Microphone Design

A few of my last posts have been commented to be "straight from lecture", as in me copying what my teacher said and posting it. I realized I was definitely inspired by my classes to write some of the previous posts, and now that I am out of that class I feel I can write freely again.

To remove myself from thinking too hard about it, I'm just going to write about a subject I have never had a class on so there can be no lame copying and to everyone it will be new information.

Todays topic is microphone design. This is an extensive subject and I am not even going to scratch the surface. There are two design elements to a microphone that I am going to try and explain: electric and acoustic.

ELECTRIC:

Microphones, in their basic form, are simple to understand. You have a capsule that converts acoustic pressure into an electrical signal. What all microphones do is split that signal into two amplified versions flipped 180 degrees from each other. When two signals are flipped 180 degrees out of phase from each other and sent down the XLR cable, inductance may occur. If an interference is picked up by the cable it will cancel itself out when the two signals are flipped back in phase.

The circuit of the actual microphone varies, of course. I don't understand electronics all that well, which is why I search online to find microphone schematics. The little bit I know helps me choose a circuit I want to build, and I'm on my way. (The circuit I am currently studying is a tube microphone that needs phantom power instead of a power supply. We'll see if it works.) One thing the circuit needs is shielding around it. Correct me if I am wrong, but I believe this acts as more of a protector against interferences in the air surrounding the mic such as electromagnetic disturbances when a cell phone receives information, and not so much a ground for the microphone's circuit to send unwanted signal.

ACOUSTIC:

The acoustic properties of a microphone are critical. Sound can be manipulated in many ways, and if the design of the microphone is incorrect, that sound can be hitting the capsule in unwanted ways. The capsule should be separated by a barrier of some kind from the rest of the body. The body of the microphone is more or less a hollow tube where sound can become trapped and reflected back into the capsule. I had just built a microphone out of a 2" brass tube, and put a barrier further down into the body so that the capsule was right above it. 2" of circular tubing creates a standing wave of 6.78kHz which bounces straight back into the capsule and causes vocals to sound very nasal. I put a barrier cutting off the capsule from the tube and prevented that standing wave from ruining my microphone.

The grill is possibly the most important acoustic element in the physical microphone design. The way that sound propagates past the thickness of the mesh wire and the width of the spacing between the wires is a tricky relationship to understand. Throughout my research on it, I have come to find that the larger the spacings in the mesh, the more open the mic will sound, and on the contrary, the closer the spacings, the more direct your mic will become. I have no idea what wire size has on the impact of the sonic quality. I can say this, the thinner wire will be easier to mold, but easier to dent. I suggest a combination of the two, with the thinner wire and thinner spacing mesh on the inside, while the thicker wire with larger spacing is on the outside.

Neumann has a patented grill shape that they use for their large capsule side-address condenser microphones. It is a rectangle shape at the top, and as it gets to the body of the microphone, it becomes a cylinder. This shape allows for zero standing waves to become trapped inside the grill of the microphone. AKG 414s have a similar design. My dad suggested studying the design of the stealth bomber, which was designed to minimize radar reflections as much as possible. (If only I can figure out how to do that in to the inside of the enclosure...) Some designers will put acoustical treatment inside the grill, such as foam padding to absorb the sound, with a hole cut out in front of the capsule for the sound to come in.


That's sort of all I can tell you for now. I don't understand much more of it, but hopefully you can now understand microphones a little bit more now because of this. Perhaps not on the building level, but I sincerely hope that you have a curiosity for how the equipment you use works.