Acoustics in Roman buildings, and later in European church buildings, got help from acoustic pottery that was stuck into the walls during building. Churches seem to have had them positioned in the walls or windows of the choir (and sometimes the nave), because "they made the chanting resound more strongly." There were often holes in the walls that went right into the mouths of pottery vessels. The vessels would spread out behind the wall. Sometimes the vessels had ashes in them to absorb sound.

There are two old articles about this at Lacus Curtius: "Acoustic Pottery" by Abbe Cochet, and "Acoustic Vases" by Robert Rolt Brash. Elfinspell has a later article on "Acoustic Jars" that is part of a book. It opines that if you find horse skulls in the walls, those aren't pagan survivals but substitutes for crockery.

Here is a new article from 2007 that deals mostly with Greek churches using them, and where. It questions whether acoustic vases are resonators or what. They find that they were useful in some churches, particularly in areas like domes and corners. However, most churches didn't have enough of them in the right places, and they were probably being put in by guess.