This fragment from "Ariadne Musica" by J C F Fischer illustrates the Diapason rank on the Estey.
Every Estey organ of any size had a "Diapason" rank. On single-manual instruments the name Diapason would be used only for the bass section (below Middle C) with the name Melodia applied to the treble section. In the Estey Style T this is a full-compass rank of 61 notes beginning two octaves below Middle C. This is the basic foundation rank of the American suction reed organ.
The reed tongues are of medium scale, with moderate degrees of twist and curvature, which are voicing features. Adding twist and curvature to the tip of the reed tongue suppresses upper harmonics, making the sound somewhat flutey. The scale, or width, of the tongue determines the amount of fundamental plus overall volume. A wider reed tongue pumps out a fuller sound, while a narrow tongue delivers a softer, thinner sound. The Diapason rank is in the middle of these extremes.
Here I've coupled the Diapason to the Pedal Dulciana 16, which uses identical reeds.
In the final bars the left hand delivers the last entry on the Swell Oboe in the tenor octave. This stop uses narrow scale reeds, which reduces the fundamental. The reed tongues have no twist or curvature, which maintains the rank's brilliance.
Where the Diapason rank is extremely common, the Oboe rank is relatively rare and provides a wonderful contrast with the Diapason. The sound of both ranks coupled will be illustrated in another example.