R/E/P > Klaus Heyne's Mic Lab Stickies

The Trouble with Microphonics In Condenser Microphones


Microphonics encountered in the use of microphones, especially in sensitive condenser microphones, is a relatively unexplored and little-understood defect that should be addressed.

A microphone becomes microphonic when any of its components other than the capsule’s membrane resonate in response to sound waves, and the parasitic signal thus triggered interferes with the signal captured by the capsule.

The source for microphonics in microphones can be grouped, with suggestions how to diagnose and eliminate the problem:

Tubes. The assumption that tubes selected for microphones are quieter, longer-lasting, or have other superior qualities compared to non-selected ones is false. F. ex. Neumann’s EF86, AC701, 13CW4 or VF14 M selected for low-microphonics, and identified as such with a special label or a stamp, are usually identical with the non-selected version, except their filament construction (heater, cathode, plate and other associated metal structures) went through a ‘pinging’ (mechanical excitement) test. Samples with lower than average microphonic tendencies were then selected out and marked as suitable for use in mics.

Pinging a tube by flicking a finger against its body midway, while holding the tube to your ear, is an excellent method to pre-select a microphone tube*. Nothing fancier is needed to predict whether the tube is likely to be triggered into ringing or resonating inside a mic.

All tubes resonate or ring to a degree, due to the various unsecured wire filaments inside, so the goal is to find the specimen with the lowest resonance. If even a light knock will trigger extended ringing, the tube, once installed, and its wires red hot, will become a serious obstacle to a clean recording.

*The VF14 tube is a special case: all of them ring when pinged. So Neumann used a cutoff definition for intensity and duration of the resonance: if the tube rang louder than x or longer than y when agitated by a calibrated mechanical impact, it did not receive the "M" classification (M for mikrofoniearm, or low microphonics) The only practical way to determine whether a VF14 has low or excessive microphonics is by installing and warming up the tube in the mic, then pinging the body.
I usually reject the tube if it rings longer than 2 seconds.

Passive components.  When resistors, capacitors, switches, wires and other components inside a mic are not sufficiently mechanically secured, they will ring and add microphonics to the output signal of the mic. Usually, good mounting and soldering techniques will prevent components from ringing, but sometimes free-standing parts need to be additionally secured with an adhesive that connects them to adjacent components or to the circuit board, to stop vibration.

I use Eclectic E6000 to prevent components from vibrating. This Styrene-based glue stays flexible, can be removed completely without residue and with one pull, and has an extremely high electrical resistance, which will not affect super high impedance circuits in condenser mics.

Other often overlooked causes for microphonics.

* Mono-filament wires. They can vibrate like a bass guitar string and should not be used as capsule lead-outs, or anywhere else in a microphone

* Mechanical switch contacts in attenuator and pattern switches. They should not be free-floating, but their contact tongues
   should rest against a firm surface

* Mesh layers of double or triple-layer head baskets.  One of them should be slightly bent inward or outward, so that one layers rests
  firmly against adjacent ones, to prevent resonating                                             

The ultimate test whether you successfully removed all sources of microphonics: hold the mic (unplugged) against your ear and knock its housing with your knuckle. If all you can hear is a dull thud, you are done.

You will appreciate the added clarity the mic delivers once microphonic artifacts are removed from the signal.

© Klaus Heyne 2020

Don't forget the mic housing itself, especially those made of brass can ring like a bell.
The typical Neumann style felt damped spider clamp helps with that, opposed to the screw-on connector-only mic stand mounting.

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