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
https://audioxpress.com/assets/upload/files/kolbrek2885.pdf
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Horns act as acoustic impedance matching transformators
transformating high pressure low speed to low pressure high speed sound waves or vice versa.
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Note that the sizes that are printable on common personal 3D printers are only suitable for rather high frequencies (even for whistles).
Well as it turns out low frequencies get amplified as well.
See: http://www.thingiverse.com/thing:389700
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Example horn