The View Up Here

Random scribblings about kites, photography, machining, and anything else

Finishing the DIY Microphones (v.1.1)

Posted by Tom Benedict on 14/07/2015

The more I thought about the hot glue closure on the back of the microphones, the less I liked it. Don’t get me wrong. It works well. But it’s… permanent. I know the BT-EM172 capsules are only $10, and I know the rest of the microphone is largely scrap-boxed, but I hate to make a thing that can’t be serviced when it needs it.

So I re-designed the enclosure to include an end-cap. It’s drilled out 1/4″ to take a cable grommet, and has three #2-56 screws placed every 120 degrees around the periphery to hold it in place.

BT-EM172 Microphone Enclosure - Exploded View

The end caps took about fifteen minutes apiece to make, and were a comfortable fit in the back of the microphone bodies I made previously. Unfortunately, drilling and counter-sinking the screw holes for the end cap meant I needed to re-coat the microphone bodies along with the end caps. Since I had to re-coat them, I added grooves to each mic body to accommodate a Shure RK183T1 lavalier clip. I’m pretty sure a generic clip for a 9/16″ diameter mic would’ve worked fine, but these turn out to be tough to find. There are several listed on Ebay, but if you look at the metric equivalents, the specs say they fit something around 7-9mm in diameter. 9/16″ is closer to 14mm, so I think something was lost in translation. The clips from Shure will fit. (For shure! Har!)

Countersinking Screw Holes

I wasn’t happy with my previous coating job, so I came up with another way to apply the coating. I shoved each part onto a wooden dowel of the appropriate diameter (3/8″ for the end caps, 1/2″ for the mic body), and chucked it in a drill. I applied the Cerakote with the drill spinning. This gave each part a very uniform coating, and let me hit every outside surface without running into my fixture. I loaded the parts in the oven, dowels and all. On a whim I coated the screw heads, too, so I wouldn’t have shiny stainless screws in a black microphone body. Unfortunately the spray gun malfunctioned, so two of the mic bodies didn’t turn out as nice as I’d like. I slated those for the pseudo-SASS array, where they won’t be seen, and saved the two “good” ones for lavalier mics. Note to self: test the spray gun before loading product into it!

Parts Ready to Cerakote

Once the Cerakote cured it should’ve been a simple matter of assembling each of the mics. But I love to fiddle. I assembled the two for the pseudo-SASS array since I already had that cable made. But I needed more cable for the lavalier mics. Even though I’m already using Mogami W3031 cable for the other mics, I ordered 100′ of Mogami W2697 from Redco Audio to use for the generic lavs (only 20′ of which I plan to use). W2697 is almost identical to W3031, except for the way the shield is constructed. W3031 uses a braided shield. W2697’s shield is served (wrapped). Electrically they’re identical. But a served shield is easier to work with when making cables. I’ll have to wait for the cable and clips to come in before finishing the generic lavs.

Completed Mic Bodies

Rather than waiting like I did with the mono mic I built out, I grabbed my pseudo-SASS array and my recorder, and hiked out to the rocks south of Hapuna Beach. The last time I was there the waves were big, and made big, dramatic crash-bam-booms on the rocks. Of course that was in the winter. The summer wave pattern is a lot more bathtub-like, so the sound was a lot more subtle. Still, I ran several side-by-side comparisons of the pseudo-SASS against the built-in mics on the Tascam DR-05. I put together a set of 30-second clips comparing the two. The recording has eight tracks, alternating between the DR-05 built-in mics and the BT-EM172 array, done at four locations. When listening, keep in mind that the gains are different on the two mics, as are the frequency responses. I did no processing on the tracks aside from cutting and fading, so some tracks are louder than others. That’s a function of my technique in the field (or lack thereof), not the microphones themselves. This test was only so I could tell how well the pseudo-SASS array was separating the two channels.

The pseudo-SASS performed well enough I want to build a real one out of some 1/4″ baltic birch plywood I have in the shop. I still haven’t tested my prototype from the air, but it’s easy enough to include 1/4″-20 sockets top and bottom so I can mount it either way. More photos and sound samples to come!

– Tom

P.S. I’m not keen on the way clips from Soundcloud show up on my web site. I’ve seen other people include Soundcloud clips on their sites that are nice, small, and easily worked with. This thing is ungainly! If you know how to fix this, please let me know.

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