Wednesday, February 27, 2008

Non technical

Ok, I need a break from posting work-related stuff on my blog. So these are some recent occurrences and things I've been thinking about.

Today, while clearing off a pile of miscellaneous junk on my desk, I found a sealed bag that still contained crumbs left over from the chocolate cookies my older sister sent me. You can't possibly comprehend my sister's cookies unless you've actually tasted one of them. With trembling hands, I opened the bag and didn't allow a single one to go to waste [thanks Paula].

The first time I went to Hawaii and I was a little kid, just me and dad went hiking down the cinder cone of Haleakala. Dad tripped and fell and gashed his knee open really huge. To my 5 year old eyes it was like a mortal wound, and blood was every where. He put like five million band-aids on it but blood was still oozing in rivers. However, dad didn't cry or anything, he acted like it was nothing. I then realized my dad was the toughest guy in the world and solemnly resolved to be like this.

While in Japan, I realized one morning that one of my monthly insurance things was overdue. I grabbed the form, flew down the stairs, hopped on my bike, and headed for downtown. On the way I had a spectacular crash that cut my thumb open pretty good. Walking into the bank, I tried to hide my twitching hand, while doing the form stuff. The Japanese are incredibly good at ignoring any form of suffering in others, be it a begging cripple or a wailing child looking for her mama (this is not the high point of their culture). The lady behind the desk however, gave me a very maternal look, reached behind the desk, pulled out some band-aids, motioned towards my hand, and patched up my thumb. I wish I sent her flowers before I left.

People who criticize "contemporary" worship music have normally only sampled 5% of it before making their criticisms. I wouldn't be surprised if most of that 5% happens to be stuff purposely dredged from the bottom of the barrel to make a point. Any one of us could write a page of lines from modern worship songs that make us want to gag. However, I could write a lot more than a few pages, filled with lyrics of modern worship songs that are lifted straight from the psalms among other places. If you want to complain, however, it's never hard to find something to complain about.

For 5 minutes, after you cut good aluminum, it shines bright white. If you cut it right, the surface reflects faint rainbows. However, it smudges and dulls really fast when you take it off the lathe and start doing anything with it. Maybe that's why I spend so much time machining. I've always been hypnotized by really really... erm... pretty things.

I restarted my long-abandoned goal of writing a computer game based on the Dink-Smallwood scripting language. See how long it lasts this time...

Ok, it's 1:00AM here, better stop before I lose all coherence

Making a mic, part 3

So last time I posted pictures of the nearly finished preamp. Several modifications were made since those pictures were taken, namely, the addition of some offset adjustment circuitry, as well as the DC supply regulators. Because the board wasn't large enough, these components were attached to a second board, and stacked together with the first. Connected to the second was also the contact (a banana plug) that fits into a slot in the back of the back-plate.
Top side of the preamp. Kind of ugly, what with the wires going everywhere and resistors splaying out all oover. When I settle on a final design, I might get to make a printed circuit-board design, which should clean things up considerably.

Stacking the second board with the voltage regulators etc. beneath the first

Banana pug for fitting into the backplate. The top is sparsely populated.

Final, assembled microphone - the circuit board is inside.

The mirror like film on the front is actually a 6-micron thick sheet of mylar with aluminum deposited on it. It is very thin but very tough stuff.

Unfortunately, ambient pressure changes fairly fast, sometimes on the order of a pascal per minute. If you have a sealed canister, it does not take long for a very large (from an acoustic standpoint) pressure imbalance to form across the diaphragm. If the mic is sensitive to DC, this can cause major problems. For this reason, an air leak is put behind the diaphragm of microphone. The leak allows the pressure across the diaphragm to equalize. The size of the air-leak determines the low-frequency cutoff of the microphone. In this case, it forms the dominant cutoff. I want this mic to be good down to about 0.001 Hz, I built this little canister here, to determine the appropriate leak:
Inside the canister is a capillary tube wrapped around a brass spool, that is long enough to determine the appropriate size of resistance.
This leak screws into the back of the condenser microphone. The reason for this is to make the back-leaks conveniently interchangeable. (I.e. suppose I want to make the back-leak cut-off at 0.01 Hz instead of 0.001 Hz, I can unscrew the old back-leak, and screw in a canister with a different length of capillary tubing). There will be problems with this related to heat-expansion of the tubing. The acoustic resistance of the tubing is very sensitive to change in diameter, and, though I have not tested it, the thermal-expansion of Teflon should be quite a bit larger than steel. Oh well, it's the best I could do with what i had on me. The 0.01 - 10 Hz region should remain reasonably unaffected, and that is the region of most interest.
(Back leak, screwed to the microphone)

So, here is the whole microphone, sitting upside-down on my desk, with a big syringe sticking out of the back.

