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Author Topic: The Trouble with Microphonic Microphones  (Read 567 times)

klaus

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The Trouble with Microphonic Microphones
« on: April 05, 2019, 08:48:55 pm »

Microphonics encountered in the use of microphones, especially in sensitive condenser microphones, is still a relatively unexplored and little-understood subject. But their detrimental effect on the sound of a mic is considerable, and should be addressed.

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

Microphonics in microphones can be grouped, in descending order of annoyance, and I give suggestions how to eliminate them:

Tubes. The assumption that tubes selected for microphones are quieter, longer-lasting, or feature other 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 same tube type, except their filament construction went through a ‘pinging’ test. Samples with lower than average microphonic tendencies were then selected out as suitable for use in mics.

Pinging a tube by flicking a finger against its body midway, while you hold 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 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, will become a serious obstacle to a clean recording.


Passive components.  When resistors, capacitors, switches, wires and other microphone components are not secured from vibrating, they will contribute 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 secured with an adhesive that connects them to adjacent components or to the circuit board.

I use Eclectic E6000 to prevent components from vibrating. This Styrene-based glue stays flexible, can be removed completely and without residue, and with one pull, and has an extremely high electrical resistance, preventing any lowering of high impedance circuits in condenser mics.

Other often overlooked causes for microphonics.

* Mono-filament wires. They should not be used as capsule lead-outs. They can hum like a bass guitar string

* 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 the layer rests
  firmly against an adjacent one, to prevent resonating                                             

The ultimate test whether you successfully removed all sources of microphonics: hold the mic 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 2019
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com
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