What Is An "M7" Capsule?
This is only a short history of the M7. (For additional info, check some of the Microtech Gefell sites.)
The M7's history, design and description is a bit confusing, because neither Neumann/Berlin nor Neumann/Gefell, the two legitimate companies that legitimately can claim having manufactured the original M7 PVC capsule, ever bothered to trademark "M7".
So now several manufacturers freely use the term without necessarily much regard for authenticity towards the original M7's features, dimensions or materials, let alone sound.
To distinguish:
1. The original (Neumann East/West/Gefell) M7 capsules were made with PVC diaphragms (ca. 8-10µ thick) - the only version that in my estimation can legitimately be called "authentic". With few exceptions (see below) it still has the dimensions, and material characteristics of the original invented in the 1930s. But through the years and location of manufacture, versions of the brass back plate design vary slightly in build, and therefore, in tone and character.
For identification, there are slight design differences among and between Neumann Berlin and Gefell back plates. One is visible at the outer edge of the diaphragm: post-war Neumann/Berlin M7 feature three distinct ridges (bearing edges), where the Gefell M7 to this day have two, just as those found in pre-WWII M7. Then there are different gaps between the ridges and widths of the grooves in between, as well. Another difference is how the sack-holes are arranged. The easiest way to visually distinguish Gefell from Berlin M7 is the center lead-out screw, which is tiny in the Gefell version - .2mm smaller compared to the M7 made in Berlin
2. Various manufacturers (including Gefell) make an M7-looking capsule, using an M7-inspired or copied brass backplate, but using the much easier to manufacture Mylar/Polyester/"PE"- diaphragm (mostly 6µ thin) instead of the laborious PVC diaphragm which only a small handful of specialists know how to cast.
In the case of Gefell, at least the brass backplate of this polyester capsule is "100% original M7", whereas all other manufacturers copy, more or less successfully, that backplate design.
But there comes another problem for copycats: what was magic synergy between an 8-10µ PVC diaphragm with a peculiar resonance behavior and the original backplate construction sounds at best mediocre, often coarse and strident when combining that backplate design with a modern 6µ polyester diaphragm: the sound is off, decidedly not magic, and all those who copy that particular diaphragm/backplate combination end in a cul-de-sac with no way out.
If you want the sound of a true M7 today, there is only one way to get it: Buy one of the very few, still intact, old stock Neumann/Berlin PVC M7 (made up until 1959), or a Gefell PVC M7 made until about 2005. (Current M7 PVC capsules by Microtech Gefell exhibit noticeable low-end loss, and other timbre changes, due to new PVC material they are using, and the retirement of the long-serving specialist who used to cast their membranes.)
Everything else, including Gefell's valiant effort of marrying the M7 backplate and diaphragm dimensions with modern diaphragm material using pre-manufactured polyester films, will most definitely sound different.
Oliver Archut added this bit of history:
"I would include as "original M7" - those made by MWL (Mechanische Werkstätten Lensaal) from 1945 to 1947 for the NWDR. (North West German Radio.) Because after the war the Neumann company was now located in Soviet-occupied Germany, with no access to their West German customers, MWL made these M7s for the (West German) broadcasters to the original Neumann blueprints.
After Georg Neumann moved from temporary headquarters in Gefell back to (West) Berlin, MWL stopped production. Aside of the markings on the MWL capsules, these M7s are absolute identical to the Berlin-made M7 capsules before 1942."
And David Satz has much to add about the date of introduction and its true inventors:
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 Elektroakustikarticle 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.