Where
c: velocity of sound (=340 m/sec)
S: cross-sectional area of the narrowest sound passage (throat or mouth)
V: volume of air chamber = box inside volume - (horn volume + volume of reinforcements)
L: maximum length of spiral
L = nLi + Lt/(n + 1) : this is an approximation.
n : rotation number of a spiral sound passage (= rotation number of partition plate or fins - 1)
Li: inside diameter of a tube or peripheral length of a cabinet
Lt: length of a spiral casing