The capacitor microphone is an incredible feat of precision engineering. We go behind the scenes in Vienna to find out what makes it possible.
Choice can be an illusion. Browse any retailer’s catalogue, and you’ll see a dizzying variety of capacitor microphones. What you won’t see is that the most important component in many of them is almost identical. Most microphone manufacturers don’t make their own capsules, but buy them in from a small number of Chinese factories. These are almost invariably copies of the Neumann K67 and K47 designs.
When Martin Seidl started a microphone company, therefore, he recognised that building capsules in‑house would provide a key point of difference. And, having recruited key personnel from AKG’s Vienna HQ, he had access to unrivalled expertise. This shared experience suggested a starting point for Austrian Audio’s first new capsule design. No‑one had a better understanding of AKG’s classic ‘brass ring’ CK12, as used in classic mics like the C12, C12A, C24, C414EB and Telefunken ELA M251. But there was a problem. The CK12 had been discontinued for good reasons. Handmade to incredibly fine tolerances, it had always been extremely difficult to manufacture, to the point where Martin estimates that AKG’s yield of functional CK12 capsules was as low as 15 percent. In order to be competitive, Austrian Audio would have to adapt the CK12 design so that it could be manufactured reliably and consistently, without losing its special sonic qualities.
Back in the late ’70s, AKG had attempted the same goal with their ‘nylon ring’ capsule, which replaced the iconic brass ring with a white plastic surround. In theory, the new capsule should have sounded similar to the CK12, since it retained the same backplate design. In practice, it didn’t sound remotely the same. “The sound of the CK12 hasn’t been matched by anyone,” agrees Martin. “As hard as AKG worked on it, they never matched the sound, despite taking care of all the dimensions. It was this mix of the dimensions, the mass of the product and the material used that made this smooth, but very clear sound. Never harsh in the highs, but still very open.”
Austrian Audio’s research convinced them that the inferior sound of the ‘nylon ring’ capsule wasn’t down to the method of tensioning the diaphragm, nor its simpler clip‑together construction. Rather, replacing the heavy brass with lightweight plastic had made the capsule less stable and more prone to moving as a whole. “If the capsule is too lightweight, it changes the sound, no matter if the dimensions are right,” explains Martin, “because you want the membrane to move, not the capsule as a whole. So we experimented with a lot of different materials, to see how could we get this mass back to the old brass weight. You have to find a material which, A, has the behaviour you want and, B, you can manufacture in a way that’s sustainable or reproducible. Because we wanted to fix it only at three points, it needed to be stiff enough to hold the capsule in place without deforming. We experimented with metal and with various kinds of plastic. Plastic never satisfied us, and metal had all the disadvantages of manufacturing costs and tolerances within production — which are doable, but very expensive if you make them one by one.
“Then one day, our mechanical engineer turned up and said, ‘What do you think about this?’ And he had a promotional item from a ceramic manufacturer, which was a bottle opener made of ceramic. It was extremely rugged and robust, it was stiff enough, and it could be manufactured to extremely fine tolerances. It behaves like brass and has the same sort of weight as brass, but it even has an advantage over the original brass because it is an insulator. And now we can recreate it in the production, stably enough to get, let’s say, a yield of 80 to 90 percent.”
Martin Seidl: "It’s very easy to make a mic that has a good frequency response on a measured distance on‑axis. But show me one recording situation where you have only an on‑axis source!"