Co-designing urban soundscapes with citizens and experts: Evidence from a field experiment

The growing role of expert input in participatory environmental planning and design has raised concerns about citizen voice. However, there remains limited causal evidence on how expert input, combined with public involvement, influences deliberation and trust. We address this democratic tension through a citizen–driven soundscape co-design experiment embedded in a soundscape investigation in central London. In this work, participants were presented with a range of aural–visual scenarios, including emerging acoustic–metamaterial (AMM) and acoustic–wall interventions for a public space, and collaboratively generated 144 alternative soundscape proposals. We evaluated these proposals using the Soundscape Perception Index (SPI) against ‘Vibrant’ and ‘Calm’ targets to assess how group deliberation and expert input affect design performance. We found that the designs created by citizens in groups consistently outperformed those developed individually, highlighting the value of group deliberation in participatory soundscape design. Structured expert input further improved group outcomes, particularly for the more challenging ‘Calm’ target, while showing minimal influence on individual designs. These findings suggest that expert contributions are most effective when embedded in a partnership model that promotes citizen agency. Building on these insights, we propose an SPI-based, engineering-informed evaluation framework for participatory soundscape design that makes the effects of participatory process conditions on design outcomes measurable and comparable while transparently integrating expertise with citizen deliberation.