Exponential Horn

thingiverse

For a unique outdoor recording project I needed a small offset exponential horn that transitioned smoothly from round to square. The rounded end connects directly to a 9.7mm OD round electret mic (PUI Audio AOM-5024L-HD-R) and the other end matches a square opening in an enclosure perfectly. You may notice it's not axially symmetrical: the small round end is offset from the centerline, so I can mount it horizontally but with the mic element towards the top. That way any rain that might come in the open end would tend to run down and away from the mic naturally. Initial testing shows this horn is highly directional in the 1 kHz to 5 kHz range, with approximately +18 dB gain (a rough estimate of 20 degree beamwidth) relative to the same element used bare with no horn, based on measuring noise of crickets near 2 kHz. At low frequencies, it rolls off to no relative gain as expected. It may be a bit tricky to print due to the low, nearly horizontal slope on two sides near the base, but I managed it successfully. The interior had a bunch of stringy threads that needed cleaning up, although I doubt they affect the sound quality. In fact, I later added some wispy material (like pillow stuffing or fake spiderweb Halloween decor) inside the horn to reduce wind noise significantly. Here is a recording from it https://youtu.be/hPLfLMcTJE8 of two cars passing by on the road in opposite directions. Try listening in headphones for an immersive experience. You can hear them coming some way off, and some leaves rustling along the road also. Here is a night-time thunderstorm: https://www.youtube.com/watch?v=aBam2T9lZUk I'd be amazed if anyone else wanted exactly this design, but maybe it's close enough for the OpenSCAD file to be adaptable to your specific needs. UPDATE: It's been outdoors for over a year now, in a box just under the roof eaves. Electret mics generally fail in damp and condensing-moisture conditions like what we've had here, but it seems that the heat from the R-Pi CPU inside the box right next to the electrets has kept them dry and working fine. UPDATE2: Slightly improved geometry and fit to 9.7mm mic capsule added as Exp-Horn2. This one is symmetric by default, although the SCAD file does allow an offset like v1.