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What is an "M7" Capsule?

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klaus:
It may be a good idea to define "M7 capsule", considering that more than one manufacturer currently uses the term which originated as a Neumann product almost 100 years ago.

The original M7 capsule design was first developed in the early 1930s by Georg Neumann with the help of Erich Kühnast (his son later took the helm of Neumann/Gefell/Microtech Gefell, now retired), though the term was not used at the time. Please read below David Satz's corrections to the date of introduction [date of "first confirmed mention of an M7 in a Telefunken catalogue" is 1939, according to David's research] and who he believes to be its true inventors.

It was a dual-diaphragm, single backplate  construction capable of three patterns (cardioid, omni, figure-eight) when correctly polarized. Its first verifiable commercial application was in the U47 (1949), U48 (1957) and M49 (1951). M7 had cast PVC diaphragms and a characteristic hole pattern to the backplate which to a large degree is still present in the polyester-skinned K47 which succeeded the M7 in the late 1950s and which is still being produced by Neumann today.

After Neumann/Berlin switched to Mylar K47, Neumann/Gefell, later renamed to Microtech Gefell, continued producing its version of the PVC M7, to install it in all of its large Diaphragm mics. Around 2000, the German government clamped down on environmental toxins, including the PVC solvent Microtech Gefell used to make these capsules, forcing MG to reformulate the PVC mix.

MG also added a PE- (polyester) skinned version of the M7 to its lineup, using it in some of its LD mics.
AFAIK, only the flagship model, 92.1S, still uses the PVC version M7.

Gefell's PVC M7 made prior to approximately the year 2000 were every bit as sexy and emotionally attractive-sounding as the original Neumann/Berlin version, yet with a slightly different timbre, which changed (at least to my ears) dramatically when MG was forced to reformulate the PVC mix and their PVC capsules never sounded the same again. If you are ever in doubt which version you have before you: the Gefell version used and still use an M1.2 center lead-out screw, while Neumann always has used M1.4.

PVC M7 diaphragms were recreated by Siegfried Thiersch in Möschitz, East Germany, in the 1990s (Thiersch had resigned from MG, where he was in charged of nickel diaphragms). He first offered reskinning of original Berlin and Gefell M7 capsules, later he also started offering complete M7 capsules with PVC ("Blue Line" and PE ("Red Line") skins. STM (Thiersch's monogram) no longer offers the PVC version. As Thiersch also could no longer use the original toxic PVC material after 2000, its capsules too have the mid-forward emphasis of the post-2000 MG M7 capsules.)

All current copy versions of the "M7" (which Neumann neglected to trademark) currently manufactured use polyester/Mylar® material for its membranes.

Aside of one manufacturer who will remain nameless, to my knowledge, the original M7 PVC backplate dimensions and acoustic network were never modified by any of the copy companies to accommodate a polyester skin of roughly half the thickness of the originally 10µ PVC membranes.

In sum, there are many versions of "M7" in circulation today, and to my ears they all sound different from each other, with none sounding identical to the original M7 PVC Berlin or Gefell version.

Please add any corrections or other relevant M7 facts.

consicerecording:
I've been pondering about the 'nameless' M7 capsule maker mentioned in this post. Is it possible that Dany Bouchard/Poctop, known for his D7 capsule and his contributions to the DIY microphone community, could be the one? Could you share if there's any collaboration between you and this capsule maker for another KLAUS HEYNE EDITION tube microphone, perhaps similar to your past project with Dirk Brauner (VM1 KHE) &  Cathedral Guitar/Oliver Archut (OA-1)?  Additionally, I'm curious if there's any possibility of another classic KHE Tube Mic in the pipeline.

Thank you for taking the time to address my inquiries.

klaus:
Hello Mark Anthony,
I currently have no contract with any manufacturer for issuing another mic or mic components under my name. If and when that changes, I will post it here.

David Satz:
just for the record, I know of no particular evidence that the so-called "M 7" capsule was designed by Neumann and/or Kühnast, nor do I know of any evidence that would support the 1932 date even though Neumann still gives it in their chart. I say "so-called" because names of the type "M 7" and "M 8" were the Reichsrundfunkgesellschaft's way of referring to complete microphones, not capsules, to which they had granted type acceptance for purchase by broadcasting organizations within the Reich. Neumann's own names for these capsule types were CM 8 and CM 7 respectively. (Yes, the figure-8 came first.)

No microphones with these capsule types appear in any catalogs or price lists that I've seen or heard of prior to 1936. Until then, all Neumann condenser microphone capsules were single-diaphragm, diffuse-field-equalized pressure transducers. We might call them "omnidirectional" except that they were so large, their pickup pattern started to become narrow already in the upper range of voice frequencies. This general type of capsule (based on the classic Western Electric designs) dominated the condenser microphone market for its first 20+ years, but has faded into obscurity in the stereophonic era.

The actual inventors of the two capsule types were almost certainly Dr. Hans Joachim von Braunmühl and Walter Weber, working at the RRG. They patented both designs in 1935 (the first page of the German patent is attached)--or actually, there seems to have been a March, 1935 filing for just the figure-8 capsule design, which was then superseded in September by the filing for both designs. The next month the same two researchers explained the workings of the capsules at length in an article published in the journal Hochfrequenztechnik und Elektroakustik. (I wanted to attach several scans from this article, but the board's maximum file size for the sum of all attachments to any one message has somehow gotten set to only 512 KB, so there's only room for the first few paragraphs.)

These same two men also wrote a textbook, Einführung in die Angewandte Akustik (Introduction to Applied Acoustics), the manuscript of which was handed in to the publisher S. Hirzel in Leipzig right around when the above article was published. It's a snapshot of a particular moment in condenser microphone history: Its section on condenser microphones mentions only pressure transducers, and features a photo of a Neumann bottle microphone with a pressure capsule--but then at the end of the subchapter on microphones, a section seems to have been added in at the last minute describing pressure-gradient condenser microphones and saying that they'd been developed only quite recently ("in neuester Zeit"). No photos of any such capsules, or microphones using them, are shown in the book, however, while the Hochfrequenztechnik und Elektroakustik article shows prototype capsules that don't look like they're from Neumann.

None of that is consistent with their already having been introduced commercially some years before. Maybe most people don't care very much about history, but that's no excuse for perpetuating false history.

P.S.: I've seen many dozens of photos of Neumann bottle mikes being used before the end of World War II, but all the photos clear enough to allow the capsules to be identified show pressure capsules (CM 5 or CM 9). Does anyone know of any clear photos showing an M 7 (or for that matter an M 8) being used on a CMV 3a prior to 1945, other than catalog photos? It seems as if the M 7 may have achieved substantial use only once it was incorporated into the U 47 microphone of the postwar era, but I don't know whether that conclusion is supportable or not; it's just an "I wonder" kind of thing.

afterlifestudios:
Very interesting, David. When you mention “the 1932 date” are you referencing this spreadsheet, allegedly from Neumann in 1999? 

https://funkwerkes.com/web/wp-content/techdocs/MixedProAudio/Neumann-Microphone-Capsules.pdf

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