Air modes of the Bacon internal resonator banjo
  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. 
David Politzer
physics.pop-ph