Thursday, January 10th, 2013 | by matthew mcglynn
Ever wonder what’s inside a microphone capsule? I’ve disassembled more than I should probably admit, including the latest, a small-diaphragm MXL capsule. This capsule is used in the 603S and 604 pencil mics, and numerous other mics including most of the 990 series. See a full listing of microphones with the 603S capsule.
What’s inside the MXL 603S capsule?
From left to right:
- The housing or shell, which holds the rest of the parts all stacked together.
- The diaphragm: a thin sheet of Mylar film, metallized with a thin layer of gold, stretched onto a metal tensioning frame.
- The spacer shim: this is all but invisible in this photo. It is a thin plastic washer that sits between the diaphragm and the backplate.
- The backplate assembly: a machined brass disk, set into a plastic carrier ring.
- The chamber assembly.
- The lockring.
We’ll explain more about all of these shortly.
How can I disassemble the MXL 603S capsule?
The lockring, visible at the rear of the capsule, can be unscrewed. You need to find two sharply pointed probes, or a pair of tweezers (hat tip for that recommendation: Henry Spragens; see the link to his MXL 991 mod blog post below) and twist them simultaneously to provide enough torque to loosen the ring. Be aware that this disassembly will probably render the capsule unusable.
Can the capsule be put back together?
The capsule’s components can certainly be stacked back up and screwed back together. But if any dirt, dust, hair, dander, etc., gets lodged behind the diaphragm, then the capsule will not sound good (if indeed it works at all).
The capsule, in depth
Let’s address the overall assembly first. Stacking disparate components into a shell, and clamping them together from behind, is not an uncommon way to build a capsule, especially a small-diaphragm capsule. I suspect most people are more familiar with large-diaphragm capsule designs, simply because their structure is obvious from inspection: they use front and rear diaphragms clamped onto a central backplate.
But there exist large-diaphragm models that follow the stack-and-clamp approach too; see for example Shure’s large-diaphragm designs.
The diaphragm of this mic is pretensioned onto a metal ring. This one appears to be made of brass. The membrane material is a thin Mylar film, metallized on one side with gold. The thickness is stated by MXL to be 6 microns.
In a functional microphone, this diaphragm would be smooth, flat, and ideally, clean. The one pictured here is contaminated, having been exposed to the decidedly not-clean environment that is my office, and punctured on two sides — scars from the calipers when I measured the inside diameter of the ring.
The thickness of the metal frame ring is 0.97mm. The diaphragm is mounted to the bottom surface of the ring (as seen from the perspective of an inbound on-axis soundwave).
In this design, the diaphragm is grounded to the housing of the capsule, because the metal ring is in physical (and electrical) contact with the housing.
Sandwiched below the diaphragm is a very thin, 12–18 micron thick spacer made of thin flexible plastic. This serves to create a volume of air between the diaphragm and the backplate. The frequency response of the capsule is determined in part by the tension of the diaphragm, but also by the volume of air beneath the diaphragm. Changing the thickness of this shim, and thereby changing the volume of air between diaphragm and backplate, is one of the ways the capsule’s response can be tuned.
To be clear, although the tensioned Mylar diaphragm has a resonant frequency independent of the rest of the assembly, the behavior of that diaphragm within the assembly is necessarily changed by the other components. You could think of this like a drum; a one-headed drum could be tuned to 440Hz, but as soon as you put a bottom head on the drum, the drum’s final resonant frequency depends on the tension of both heads as well as the volume between them. (A drum’s resonant frequency depends on shell factors too, of course, but that goes beyond the utility of this metaphor.)
The backplate proper, as in large-diaphragm designs, is a disk of machined brass. Its surface profile is the result of complex analysis, engineering, and compromise. Backplates have a combination of blind and through holes and/or grooves or slots that create space for a particular volume of air. The air provides damping to the diaphragm, influencing its resonant frequency and therefore frequency response.
The through-holes allow sound from behind the microphone to counteract vibrations imposed by soundwaves on the front of the diaphragm, thereby creating cancellation. As you might recall from our groundbreaking video about the engineering secrets of the Shure SM57, providing a rear entry path for audio signals is the primary way to create a single-element Cardioid capsule.
The backplate must be metallic, because the point of the transducer is to create a voltage change based on physical vibration. The diaphragm and backplate form a capacitor; they’re charged differently, and output a varying voltage based on the distance of diaphragm to backplate.
In this capsule, the brass backplate is set into a plastic ring because the backplate must be insulated from the housing. The diaphragm ring is grounded to the housing, so the backplate needs to be insulated from both.
This particular backplate design was reportedly inspired by the Neumann KM84 backplate, which uses an innovative crossed slit design. The MXL backplate instead uses grooves in a concentric ring arrangement. In the early production runs, the middle ring is perforated, as shown here; more recent production runs show altogether higher-quality machine work, but no perforations in the middle ring. This changes the air volume of the capsule and therefore its frequency response.
This capsule had two metallic (copper?) washers. These are stacked between the back of the backplate assembly and the chamber assembly. Like the plastic shim beneath the diaphragm ring, these spacers help create a volume of air behind the backplate.
Behind the backplate is a perforated plastic disk. This forms part of the acoustic labyrinth that delays audio signals entering the rear of the capsule. There are eight holes drill through this disk, plus a 9th for the terminal that connects the backplate to the mic’s circuit.
Mark Fouxman of Samar Audio, who has studied and modified these capsules extensively, calls this piece the “phase shift network.” According to Mark, all of its parameters — the depth of the chamber, the number, placement, and size of the holes — affect the capsule’s polar pattern and frequency response.
Sealing off those holes would yield a roughly omni polar pattern, because a solid plate here would prevent sound from entering behind the diaphragm. That said, a capsule designer would cringe at the idea of sealing these holes to produce an Omni capsule. A better Omni design would require a more carefully planned damping and resistance network behind the diaphragm, higher diaphragm tension, and potentially other changes, including (depending on who you ask!) a modified backplate.
The backplate is connected electrically to the microphone circuit via a brass terminal screwed into the rear of the capsule. The terminal goes through the center hole in the chamber assembly, through one or more shim spacers, and into the threaded center hole of the backplate. In the MCA SP-1, MXL 990, and similar microphones, a wire connects this terminal to the printed circuit board.
The lockring is a threaded metal ring that clamps the assembly into the housing.
The design of this particular nose has invited criticism, because it creates a tunnel in front of the diaphragm. The screen piece is approximately 1mm thick. It sits on a shelf formed into the housing; this rim is ~1.5mm thick. Below that would be the diaphragm assembly, which has another 1mm offset, for a total of 3.5-4mm of depth between the frontmost edge of the grille and the surface of the diaphragm.
Theoretically, soundwaves could bounce around within this 3.5mm deep tunnel, causing reflections and coloration that would be picked up by the diaphragm.
Read more about modifying the nose of the 603S capsule in Henry Spragens’ blog post: Modifying the MXL 991.
Why would you wreck a perfectly good capsule?!
We have long reported three capsule measurements for condenser microphones, to help consumers understand and predict a given microphones’ behavior:
- The diameter of the suspended area of the diaphragm.
- The outside diameter of the backplate. This number is less revealing, but we include it because some manufacturers report this value when describing a capsule.
- The gauge of the membrane material.
We had in some cases misstated the measurements for this capsule, having quoted misunderstood specifications found elsewhere. But no more. For the record:
- Diaphragm diameter: 17.1mm
- Backplate assembly diameter: 19.9mm
- Exterior housing diameter: 22.4mm