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Archive for December, 2017

DIY Stereo Parabolic Microphone

Posted by Tom Benedict on 21/12/2017

I’ve had a parabolic dish I bought off Ebay sitting around for the better part of a year. The whole idea was to build a parabolic mic out of it based on the dishes from Telinga, but for one reason or another I got stuck in the design-and-abandon phase and never got to the just-cut-metal phase. So the dish sat, collected dust, and didn’t do much except confuse my cats and annoy my family.

At some point in there I picked up eight EM-172 capsules in two matched sets of four from FEL Communications aka Mic Booster along with a bunch of yoga blocks. My plan with these was to make a pair of quad capsule foam SASS arrays similar to Vicki Powys’s for recording surround sound. I got stuck in design-and-abandon on this project, too.

In the end, though, it all worked out for the best because of yet another project. Only, this one actually reached completion, mostly because I was doing it for someone else. (There’s nothing like a deadline for getting a designer off top dead center.) The project involved designing and 3D printing some mating threaded parts. I modeled the threads in CAD, put the clearance into the model, and printed the parts. The threads meshed first try. W00t!

That was reason enough to celebrate, but it was also reason to scratch my chin and go, “Hmmm!” Being able to 3D print threads solved one of my issues with the parabolic mic: how to attach the handle to the dish. While reading this article from Avesrares on a DIY parabolic dish I had my second epiphany: Their stereo parabolic mic used four matched EM-172 capsules… just like the ones I had sitting in an envelope. Thank goodness I hadn’t built those SASS arrays, either!

So with technique and components in hand, I set about making my own stereo parabolic mic.

The Dish

One difference between my DIY parabolic mic project and many others is that I made no attempt to fabricate my own dish. There’s an Ebay seller who makes 22″ Lexan dishes for under $50USD. Shipping to Hawaii was expensive, but the dish itself was pretty affordable. This was where the whole project started.

The Handle

I like how the Telinga handles attach to their mics. The Avesrares article mentioned using auxiliary drill handles in a similar fashion, so I searched Ebay and found one of a suitable size for under $10US. The next step was to model a corresponding threaded tube and two screw plates to attach the handle to the dish.

DIY Parabolic - Clamp Tube and Collars Render

I wanted to be able to build interchangeable microphones, so I sized the inside diameter to take 3/4″ schedule 40 PVC pipe. The screw plates are 4″ in diameter – about the maximum my 3D printer can handle. The screw pitch is 14 TPI (sorry, metric system), and was sized to fit the outside diameter of the tube. If I remember right the threads left 10% flats top and bottom, and I left 0.020″ clearance between the tube and the screw plates. (They meshed great, too. W00t!)

Since these are sandwiching a parabolic form, the inner surfaces of the screw plates are curved to match. I added a layer of craft foam inside and out in an effort to provide some damping for handling noise (which it didn’t) and to make a nice snug fit (which it did.) Everything went together fine.

DIY Parabolic - Clamp Tube and Collars

The Microphone

In an article written by Klas Strandberg from Telinga Microphones, he explains that the focus of an acoustic paraboloid isn’t a precise affair. The mathematical focus of the paraboloid will be the point of best focus, but the size of the region of focus is frequency-dependent. I’d try to paraphrase here, but the article puts it so well there’s really no point. If you’re interested, read his article.

The upshot is that you can, to some degree, adjust the EQ of a parabolic mic by shifting where the capsules are with respect to the point of best focus. I knew I wanted some latitude, but I didn’t know how much range I might need. So I cut the 3/4″ PVC pipe a little long, reasoning I could always cut it down later.

One bonus to using 3/4″ PVC pipe is that Neutrik XLR connectors are a nice snug press fit into the tube. Since I was planning to make a stereo microphone, and since I’d already built an XLR-5 to twin XLR-3 splitter cable for my MS Alice microphone, I used an XLR-5 connector for this mic as well. While not the most elegant setup for this connector, it was an expedient solution that let me get on with the business of building the rest of the microphone. I can always revisit this later.

(As a quick aside, “I can revisit this later” is designer code for “You’re going to see this in the final version.”)

DIY Parabolic - Handle, XLR5 Connector, and Reflex Sight

My first design for the business end of the microphone followed the logic in the Avesrares article: a baffle that splits the dish left-from-right and a second baffle facing the dish. Unfortunately my dish has a shorter focal length than its depth, so adding the second baffle would’ve cut off about half the surface area of the dish.

Instead I went with a baffle similar to the one used on the Telinga microphones, except that instead of using the microphone capsules as pressure zone transducers (PZT), I stuck to familiar territory and arranged them as boundary mics. I’m not 100% sure this was an ideal approach since it moves the diaphragms of the capsules away from the axis of the dish, limiting how close they can get to the point of best focus in the radial direction. Still, it worked.

