demonstration of this concept, in collaboration with the School's Lighting Lab, was installed over the Wilkinson Building entrance in late 2023.  Designed by Jonothan Holmes, Associate Professor Emrah Baki Ulas, Associate Professor Densil Cabrera, and Dr Wenye Hu, this installation represents the first publicly accessible realisation of focusing acoustic retroreflectors.

Future directions for sound at Sydney 

In late 2023, an ARC-funded research project commenced, led by Associate Professor Densil Cabrera, Associate Professor Dagmar Reinhardt, Dr. Shuai Lu, and Dr. Manuj Yadav. This project aims to further explore the use of acoustic retroreflection in architecture to optimise speech environments. Funding will support innovative developments, demonstrations, and human experiments related to retroreflective treatments.  

Other exciting work in this field includes PhD candidate Jonothan Holmes, who is exploring the design of new focusing retroreflector geometries, while Dr. Yoshimi Hasegawa is collaborating on applying focusing acoustic retroreflectors as passive echolocation beacons for individuals with visual impairments.

Leveraging focusing retroreflectors holds significant promise in enhancing everyday acoustic environments for communication, comfort, and wayfinding, and academics at Sydney are leading the way.

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