Acoustic jar

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 the Roman writer Vitruvius, who described their use in Greek and Roman theatres. No examples from the ancient world have survived, but examples from the Middle Ages are found in about 200 churches, about half of them in France.