The horn is designed as a pure plane wave exponential horn matching an Altec 288B compression driver. This is a simple version of the Reciprocal Flare geometry described by Abraham Cohen, using conical expansion in one plane. The horizontal and vertical expansion is shown to the left. To test this, and also to gain more experience in how horns work and how things are in practice, I started to build a test horn. Real wavefronts in horns are neither plane nor spherical, but something in between (,).Įxponential expansion of the wavefront is what we want, and we want the horn to be shaped in a way that allows this. With full size horns this is a closer approximation to how the horn works in practice, even if it is still not perfect. Tractrix type horns are calculated assuming spherical wavefronts, and considering this, they are an approximation to an exponential expansion of the wavefronts. So what we really have then is not an exponential horn, but something different. But when it comes to the midrange and full size mouths, the curving of the wavefronts is large, and the difference between the real and theoretical expansion is much greater. But bass horns are shortened at a point where the wavefront curvature is comparatively small, and thus the horn contour calculated for a plane wave is not very far from the real contour experienced by the travelling wavefronts. The larger the angle between the horn wall and the axis, the more the wavefront curves. These formulas are based on the assumption that the wave fronts in the horns are plane. Midrange horns are taken to the full mouth size, as they radiate in free space.Įxponential and hypex horns are usually calculated from the given formulas. Why is that? Let's look at the differences between the way horns are used in these two different ranges, and the difference between expo/hypex and tractrix/LeCléac'h.īass horns are usually shortened, depending on corner placement, wall-floor or wall placement, corner placement being the most common, as this gives the shortest horn. A 200Hz Exponential Midrange Horn for Altec 288BĪn exponential midrange horn? You must be kidding! Tractrix is what is used in the midrange, or else you get 'horn sound'!īruce Edgar has said, and it is common knowledge in the DIY community, that exponential and hypex horns work in the bass range, but tractrix and LeCléac'h horns work better in the midrange.
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