An abat-son is an architectural device constructed to reflect sound in a particular direction. Typically it takes the form of large louver

Louver

A louver or louvre , from the French l'ouvert; "the open one") is a window, blind or shutter with horizontal slats that are angled to admit light and air, but to keep out rain, direct sunshine, and noise...

s which direct the sound of church bell

Church bell

A church bell is a bell which is rung in a church either to signify the hour or the time for worshippers to go to church, perhaps to attend a wedding, funeral, or other service...

s from a bell tower

Bell tower

A bell tower is a tower which contains one or more bells, or which is designed to hold bells, even if it has none. In the European tradition, such a tower most commonly serves as part of a church and contains church bells. When attached to a city hall or other civic building, especially in...

toward the ground.

In general use, any louvers in a bell tower are commonly called abat-sons, whether they are designed to redirect sound or merely to prevent ingress of water.