Nowadays, those who administer churches and cathedrals tend to use electronic amplifiers to enhance the sound quality of their choirs and musicians. But long before electronics – back in the 12th century in fact – the caveau phonocamptique constructed at Noyon Cathedral in France, successfully resolved the same problem.
Builders constructed a series of vaults, especially designed (it seems) to physically amplify various notes (presumably sung by choirs) using passive reverberation. Tuned ‘jars’ built into the cathedral walls and basement, had holes which allowed sound to enter, reverberate, and return to the main space of the cathedral.
There have been several extensive investigations of the caveau, authored mostly in French, but there is also a recent paper by professor Andrew Tallon, of Vassar College, New York, presented in English :
“The caveau phonocamptique, true to the name coined for it by Moët de la Forte-Maison, appears to have been intended as a monumental sound amplifier. It was conceived in a such a way that the vocal production of a cleric or group of singers, situated perhaps at the center of the crossing, could have entered the chamber through the aperture in the floor, been treated by the array of vases, and been returned by the same route. It may well have been intended as a clever scheme — if an illusory one — to increase choral output without having to engage a greater number of cantors and choristers, at significant cost.”
See: Acoustics at the Intersection of Architecture and Music: The Caveau Phonocamptique of Noyon Cathedral, in the Journal of the Society of Architectural Historians, 75, no. 3 (September 2016).
The paper can be read here in full.