Neumann History
Neumann: A Name Stands for Quality and Precision
Despite all the progress in machines and production technology, manufacturing a
Capsule Building – A Science in Itself
The performance of the condenser microphone, now manufactured in an extremely wide range of models, remains largely reliant on the precision engineering involved in capsule production.
The common centre electrode found on a double diaphragm capsule contains a large number of critical drill holes, some of which are blind. The depth of these blind holes determines the volume of air trapped behind the diaphragm. This volume, which inhibits the movements of the diaphragm, determines the transducing capability of the condenser microphone.
The dimensions of the holes, and their accurate machining becomes even more crucial when the electrode is produced in two halves. With this design the two halves of the capsule can be electrically connected, and similarly separated, by means of an isolating intermediate layer, thereby making it possible to switch the directional characteristic with the available polarisation voltage.
To smooth the surface of the electrodes two different processes are employed. For microphone capsules whose surface lie on one plane a lapping process can achieve a surface flatness of
0.3µm and a plane parallelism of +/– 1 µm be- tween the front and the back of the electrode. In some cases a capsule’s surface may be in two planes. This may be because the distance be- tween the diaphragm and the electrode has al- ready been determined by the second plane of the electrode. In such cases the finishing is per- formed on special lathes.
After lapping or lathe finishing, the holes must be deburred, followed by a visual inspection using a powerful microscope.
Diaphragms are made from a 6.3 µm thick pol- yester foil, such as Mylar. This is first attached to brass rings, then put into a container which holds it while gold is applied under vacuum to a uniform layer 300 Angstroms thick (0.03 µm). The external diameter of the capsule is approximately 34 mm. The diaphragm is fitted approximately 40 µm in front of the electrode and is
6.3µm thick. When a sound pressure of 1 Pa is applied the diaphragm movement is no more than 10 nm. By comparison, the wavelength of violet light is 400 nm.
The mechanical advantages being achieved under these microscopic proportions is best put into perspective by illustrating thus: if a microphone capsule were to be given a scale on which the amplitude for 1 Pa were represented by 1 mm the capsule under manufacture would have to have a diaphragm spacing of 4 m, and the diameter of the capsule would be more than 3 km.
One type of capsule, the KK 88 from the KM 88 microphone, uses pure nickel as the diaphragm material 0.0007 mm thick (0.7 µm).
On assembly of the capsule aluminium foil spacer rings, 40 µm thick are attached to the middle and the edge of the electrode. The
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