Exponential Horn

For an outdoor recording project I wanted a small offset exponential horn that transitioned from round to square. The round end connects 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.

You may notice it is not axially symmetric: the small round end is offset from the centerline, so I can mount it horizontal 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.

Initial testing shows this horn is highly directional in the 1 kHz to 5 kHz range, with maybe +18 dB gain (rough guess 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, of course it rolls off to no relative gain.

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. The interior did have a bunch of stringy threads to clean up, although I doubt they affect the sound. In fact I later added some wispy material (like pillow stuffing, or fake spiderweb Halloween decor) inside the horn to reduce wind noise. 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. 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, but maybe it's close enough for the OpenSCAD file to be adaptable to your 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. This exponential horn was key to letting the sound in while the wispy stuffing inside slowed air motion, allowing a temperature and relative humidity gradient along the interior length of the horn to keep moisture away from the electret.

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.

UPDATE3: I added two shorter, curved versions, that bend over. "Exp-Horn-Curve-975" has a 9.75 mm (ID) round opening at the top, and a square base 55 mm on a side. Those values can be adjusted in the SCAD file. "ExpHorn975-pocket" is the same curved horn on the outside, but tapers to a smaller 3.46 mm ID throat, and the 9.75 mm ID at the top is a shallow pocket, matching one specific microphone capsule.

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