  Sound measurements on a sequence of related, similar constructions with slightly different dimensions confirm a simple picture of the air modes of the internal resonator banjo's body. For the purpose of this study, the air modes are decoupled from the soundboard (i.e., [drum] head) modes by replacing the head with 3/4" plywood. The resulting characteristic features survive the strong coupling of the air modes to the head and are in accord with the qualitative distinctions recognized by banjo players. 
Air modes of the Bacon internal resonator banjo