The View Up Here

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

(Yet More) Microphone Tests

Posted by Tom Benedict on 19/12/2016

Since writing my last post Homero Leal pointed out that I could mitigate some of the harshness of the Alice microphone (for field recording, mind you) by adding a capacitor across the 2.2k drain resistor. I didn’t have the size I needed (6.8nF), but I had everything to make a similar modification by adding an 8.2nF capacitor in series with a 750 ohm resistor, both across the 2.2k drain resistor. All of this is spelled out in Ricardo Lee’s ChinaMod+U87.doc file on the micbuilder forum.

Alice with ChinaMod U87

I walked out into the park behind my work, recorded for about five minutes, and headed back inside to modify the microphone. After adding the HF EQ mod I walked back out into the park and recorded again.

Prior to the mod my recording had a background hiss that sounded like microphone self-noise. I knew from testing the mic inside my car that it’s not, and is actually a sound from the environment. While testing the mic with a 22″ parabolic dish a couple of weeks ago I panned around to try to identify the source. I’m almost certain the hiss comes from the sounds of tree leaves rattling against each other in the wind. It only takes a breath of wind to make the leaves rattle, so the sound is almost always there. After the mod, that background hiss was reduced quite a bit. Enough so that I wanted to try it more rigorously out in the field.

Last night conditions were almost perfect. We had a storm system rolling in, the air was still, and the sky was overcast. Perfect conditions for people to stay home, get off the road, and let people like me lurk in the shadows with headphones on. I packed both my Alice microphones along with my SASS and Olson Wing, and headed out to an old cane haul road to record coqui frogs and insects. I was rained out in the end, but even that worked to my favor.

The tests!

Alice with HF EQ vs. Stock Alice

This is an A-B test between the Alice with Ricardo Lee’s HF EQ mod (thanks for the pointer, Homero!) and an unmodified Alice. The mics alternate every ten seconds, with a two-second cross-fade. That’s probably excessive on the cross-fade, but c’est la vie. Keep in mind there was very little wind during the test, so the difference is subtle. But it’s there.

SASS vs Olson Wing

While I was there I also tested the SASS against the Olson Wing. In this case both were populated with Primo EM-172 capsules. After I got home I realized I had wind protection on the SASS, but none on the Olson Wing. So this isn’t a fair test of frequency response, but it should be a fair test of the depth of stereo imaging, and to some degree, sound localization. (The frogs really don’t move around that much.)

I was content to let this setup run for a while, but it started to rain. Without any rain protection on either array, I knew the rain would eventually soak the mics. So I packed it all in and pulled out my rain gear.

Rain Gear

I’m still trying to get a good, clean recording of rain. A while back I took a tip from Gordon Hempton and built a microphone rain shelter. It’s a hard aluminum plate covered with two inches of non-woven air filter material. The aluminum plate keeps the mics dry, and the filter material diffuses the rain drops to a soft “fuff” sound. I also added a layer of carpet foam underneath to cut down on the residual “fuff” sound. It’s set up to take my DIY shock isolator, a small ball head, and my ORTF bar. (Sorry, no pictures of the whole setup just yet.) With the whole mess set up on a tripod or c-stand, it protects the mics from rain while minimizing the sound of the drops hitting the rig.

Finally finally I had a chance to use it in the field. And it worked! It worked great!

Only problem is that I managed to damage one of my EM-184 cardioids while testing the Alice mics. It barely responded at all, and produced a deep wumping noise in the recording instead. So the stereo recording is rubbish, unfortunately. I thought the wump sound was the mic picking up rain drops hitting the tripod legs, so I switched to a c-stand, re-arranged, tied up cables, did all sorts of things. None of it helped. After about half an hour I finally admitted to myself that the mic wasn’t working, and packed it all in.

But the rain gear worked! It worked great!

And once I dried the EM-184 mics out they worked great again, too. (Lesson learned:Don’t let it rain on your mics. DOH!)

All in all it was a good night of testing. I have one other test I’d like to do with the two Alice mics (ocean waves!), and I’d like to do one more side-by-side of the SASS and the Olson Wing to see if I can shorten the length of the Olson Wing and still get a good boundary effect out of it. But I’m pleased as punch with the rain gear.

Tom

P.S. I also learned that I need to finish this project before watching another season of Stranger Things. There’s something about driving way the hell out on some abandoned road to some spot in the woods in the middle of nowhere with fog and rain and nothing but the buzz of the insects and the calls of the frogs to… WHAT THE HELL WAS THAT?!

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8 Responses to “(Yet More) Microphone Tests”

  1. Michael said

    Hello,

    I am trying to do a similar alice mod but am fairly new to the mic building world I have looked at the chinamod+u87 pdf but am a little confused. When you say that you put the “8.2nF capacitor in series with a 750 ohm resistor, both across the 2.2k drain resistor” does that just mean that you have soldered them both to this resistor? or that they were something else entirely? any help would be appreciated,

    Thank you

    • Tom Benedict said

      That’s it. I soldered the 8.2nF and 750 ohm together in series, clipped the leads to length, and soldered that across the 2.2k drain resistor as if it was a single component.

      I can take a picture, if you like, and add it to the article. Let me know if that would help.

      • Michael said

        Thank you for the reply Tom, A picture would be incredibly helpful I am a very visual person. Also you have a great blog and I have enjoyed reading many of the posts and listening to your wonderful recordings.

      • Tom Benedict said

        Thanks!

        I’ll photograph the mic when I get home and add it to the post.

        Yeah, I’m a visual and hands-on learner. I love words. I read constantly. But pictures answer a lot of questions. More than happy to add one.

      • Michael said

        Thanks again, I think an Alice might be my next project. I am currently awaiting some 158’s and 172’s that I recently purchased from Frogloggers and I am interested to hear how those compliment the soundscape up here in Maine.

  2. […] Back when I built my first Alice microphone I intended to use it for field recording. I took it out numerous times, but compared to other microphones I already had it always seemed noisy. Some testing indicated it wasn’t noise, just over-sensitivity to the higher frequencies generated by even the slightest wind in trees. Homero Leal pointed out that Ricardo Lee had documented an EQ modification to the Alice circuit on the Yahoo! micbuilders forum that could tame some of the high end sensitivity, so shortly after that I modified both of my Alice mics. […]

  3. Nick Snowdon said

    Tom, with the Olson Wing / EM-172 that you used, how big a wing did you make? I appreciate the width needs to be reasonably standard but what about the depth / height considering the capsules are small? Note that I am just starting out so please forgive a newby question.

    • Tom Benedict said

      I’m probably the wrong person to ask, but I can describe what I did. I based the dimensions of mine entirely off of the information on Curt Olson’s site and Vicki Powys’s site. Here’s the URL on Curt’s where his wing design shows up: http://www.trackseventeen.com/mic_rigs.html

      Vicki’s site seems to be down at the moment, but you can reach her page on the Wayback Machine here: https://web.archive.org/web/20161019193807/http://www.caperteebirder.com/index.php?p=1_17_recording-gear-1

      If I remember right mine had 1.5″ wide and 1.5″ high wings, spaced 5.5″ apart and 7″ deep. I think I had a 1.25″ set-back from the front edge of the array to the diaphragm of the EM-172 mics. I tipped mine forward by 30 degrees from horizontal, as Curt suggested on his page.

      And no worries about just starting out. I’m perpetually on the steep end of one learning curve or another. I’m happy to help out if I can.

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