DIY Parabolic - The Giant Lollipop - Render

Originally I’d intended to make solid baffle plates on the 3D printer, but these would’ve been costly in terms of material and time. Instead I took a tip from the SASS I built and made them only 1mm thick. This made for a very thin, unfortunately flexible plate with a high resonant frequency. I backed it with Dynamat, which does an excellent job of damping vibrations, and sandwiched dense latex foam between the two plates to provide further damping. The whole stack wound up being acoustically dead to handling noise. (But more on the handling noise later.)

Unfortunately, before I took any pictures of the finished parts I stuck open celled foam on either plate to provide wind protection. So the only photos I have look like a big foam lollipop.

DIY Parabolic - The Giant Lollipop

I wired the capsules using the same pattern as Vicki Powys’s SASS, except instead of wiring them into a 3.5mm TRS plug, I wired them into the XLR-5 connector using David McGriffy’s and Ricardo Lee’s Simple P48 circuit. Tuning the resistor for the Simple P48 circuit took some experimentation since each circuit was now driving two capsules instead of one. I’d quote the value of the resistors I wound up using, but there’s really no point. If you choose to build one of these, best to tune the resistors to get the bias voltage you want for your capsules.

With everything closed up, the microphone slides into the clamp tube on the dish and locks into place by twisting the drill handle. Voila, the DIY stereo parabolic microphone.

DIY Parabolic - Finished and In the Field

The Bling:

With such a narrow pattern microphone, I erroneously thought I’d need some way to aim it. (I’ve since used it in the field enough to know you aim these things by ear and not by eye.) One of the remnants of a previous project of mine, the camera helmet, was the small reflex sight I’d used for aiming the cameras. I mounted a short length of Weaver rail to the drill handle using a 3D printed adapter, and clamped the reflex sight to the rail. Voila, a neat looking, but ultimately useless addition to the parabolic mic.

This was rendered even more useless by the next bit of bling.

DIY Parabolic - Useless Reflex Sight
Wind Protection:

After some initial testing in the field, I found I wanted the option of another layer of wind protection. The foam on the lollipop was nice, but it wasn’t really enough to stop the kind of wind you get outdoors. (Foam is never enough to stop real wind. Like ever. Eventually I will learn.)

DIY Parabolic - Wind Protection

Chris Owens wrote a really good article describing the wind protection he made for his Telinga parabolic mic. I followed his directions to the letter and wound up with a very serviceable (and machine-washable!) cover for my mic.

DIY Parabolic - Wind Protection Detail

He used his dish to measure a circle of cloth, and rolled a hem at the edge that holds a length of shock cord. Once the hem is finished, the shock cord is pulled tight(ish) and is tied off. Chris Owens cautioned not to pull the shock cord too tight or the dish will warp. I made mine just snug enough to make sure the cloth stays on unless I’m ready to pull it off.

 

Post Script:

Now that I’ve had the chance to use my dish in the field I’ve found a number of shortcomings I’m planning to address in the future:

  • Anything you can’t put a shoulder strap on is lame – My dish is incredibly awkward to carry on a trail. Klas at Telinga solved this by making 1mm thick dishes that can be rolled up and carried in a shoulder bag. I’m sticking with my dish, but I have to add a shoulder strap. This is irritating.
  • Handles that can twist bother me – The drill handle I chose tightens (and loosens!) by twisting the handle. I’ve had it try to come undone in the field a couple of times when turning the dish. I’m planning to replace mine with a Bosch handle that uses a thumbscrew.
  • Handling noise is lame – I’ve been chasing handling noise on the BM-*00 conversion mics I’ve been building with some success, but this thing has it twice as bad. You can’t just shock-isolate the capsules. You have to shock-isolate the dish as well because it basically acts like a great big sounding board. Anything that’ll conduct handling noise to the dish shows up in the recording. A cushioned handle like the ones on the Telinga dishes would be a step in the right direction. For now I’m wearing gloves any time I use it.
  • I’m not 100% happy with the mic setup – The first prototype mock-up I did with this dish involved mounting my Alice M-S at the focal point of the dish. This worked way better than I suspected, and provided a better stereo image and a better mono image than the mic I’ve got in there right now. I’m thinking of recovering my four EM-172 capsules so I can go ahead and build that SASS surround mic, and building a dedicated mid-side mic to live in the dish.

 

And now for the rest…

First, A Sob Story:

I like to include audio samples in my build articles. Unfortunately I can’t for this one. Not yet, anyway. A little over a week ago I got some kind of outer ear infection in both ears. The tinnitus I’ve experienced for the last twelve years or so is about a bazillion times worse at the moment, and my right ear has lost about half its frequency range. I tried to edit a recording I’d made of a katydid and realized I couldn’t even hear the katydid in that ear. (And it’s not like they’re subtle or anything.) Even worse, my two ears currently hear broad-spectrum noise as two completely different sounds. Until this infection clears up, my ears are screwed. I can record, but I can’t edit. Sorry.

