Tractrix Horn Generator

(for now keep crop factor >0.9 in customizer - for reason see below)
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This is a generator for horns with tractrix crossection.
Those are not the same as spherical wave horns. See here:
https://www.grc.com/acoustics/an-introduction-to-horn-theory.pdf
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https://audioxpress.com/page/audioXpress-Supplementary-Material
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under: Horn Theory: An Introduction, Part 1
https://audioxpress.com/assets/upload/files/kolbrek2884.pdf
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and: Horn Theory: An Introduction, Part 2