the rate of expamsion determines the lowest frequency of the horn. One selects which part of that horn shape one needs. At the throat end move along until the throat macthes the Sd/Throat ratio required. Aty the mouth end move alomg until the throat area gives the correct area for the lowest frequency and space loading. then you have fixed the length. If it's too short it won't play right. If it's too long then choose a higher lowest frquency.