Second, A Note on the Photos and a Thank You:

No, that’s not me holding the mic in the field. I was the one tripping the shutter. (I know which end of the camera I like to stay on.)

About half the pictures in this set involve wet gear. This is one of the hazards of recording in Hawaii: it rains. Rain is not ideal for recording with a parabolic mic. Each rain drop on the dish was picked up by the mic, and there were plenty of rain drops. I did make some recordings during the photo session, but because of the rain none of them really showed off the mic very well. Everything survived fine, though.

The black audio bag worn by the recordist in the photos is the brainchild of Andrew Jones, described in an article he wrote on WAV.REPORT, Tutorial: “DIY” Audio Drop Bag. Mine’s only about half finished (it still zips shut) but it’s a really good article and an unbeatable bag for the price. Thanks, Andrew!

Third, An Apology:

Normally at the end of my build articles I like to link to all the necessary bits and pieces, including any parts I designed. I’m not 100% comfortable doing that for this project because it’s still such a work in progress and there’s so much work still to be done to make it what I want it to be. But if you’re dead set on building one of these, feel free to contact me.

 

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Useful Little Clampy Things

Posted by Tom Benedict on 20/12/2017

A while back I needed the ability to put a 1/4″-20 threaded stud… Somewhere. Anywhere. Everywhere. I tried some things on the 3D printer, but realized none of them would be as robust as I needed these to be. So…

I turned to Ebay!

A Useful Clamp

Turns out you can get insanely cheap clamps by searching on “super clamp”. Some come with T-bar handles, some with offset handles like this one. Some come with articulated arms (which I used for mounting cameras on our vacuum chamber for work). And some just come with 1/4″-20 and 3/8″-16 threaded holes. Mine came with threaded holes.

I added the 1/4″-20 threaded stud by chopping the heads off some bolts and cleaning them up with a grinder and a die. The thumb screws came off of a camera hotshoe to 1/4″-20 adapter (also courtesy of Ebay.)

All in all these things came together for less than $6 apiece.

A Useful Clamp with Shock Mount

My original purpose for these was to be able to space mics anywhere I wanted across a bar for trying different stereo recording techniques. To do that I needed to be able to mount any of my self-contained stereo setups, all of which take a 1/4″-20 thread, or to mount individual shock mounts like this one.

A Useful Clamp with Ballhead

But there’s no reason to stop there. Pop a small ballhead on one of these and you can mount a camera, a handheld sound recorder, an off-camera strobe, or whatever else strikes your fancy.

The only problem I’ve found with these is that the handles like to come off, leaving the clamp firmly attached to whatever you cranked it onto. If you decide to get any of these, be sure to test them first, and if any handles come off, re-install them using red or green Loctite. (I had to re-install three of mine!)

I wound up building five of these, but now I wish I’d ordered more.

P.S. Since writing this, I’ve been told this is all pretty standard fare for lighting and grip gear on a film set, and that re-purposing lighting gear is a cheap and easy way to build solid recording setups. My take-away: 1 – There’s nothing new under the sun with this article. 2 – My butt’s gonna go numb the next time I sit down with the B&H catalog or start surfing their web site! 3 – Neither of these is a bad thing.

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Clean Clean Clean!

Posted by Tom Benedict on 13/12/2017

I’ve spent the past year or so at work helping prepare for the arrival of a new instrument. One of the requirements is for the room to meet ISO 8 standards with a goal to reach ISO 7.

The reason for the requirement is that the instrument is one big cryostat – about 2m in diameter and about 3m long – and will be bolted to the floor. Once it’s installed, the only way to service it is to open it up wherever it is. It can’t go in the clean room, so we brought the clean room to it.

ISO 8 roughly translates to Class 100,000. It allows for 29,300 >=5µm particles per cubic meter, 832,000 >=1µm, and 3,520,000 >=0.5µm particles. ISO 7 , our goal, knocks that down by a factor of ten (2,9300 >=5µm, 83,200 >=1µm, 352,000 >=0.5µm, Class 10,000).

 

Coude Room Particle Counts 2017-12-13

As of this week we’re hitting under 175,000 for 0.5µm, 75,000 for 1µm, and under 3,500 for 5µm with upwards of ten people in the room, two of them installing weather stripping on the doors. That’s well inside ISO 7 with a shot at ISO 6 once we finish all the work, provided we can demonstrate counts at 0.3µm and smaller. (That instrument is in use in another room at the moment.)

When no one is in the room, the particle counter doesn’t register any particles at all. W00t!!

Just a happy point in a long work schedule. Details of the work on the room will be presented in a paper at the SPIE Astronomical Telescopes + Instrumentation conference in Austin, Texas in June, 2018. The instrument itself will show up at the end of January, 2018, so we’ll have plenty of time to put the room through its paces during integration and commissioning.

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