(for now keep crop factor >0.9 in customizer - for reason see below) . 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 http://audioxpress.com/ax-supplementary-material/ under: Horn Theory: An Introduction, Part 1 http://www.audioxpress.com/files/2008/03/kolbrek2884.pdf and: Horn Theory: An Introduction, Part 2 http://www.audioxpress.com/files/2008/04/kolbrek2885.pdf Horns act as acoustic impedance matching transformators transformating high pressure low speed to low pressure high speed sound waves or vice versa. . 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 . Example horn: cutoff frequency: 4500Hz inlet inner diameter: 20mm inlet outer diameter: 24mm inlet pipe length: 5mm horn length cropped to: 80% cropped horn mouth diameter: 126.5mm complete horn length: 129.9mm tractrix segment length: 124.9mm . known problem: customizer can't deal with this file :( found out: crop factor above 0.9000 works