Helmholtz absorbers have been used for some time in built spaces. Look for a pattern of slats or holes like perfboard, often on the rear wall of the space. Lecture halls, museums, performing arts, and the like.
The author acknowledges present use and also gives intriguing historical examples from Vitruvius and and Da Vinci.
An acoustic jar, also known by the Greek name echea (ηχεία, literally echoers), or sounding vases, are ceramic vessels found set into the walls, ceilings, and sometimes floors, of medieval churches. They are believed to have been intended to improve the sound of singing, and to have been inspired by the theories of Vitruvius.
https://www.youtube.com/watch?v=y9-p4AkgVU8&t=407s