Making a Condenser!

Ever wanted to see what's inside a condenser microphone? I rarely get a chance to take photos in the lab, but I got (hazily, but I'll take it as a yes. This is all unclassified anyway...) permission to show these. I've always wanted to make a condenser microphone, and since I had enough good reason, Dr. Gabrielson gave me the go-ahead to make an infrasonic condenser mic. One of the main reasons is, they are hard to find (sold as ultra-accurate barometers, not microphones), and buying one costs a fortune. $10k+. This won't have quite as low a cut-off as the barometers, and will have a much higher resonance, I'm guessing. But, it's a start.

I'm designing as I go here. I didn't start out with a pencil/paper and lots of dimensions, cause I wanted to be flexible with the design. But this was one of my hardest challenges yet. The dimensions have to be perfect, and are pushing the limitations of the accuracies of the equipment I'm making it from (The diaphragm has to be 1/1000th of an inch from the backplate!). Also, I'm using a 3-jaw chuck that is way out of alignment, for most of it...

But, if you want to see what I'm up to, half of my time, here goes. Making a condenser microphone! [I'm skipping a lot of steps, but you will get the point]

Step 1: - selecting the body
Find a suitable piece of tubing from the scrap pile (thick tubes are expensive!). The tube needs to be about half an inch thick, with an inner diameter of at least 2.5 inches. I found a suitable piece of aluminum.

Step 2 - Drill holes that need to be aligned with the axis of the body
Here the piece has been mounted in a [really great] clamp that allows you to spin the piece a fixed number of degrees. I'm boring the holes with the Bridgeport. The diaphragm clamp rings are going to be made out of the same piece as the body, so the drills for tightening and tensioning screws are being drilled.

Step 3 - Face the tube, bore the body on axis, and turn the sides
This is the lathe. You can see the 3 jawed chuck, and the end-bore is currently mounted. Everything needs to be on-axis, so I'm re-boring the tube's inner surface. It has already been faced, and turned (the front and outside have been already cut and polished).

[Don't stick your finger in the chuck!]

Step 4 - make the diaphragm clamp rings
Smoothing the edge with a file, after the appropriate inner diameter has been cut.

This is the parting tool. My old foe. This things break so easily. You can see this one is already broken, but I'm using it anyway (less to lose, I guess). I'm cutting off the top clamp ring here.

The top clamp ring is finished. There is a slight cut-away on the inside that will match a lip on the next ring. This will crimp the diaphragm in place.

You can see the lip, here. This will be made into the next clamp ring.

Cutting the next ring, with the parting tool. I need to catch it, which is why I have my hand there.

The finished camp ring, and the outer body of the capsule.

Step 5: Make the insulator, that will hold the back plate.
So... I'm using PVC here. This is not a good material for a back-plate insulator, if you want a stable sensitivity (thermal expansion...). You should use something like glass, or a ceramic of sorts. But I didn't have a bar of machinable ceramic on hand so... make do with what you got, you know.


Rests on a lip machined about half way down the body.

Step 6 - Make the back plate.
Found an aluminum scrap piece with a narrow neck. Great! I can use the collet instead of the 3-jaw chuck! It will be much easier to machine this piece. Made lots of these stringy aluminum worms...

The back of the back-plate. The collet is great, because you can take pieces out of the lathe and put them back in, and they are still on axis. In a really good mic, you probably don't want such a large backplate, because I'm going to have a large resulting capacitance, with the smaller distances to the body. Again: oh well.

The front of the backplate.

Backplate mounted in the insulator. The diaphragm will be stretched 1/1000'' above it, but this is still a long ways from complete. I'm just checking the alignment of the pieces.

View from the back

With the clamp rings added...

Now - to fix the backplate to the insulator, a clamp ring needs to be made.

Step 7 - make a clamp ring
Remove the collet

From the lathe...

Re-attach my old foe... the three jaw chuck!

Found just the piece...

The finished ring. Skipping a lot of steps here.

Step 8 - fixing the clamp ring to the insulator and backplate
Move back over to this cool, old school machine.

This is the insulator, sandwiched between the backplate and clamp ring. I'm drilling holes every 120 degrees, that will attach the clamp ring to the insulator, and to the backplate.

Almost done... the drill bits sticking up are to keep the plates in place

"Tapping" the holes. This allows me to put screws in the holes.

DISASTER!! A tap broke! I'm not used to using 4-40 taps, so I busted it. These are nearly impossible to get out, once they've been busted in a hole. Uh oh.

Skipping several busted tools and the 2 panicked hours between 12:00 am and 2:00 am when I am trying to salvage my backplate... (I 'solved' the problem by drilling in from the side, hammering out the piece, and covering over the scars by making it the air-leak between the diaphragm and the capsule compliance chamber...)

Tapping the insulator, so it will stay fixed to the capsule body. Set screws will go in these holes, and be forced outwards against the sides of the capsule. Yeah... again, not a good idea. But, it's 2am, I'm not making good decisions

Nearly complete capsule, and clamp rings.

Clamp rings installed. Diaphragm has not been inserted.

View from the back. I was intending on inserting that piece sticking up, (the back of the backplate) into the collet and turning the front to the appropriate distance below the edge of the capsule. But... turns out the insulator and backplate are terribly off axis from the capsule body and clamp rings. THIS CAPSULE IS A DUD.

But, it's actually about 4:30 in the morning at this point, and I can't work anymore, so I go home. I'll solve the problem next day.


NEXT DAY

I decide to completely redo the capsule. I grab a ladder, inspect the far reaches of the scrap pile, and find a nice piece of acrylic:
Acrylic is much nicer than PVC. It is harder. It as a slightly smaller thermal expansion coefficient, but it is still pretty bad. But, sleeping on it, my design strategy has been revised.

This is the piece machined from the Acrylic. Amazing what can be done with scrap, and a non-drowsy mind

The back side

Fits perfectly into the capsule. Fortunately the set-screws from the previous disaster didn't scratch up the insides too badly

The new backplate, that fits inside the insulator

A set screw in the back holds the backplate in the insulator.

Next, the vents are drilled into the backplate/insulator. These increase the compliance of the diaphragm.


Backside...

Skipping the creation of another plexiglass clamp ring, that fixes the insulator and backplate to the capsule body. But - it is done much better, this time!


All the pieces!


Now - the microphone capsule can be assembled.

The clamp ring is installed in the back. The set screws are not pressing against a critical piece of the tube this time... The black screws on the clamp rings were just to test the alignment. The diaphragm will be installed later.

Inserting the backplate and insulator (back view).

Now, the diaphragm can finally be made. The diaphragm is made from 1/4 mil aluminized mylar. The stuff is thin (0.00025'' thick). Really amazing stuff. Dr. Garrett let me have at his secret stash ;) .

Clamping the membrane between the rings. The tension on the diaphragm is still low. The tension will come from placing it over a ring on the insulator. This lip on the insulator is machined 1/1000 of an inch above the backplate.

The finished microphone capsule!

You can't measure sound with this yet. Next up, I need to design the preamplifier (with an ultra-high input impedance), and the back-volume of the microphone. You can tune a mechanical cut-off frequency by appropriately balancing the size of the chamber with the air leak out of the chamber. Two other low-frequency cutoffs are tuned by adjusting impedances.

More to come on this project later!