Capsule Technology and Types
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| Microtech Gefell UM 92.1S studio microphone with M7 capsule |
The Famous Gefell M7 Capsule
The art of microphone manufacturing begins in the capsule and there is no capsule that has played a more important role in the development of the microphone than the Gefell M7.
Originally designed by Georg Neumann in 1932, the M7 redefined the way we recorded music and completely shifted our listening expectations. All of a sudden, this new microphone could at once make the human voice sound larger than life, while bringing a clarity and detail to instruments that could never before be realized. [an error occurred while processing this directive] The Gefell M7 is a dual membrane capsule that is mounted on a triangulated pedestal. The M7 is the only capsule manufactured today that employs PVC (Poly Vinyl Chloride) as its substrate or base material. All other ‘plastic’ based capsules employ PE (Polyester). This newer material is marketed under various brands such as Mylar and Hostaphan. These substrates are fashioned into a thin sheet and are coated with a gold surface that is eventually charged to create a capacitive effect. The term ‘metal film’ is derived from this process.
As a substrate, PVC is quite different from PE. PVC starts in a liquid form that is spread across a glass surface while PE is derived from a solid mass that is essentially rolled into a thin membrane. Because the manufacturing process involved in making PVC is both exceptionally difficult to execute and is subject to a much higher rejection ratio, it is impossible to buy thin PVC film. As a result, the PVC used to make the M7 is still made in the Gefell factory following the process that was outlined by Georg Neumann himself. Of course we now have access to more advanced technology, which allows us to attain significantly higher levels of purity. This has resulted in consistency and life expectancy to improve significantly.
The advantages of PVC are many although one may rightfully argue that these are character traits as opposed to benefits. Gefell offers microphones that are made with PE and PVC to address both perspectives.
Because PE is rolled in a particular direction, the molecular tension of the membrane becomes ‘firmly aligned’ depending on the direction it was rolled. PVC on the other hand is ‘fluid’ and as such, assumes its final molecular shape naturally. Because the membrane is more evenly tensioned, physical distortion is reduced. This ‘rubbery’ and more homogenous PVC diaphragm renders a more uniform transfer of the energy, which tends to sound more forgiving.
The M7’s PVC membrane is refined to a specific 10-micron thickness while the stiffer PE is generally trimmed to a diminutive 6 microns. The ‘heavier’ PVC further credits the M7 with its distinct character. This has side benefits of being less prone vocal ‘pops’ and less sensitive to problems caused by humidity deposits from the human voice on the membrane.
The New Gefell M9 Capsule
Because PE (Mylar) is readily available, is easier to use, and can be fashioned in to a lighter membrane, it has become the material of choice for all metal-film microphone manufacturers. The lighter PE material provides a more responsive capsule, which results in greater sensitivity and articulation.
But manufacturing PE capsules is not without its challenges. As discussed above, due to the rolling procedure employed during manufacturing process, the molecular structure of PE becomes directional. This means that it has more resilience in the ‘roll’ direction. This prescribes that the capsule cannot be automatically tensioned to a given formula, but must be hand tensioned and ‘tuned’ like a drum skin for optimum performance. Hand tensioning by highly skilled employees is probably the single most important aspect that differentiates ‘the hand-crafted Gefell M9’ versus all others.
Obviously from a manufacturing point of view, because PE can be purchased ‘off-the-shelf’ and it is also easier to work with, the M9 has the benefit of reduced parts and labour costs making it relatively affordable.
Gold Deposition
The heart of the ‘capacitive’ microphone is the pressure gradient capsule. This is made up of two charged plates that are brought closely together. When the active membrane is set into vibration from the compression of air molecules (we call this sound), a voltage differential is generated that can then be amplified, recorded and eventually played back through a transducer called a speaker.
In order to charge the capsule membranes, a thin conductive layer is deposited on the substrate. At Gefell, a thin 0.1 micron layer of 24-karat gold is evaporated on the membrane in a vacuum-locked chamber in order to get maximum uniformity. The more economical process of gold sputtering or ‘spray painting’ the capsule produces an uneven coat, which results in a membrane imbalance and the associated inconsistencies that one equates with inexpensive knock-offs. This makes correct tensioning impossible and of course is the trouble with low cost microphones.
Pure Metal Capsules
The most challenging capsule to manufacture is the pure metal diaphragm. These feature an ultra-thin membrane for maximum sensitivity. Because of their exacting nature, less than a handful of manufacturers produce pure metal diaphragms and up until recently, these have been the exclusive tenure of the test and measurement industry.
The most notable test and measurement microphone manufacturers in the world are B&K and Gefell. For instance, BMW (Bavarian Motor Works) uses Gefell test and measurement microphone arrays to monitor motor and part vibration during the manufacturing process of their automobiles. Both B&K (now marketed as DPA) and Gefell offer pure metal diaphragm microphones that have been voiced for use in music. DPA employs stainless steel for their membranes while Gefell employs pure nickel.
Nickel has several distinct advantages. Nickel maintains its performance better over longer periods and is less prone to react with atmospheric conditions such as humidity and pollutants. Furthermore, just as PE and PVC differ, steel and nickel follow the same characteristic discrepancies. Steel is rolled and therefore performs differently depending on the direction of the roll. Nickel is produced through a galvanizing process that once again is liquefied resulting in a more evenly distributed molecular structure.
Stainless steel is less expensive and easier to work with but is limited to how thin one can roll the material in order to increase sensitivity. As a result, steel membranes are usually no less than 1 micron thick while the nickel membranes in Gefell microphones are typically 0.8 micron for musical applications down to 0.4 micron for critical test and measurement.
Test and measurement microphones are most often seen as long slender microphones with small tips or probes. The reason for this is simple: Stretching these ultra-thin membranes is extremely difficult if one expects to yield a perfect transducer. The larger the diaphragm: the more difficult. Smaller diaphragms are exponentially less costly to produce and have the benefit of much less physical membrane distortion, which of course yields a ‘flat’ or more linear response. Unfortunately, this comes at the expense of being less sensitive which is why many engineers are not partial to the sound of these ‘clinical’ microphones.
The Gefell M294, M295 and M296 employ the Gefell N2 nickel diaphragm which is termed a 1” microphone even though it is only 22mm or about ¾” across. The 1” or large diaphragm categorization as used in the test and measurement world likely stems from the outer dimension of the probe or the size of the microphone’s outer capsule housing.
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