The syringe allows me to raise the pressure inside the microphone and watch the decay. This lets me find a time-constant for the leak, and allows me to characterize the resistivity of the back leak. It also allows me to see how thermo-viscous losses will be affecting the compliance of the chamber behind the diaphragm (not an insignificant thing).

Not quite done however... the signal is still pretty low-level. Also, as of right now there is still some junk in the signal left over from processes done in the preamp. This little box allows me to battery power it if I want, as well as apply some selective filters to the output.

(The inside of the box. The big empty space is for if I want to make the thing battery-powered)

Here's the box from the outside. Two outputs and power input +-gnd inputs (if it's not being run by batter).
Here's Matt, posing while I test whether or not I busted the SD card in the camera. I guess it still works. Matt is research faculty in the lab, and he's helped me with innumerable mechanical problems in the past. He also helped me get my bed from storage at the beginning of the year, so I didn't have to sleep on the carpet anymore.

Anyway, as far as the mic goes, I could keep working on the thing for a long time but I can't. There's a billion improvements I would like to make. However, the mic itself isn't my research project, it was just built to help with my project. So even if I see a dozen ways to make it better, if it's good enough I sort of need to say "it's good enough" [rrrrgh] and get on with the rest of my research. Today though, in the shower again (where, like normal, all my best ideas come to me. I don't understand this), I though of a way to possibly suppress 1/f noise in the electronics. Even Dr. Gabrielson thought it was cool and said I could have a shot at cooking up the system I described to him. So who knows... there might be a part 4 coming up sometime.

Monday, February 18, 2008

Making a mic, part2

First off, Hilary is leaving for Germany tomorrow. Traveling over-seas is cool because, hey that actually needs no explanation, but it can also be frustrating because of all the security stuff that goes on, and the fatigue involved. So pray for her trip tomorrow!

Ok, so the first post in this series was about making the condenser microphone capsule and diaphragm. I'm going to skip the part about making the body of the mic because I already showed a photo of it a couple posts back, and I didn't have a camera available when I was actually building it. So this part is going to be on the preamplifier. By the time I am writing this, I actually have the preamplifier finished up, and the final form looks quite a bit different than in this photo (you will see later). Well, I need to deliver some photos here in about 5 minutes, so i'm gonna skip the explanation and just post the pics:

some components

The board - complete with groundplane

My first amp that didn't work so well...

My desk. I installed a lamp - no hard lights!

The new circuit - looking like a rat's nest

Assembling the components

Testing - still not quite ready, but I'll show you the final version later

Using IC's is normally frowned upon by audio enthusiasts and preamp builders but in my defense, I'm not making a normal microphone preamplifier. If you want a condenser microphone to be sensitive to DC (yes, all the way to DC) then you need to do think about things differently...

Wednesday, February 13, 2008

575 Faces

Or... "575 reasons why a person ought not be apprehensive about going to live in a foreign country for a good long while." Each photo's got a story.

Music: "Trust in a sigh" by Phil Woodward

Sunday, February 03, 2008


Hi, haven't been blogging much recently, I'll try to get something up this week. Stuff's been happening of course, but mostly I just wanted to say...

What a great dad I have. I've learned so much from talking with him, especially recently, and I am really secure knowing that I have his approval. I know he would have been happy if I had stayed and gotten a job in Seattle instead of going to PSU. He would have been proud of me if I had finished with a Master's and gone to China, and I know he's happy I'm here working on a PhD. I am really convinced that the unconditional love of the Father in heaven is something that is easier to understand when you have a parent on earth that loves you the same way.

So, thank you for that. Happy 60th birthday, and I am proud to boast to everyone else that you are my dad.