musical thoughts

Here’s a brief description of my homemade Jecklin disc and how I use it.

A Jecklin disc is an arrangement that holds two matched omnidirectional mics in a particular arrangement that gives a good stereo image. It is pretty much a 1-foot disc with absorptive padding that “casts a shadow” sonically in a way that corresponds to a human head. Below a certain frequency, it doesn’t localize, but above that frequency it does, just like a pair of ears. The mic elements are spaced in accordance with the spacing of a pair of ears, and overall the fairly simple arrangement does a remarkably good job of capturing a stereo field.

You can read the original paper by Jürg Jecklin at Josephson Engineering, it’s very interesting. And it really works well.

I constructed my Jecklin disc from 1/2″ MDF (from my scrap bin) by cutting a 1 foot circle on my bandsaw with the help of a circle cutting jig. I applied some 1 inch foam (the type that would be used in cushions) from a fabric store. Using 3M Super 77 adhesive, I glued the foam to both sides of the MDF disc. I made a bracket from a scrap of mahogany with a 1/2″ slot routed into it that holds the disc upright, and I attached a mic mounting flange to the bottom of that bracket so that I can mount the disc on a mic stand. There are two flanges mounted at the back edge of the disc that hold the mic clips. I had to buy the flanges online at Parts Express. Mic clips (included with my ECM-8000s) attach to the flanges on the disc body, and that lets me position the tips of the mics at about the center of the disc, and set the spacing to the 6-1/2 inches that Jecklin recommends.

ECM-8000 mics are sold as reference or instrumentation mics. They are omni condensers that are made to capture a room sound for an analyzer, so they have a very flat response. I bought them because I wanted some instrumentation mics like that, and only afterward figured out that they’d work well (being omni) in a Jecklin array. I got the mics on ebay (new) for about 40 bucks per. I’ve heard them referred to as having high self noise, but for my application they work really wel and sound great. I doubt I’d be happier with more expensive mics at least. My Edirol box has phantom power which is a necessity with condenser mics.

I set up the array on a stand positioned in the crook of the piano and with the axis pointed right at middle C (thanks Tom for the placement advice, it works great). I get a very nice balance of room and piano sound this way. I’d like to get a boom arm and experiment with a placment closer to the soundboard.

This shows the placement as described above. I’d like to try getting it in closer to the soundboard, but I need a boom arm to do it. Plus a counterweight, because the thing is heavy.

The view from the back shows the mic orientation pretty well. And you can see the attachement of the flanges to the disc body at the back position. The clips are standard.

Another view showing the ugly foam and mediocre trimming job I did. I could make a little fabric sock for the thing to tart it up but it wouldn’t change the sound so I skipped it. Presumably this is why a commercial one would set you back 280 bucks instead of 10.

The mic spacing is pretty much like a pair of ears. The tips wind up centered in the disc.

Looking at pics of other DIY Jecklin discs seen on the intertubes, I think mine is more massive than most thanks to being made of MDF. Since the design goal of the disc component is to absorb sound, I think that’s a plus for performance. I entertained the idea of using 3/4″ MDF but the thing is really heavy as it is. I can’t put it on a gooseneck mount.