Tremulant

[3] The tremulant should not be confused with the celeste, which consists of two distinct ranks of pipes, one tuned slightly sharp or flat from the other, producing an undulating effect when they are used together.

The simplest kind of tremulant is a weighted electric motor affixed to the top of the reservoir for the division.

The tremblant doux (French: "gentle tremulant") was illustrated by Dom Bédos de Celles in his monumental L'art du facteur d'orgues.

The spring allows the flap to rebound repeatedly back into the wind supply, which creates the undulation.

The Austin tremulant consists of a large blade that spans the length of the windchest above the pipes.