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!"
The original CK12 was held together by numerous tiny screws. These were inserted and tightened by hand, making construction a fiddly and time‑consuming process. The CKR12 design eliminates this, and Austrian Audio have also developed an innovative tool that uses a laser to measure the distance between diaphragm and backplate. The sensitivity of the capsule is a function of this distance and the polarisation voltage across the membrane so, once the capsule is installed in a mic, the voltage can be fine‑tuned to deliver exactly the same overall sensitivity in every example. Consequently, there is no need for pair‑matching with OC18 or OC818 microphones, because they are all within half a dB of each other.
Consistency of polar pattern is achieved through the use of a highly specified fabric as part of the acoustic labyrinth within the capsule. “These are automotive‑grade fabrics, so they have a very controlled acoustic resistance and an extremely well controlled density,” explains Senior Acoustics Engineer Christoph Frank. “A lot of capsule manufacturers do it with air gaps, which can provide acoustic resistance, but it’s quite hard to control because every micron makes a big difference. And this is why we decided to go for a textile, but a very high, automotive‑grade controlled textile to really have a reproducible polar pattern all the time. In the old days you sometimes had, for example, poorly specified textiles and you made them denser by putting small glue dots on them. You measured and put another glue dot on, until you had the right resistance.
“The easiest way to imagine it is like this. If you have a membrane and an electrode and no resistance on the back side, it would have figure‑of‑eight characteristics. If you have it closed in the back side, you have omni. The acoustic resistance is like the fader between those. So ‘half closed’ would equal cardioid, but the really hard part is to get a very broadband cardioid response. It’s very easy to have a perfect cardioid at exactly 1kHz, where it’s typically specified, and where especially Asian products are good. But if you look at the frequency response from behind, it is not at all even. I’s not only the material: it’s all the acoustic volumes and so on that help deliver the characteristics you want.”
This acoustic resistance also helps to damp the resonance of the diaphragm, which would otherwise deliver a very strange frequency response. The frequency of this resonance is determined by the area of the diaphragm and its tension; again, Austrian Audio have come up with an innovative jig which uses a laser to measure the frequency and automate the process of increasing the tension. Once optimum tension is reached, the membrane can be glued to the ring and then cut out, ready to be joined to the rest of the capsule assembly.
One difference between the CKR12 design and that of AKG’s original CK12 is that Austrian Audio have chosen to adopt a slightly greater distance between diaphragm and backplate, which means they are typically using a slightly higher polarisation voltage. “The AKG had a smaller air gap between the membrane and electrodes,” says Christoph. “It always happens that if you have an explosive sound, the membrane will touch the electrode. That can’t be avoided, because this pop noise is producing a sound pressure level of around 160dB. It will touch, but the thing is, it’s not allowed to stick. In the old AKGs, when the membrane tension gets a little loose over time, it gets stuck to the electrode. And therefore we increased the distance — but therefore you have to use a higher voltage.” Once again, Austrian Audio have developed their own testing machine to ensure that their diaphragms don’t stick to the backplate when they encounter a burst of air.
Martin Seidl describes the CKR12 and the newer OCC7 small‑diaphragm capsule as ‘platforms’ rather than products in their own right. “We took more than a year just to develop the capsule. So we had no product to sell — and even when we had the capsules, we didn’t have a product, we had a capsule. If you start up a company like this and you need a wider range of products, how can you achieve that? If you buy capsules, you don’t know what you get. If we make our own capsules, we can manipulate them, and that’s a great advantage.
“For example, we’re using a very similar capsule in our handheld condenser microphone as in our condenser instrument mic. But as we are making it here in house, we can tune the capsule. And tuning means small changes on the back side, damping material, little things that makes the capsule perfect for the application. We don’t want to change the electronics in the audio path. Most manufacturers only do that because they don’t make their own capsules, but we can manufacture a capsule to suit the application. We can’t afford to make 20 different capsules in‑house. We only have a handful of operators hand‑building each capsule. But we can say, ‘OK, we need 1000 pieces for the instrumental mic.’ And then we can change them in the manufacturing process to be perfect for the instrumental mic. And I think that’s what a high‑class microphone manufacturer should do.”
Once a capsule has been developed, of course, many more design decisions have to be made before it can form the basis of a product. “The physics is just the first step,” says Martin. “You have to turn moving air waves into an electrical signal. The second part, of course, is the electronics: amplifying it to make it a good signal. There is a lot of art in this as well, and we have a great engineering team who are doing astonishing things on fast circuitry.”
