[ProAudio] Microphones question

Chris Caudle 6807.chris at pop.powweb.com
Sat Jun 12 13:48:36 PDT 2021

On Sat, June 12, 2021 11:15 am, Dan Lavry via ProAudio wrote:
> I was hoping that someone would talk about how to measure the electronic
> part of the mic, without contributions from the membrane. I would assume
> that one could replace a variable capacitor membrane with a fixed cap.
> The old carbon mic could be replaced by a resistor. That or similar
> methods to remove the acoustic contributions can help characterize the
> circuit (flatness, noise, impedance...).

That sounds like you are talking about characterizing the capsule buffer
circuit of a condenser mic, but previously it sounded like you were just
discussing the interaction between output impedance of a microphone (which
could be a capsule buffering circuit, or could just be passive as from a
moving coil or ribbon mic) and the input stage of a preamplifier.

> We (designers) measure all sorts of circuits. I don't intend to
> open my mics, remove the membrane and "put the circuit on neutral".

Would you need to?  Is your concern that acoustic noise in the room might
give interfering results if you just try to measure the output impedance
of a microphone as a black box?
If you are worried much about that you could make a box-inside-a-box type
small isolation chamber to put the mic inside while measuring.

> Ideally (or close to it), we need the proper acoustic and mechanical
> setup, with a clean acoustic source (frequency sweep or a chirp).

If you wanted to measure the entire system from acoustic source to
electrical output.  I think most people just let the microphone designer
handle the acoustic source to microphone electrical output part, and the
preamp designer handle the low level electrical source to higher level
electrical output part.
Working on a full end to end system measurement sounds like a lot of fun
and a learning experience, but from a practical aspect that seems hard to
justify if you have to crank out a commercial product.

I do get what you are saying though, most microphones are under-specified
(at least in the public specifications) if you want to really know how
well a particular preamp design will work with a particular microphone.  I
guess that is why there is still a (small) market for boutique amplifiers
tailored for ribbon microphone duty.

Chris Caudle

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