The caustic in the acoustics of historic interiors

In the paper, caustics are discussed as ordered forms of focus broadening occurring in concave mirrors. Presented are examples of caustics formed in natural conditions and application of the phenomenon in different fields of science and technology. Against this factual background, the possibility to observe caustics in rooms is pointed out. Special emphasis is put on large rooms of historic character, as such interiors frequently include acoustic mirrors in the form of arched vaults and concave walls. As a case study, the eighteenth-century Whispering Grottoes representing one of attractions of the Oliwa Park in Gdańsk was selected, where the phenomenon of forming a caustic was used intentionally to obtain the desired acoustic effect.