But there’s also a third aspect to microphone construction: the physical design of the housing. This needs to be as acoustically transparent as possible, whilst still protecting the capsule from dust and wind, and the electronics from radiated interference. “We started with the mechanics before we started with electronics,” says Martin. “If you ask a trained acoustician, ‘What would be the best acoustics?’, he would answer, ‘It would be this capsule flying in free space.’ So how can we create acoustically the best headbasket without having a capsule in the free field? How big do we need to make the cage? How well do we need to suspend it? And then we said, ‘OK, in every large‑diaphragm microphone, you have reflections from inside the microphone.’ Because there’s electronics down there, right, so at least you have reflections from there. We say, this is not acceptable, we want open acoustics, so we built a diffuser in here. There’s no hard space, so it moves reflections into the body. And then we started with electronics.”
The flagship OC818 is a multi‑pattern capacitor mic and, indeed, allows the signals from both sides of the capsule to be recorded separately for maximum flexibility at mixdown. Pattern switching has been a feature of capacitor mics since the 1950s, but it’s a compromise; often, the frequency response varies a lot between patterns, and the omnidirectional pickup can be approximate to say the least. The key to consistency is a good off‑axis frequency response, which is a function both of the capsule and its housing.
“If you look at the literature we are offering for the mics, you don’t get one frequency chart, you get a frequency chart per pattern,” says Martin. “We show that because we are proud that we invested a lot of work into that. 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! Show me one singer who stays fixed in front of a microphone when singing. In a piano, 80 percent of the strings are off-axis. That’s why a microphone sounds really brilliant on a piano or it doesn’t. And that’s why it’s important that the pattern stays stable and the frequency response stays stable off‑axis. And that’s what only good microphones do.
“I’m not saying Austrian Audio are the only company doing that. There are good microphone manufacturers, and I absolutely honour them, and we are proud to compete with them. But you have to invest time and, unfortunately, also money. And that’s why a good microphone will never cost €199, if you want a good large‑diaphragm condenser...”
Microphone manufacture at Austrian Audio is still a surprisingly hands‑on business, with the all‑important capsules being assembled manually on specially designed jigs. “We have two capsule production stations right now,” explains Christoph Frank. “We have clean room benches with a constant airflow, so dust can never settle anywhere. The gap between a condenser membrane and the electrode is in the range of 30 to 50 microns, which is comparable to the thickness of a hair. If there is a hair inside, there is no gap any more. That’s why dust is a big problem. There is also no fat, dirt, or liquid allowed inside the capsule, because that can cause humidity problems.”
What is heavily automated, by contrast, is testing and data management, which Austrian Audio see as the cornerstones of quality control. “You need to take a lot of care about the materials, the production process and the testing process,” insists Martin Seidl. “Because even if you use the right materials in the right way, you have deviations in the material stiffness. You have deviations within tolerances, and if you specify very narrow tolerances, you have a small yield of what you can use. Even in electronics, you buy the same parts and they perform within tolerances. On a whole circuit board, the tolerances add up, so that’s all what makes a difference between a great microphone and a good microphone. And so, yes, we deal a lot with all the measurements and we measure every product before it gets packed, in all the patterns and on‑ and off‑axis.“
The factory thus bristles with custom testing machines. One mimics the knocks and bumps that a shipped package will encounter on its journey to Australia; another reproduces a 1m fall onto a hard surface; a third recreates the impact caused by a mic stand toppling over. There are even machines that can simulate hostile climatic conditions and deliberately cause condensation to form on capsules. But the jewel in the crown is their anechoic chamber. One of the best‑specified test facilities in the audio industry, it’s available for hire by third‑party organisations, but also plays a key role in the development and qualification of their own products. For example, the frequency response of every production OC818 microphone is tested not just on‑axis but through 360 degrees, a process that takes between 10 and 12 minutes. Once the microphone is placed in the test jig, the rest of this process is entirely automated, thanks to the remote‑controlled turntable in the centre of the room.
Underpinning all of this testing activity is a custom software package called Aurora. This allows a computer to talk to almost any piece of test equipment and store the results ready for interrogation and analysis. “It’s just a program, but it can do everything,” says Christoph. “It can control soundcards, it can control these specialised test boxes, anything that can be addressed from a USB port; we can do PCB testing, we can do barcode scanning. Even if production is done in China [as it is with some of Austrian Audio’s more affordable products such as the Hi‑X15 headphones], we can look at each and every measurement. We see how many times it is measured: Was it good? Was it bad? When was it measured? It’s connected all over the world. But what you test is completely up to you, you can program everything.”