So…it’s been a while. Somehow I thought, I’d have finished the CNC writeup by now, as well as my plans for continuing work on my plasma-bar meter project. Things turned out different.
I got into my Bachelor’s thesis after completing the preceding seminars, which kept me busy since june ’13. The topic is centered around signal power based speech DOA estimation using directional microphones, very interesting stuff from the domain of signals processing. I had a very interesting time learning lots of new things, and somehow I also wanted to get hands-on with the matter since attending several advanced DSP courses during the last two terms.
While working on the thesis, I realized that microphone array systems are really nice things to have to play around with. I decided to make my own, which I did in my free time in parallel to the thesis. Let me at least show some pictures as long as I still don’t get around to working on other stuff.
A few technical details follow below.
Basically, the array consists of four Panasonic WM-61A electret capsules, which are known for their robustness under high sound pressure levels. The capsules are mounted on several 140mm long, 6mm dia. brass tube segments using aluminum sticky tape and heatshrink. The mounting method ensures no acoustical reflectors or flat faces in the near field of the microphones, which enhances the acoustical properties of the whole assembly. All sound ports are pointed upwards to keep the characteristic beam patterns of the microphones as similar as possible. According to the datasheet, this should be an omnidirectional pattern for each capsule, but the mounting causes some deformation. In this case, the bottom direction (sound coming from along the brass tubes) is somewhat sub-optimal.
To connect the capsules, I used 2-lead shielded microphone cable with a 3-pole headphone plug at the end. I applied the “Linkwitz mod” (http://www.linkwitzlab.com/sys_test.htm) to rewire the capsules for better preamplifier performance, using the described two-wire connection for the capsules. I didn’t connect the shield on the capsule side, but the capsule has a pretty good metallic connection to the brass tube, which means that the shaft is connected to microphone ground. For the meantime, this solution is sufficient, even though it would be better to connect the shaft to the shielding and keep the capsule isolated to prevent a ground loop. I will correct that at a later point since it is pretty hard to connect a wire to a brass tube from the inside, if you can’t solder for fear of the cable melting 😉 Well, I’ll figure that one out eventually. For now, humm and distortion is low enough for the purpose.
To keep the array together I cut a piece of layered wood (called “Multiplex” here) on the CNC mill, drilling holes for the brass holders in predefined distances that match the parameters of my DOA estimator project. The brass tubes are pushed tightly into the holes and the cables dangle out of the bottom. An extra hole in the middle allows the assembly to be attached to any kind of support with a 6mm screw.
The preamp is still breadboarded for now. Shielding is pretty bad (or in other words “not present”), but nevertheless it performs rather well. I get a little bit of hum from nearby power lines, but I can blank that out in software for the time being. The case (cast aluminum) is already in the making, and has also proven to be an adequate test for the CNC to demonstrate its ability to mill aluminum. Not perfect to the 0.001, but near enough. I will integrate the amplifier into its new home as soon as I get a little more free time to mill a pcb, which will be in another two weeks, when all the other stuff is finished.
For signal acquisition, I currently use an ESI MAYA 44 USB+ 4ch-in/4ch-out external soundcard, which has the nice benefit of being ASIO-capable and can be used with MATLAB and especially Simulink. The latency is not as rock-stable as with cards like the RME Fireface series used in the university labs, but with some MATLAB trickery, I can get more than acceptable results out of it.