[ProAudio] Microphones question

Bill Whitlock engineer_bill at verizon.net
Fri Jun 11 18:47:55 PDT 2021

How would you propose testing and specifying equivalent input noise for mic preamps?
Using the 150 Ω dummy source at least levels the playing field, even though it's not accurately predictive for all mics.  Specifying e and i noise separately (and their own spectrum, if you're going to be rigorous) will further confuse buyers - most of whom can barely understand why a shorted input is unrealistic.
As I recall from my tests of the SM57, its impedance varied from under 150 Ω at very low frequencies to over 300 Ω at resonance - and continued to rise at higher frequencies.  I'll try to find the data - I did the tests as research before writing Jensen AN-005 about mic splitters.
If you want to change this, I'd encourage you to join an AES standards committee and make your case.  Membership in working groups is open to all.  Working group SC-05-05 is currently trying to change the ways equipment manufacturers describe inputs and outputs - in the interests of avoiding interoperability issues and unexpected results.
Bill WhitlockAES Life FellowVentura, CA

-----Original Message-----
From: Dan Lavry via ProAudio <proaudio at bach.pgm.com>
To: crispin at crookwood.com; proaudio at bach.pgm.com
Sent: Fri, Jun 11, 2021 6:24 pm
Subject: Re: [ProAudio] Microphones question

Yes of course the input noise has to be taken into account AFTER 
amplification. So say a micpre has 120dBu noise (referenced to the 
input), with say 60dB gain the noise is at 60dBu. That is easy to 
measure and hear...

My point of interest was not about measuring microphones. But we have 
dynamic, ribbon and condenser with phantom, and a wide range of 
implementations in each category. So the use of one value resistor seems 
to be arbitrary. I mentioned earlier that the input noise is made of 
both noise voltage and noise current components. The noise current 
(today's technology) will have low impact for 150 Ohms resistor. So why 
is the resistor there? The answer is to give us a better idea of how the 
micpre works with a mic instead of a short. And so we lump all mics into 
a simple model. A 150 Ohm resistor.

I think we should remove that 150 Ohm resistor. The resistor noise is 
-130.9dBu (room T). If future technology will enable a shorted input 
micpre to reach 130dBu noise (referenced to input with acceptable gain), 
the outcome with 150 Ohm is -127.4dBu. Further down the line,135dBu 
noise (referenced to input) only improves the outcome to -129.5dBu.

I just wonder if there is some information about the real impedance of 
real mics including different types relative to that 150 Ohm. It would 
be good to have some better detail...


Dan Lavry

On 6/11/2021 1:32 PM, Crispin HT wrote:
> I'm not a mic expert, but in designing preamps over the years, we’ve found that the EIN of most mics sits around the -118 to -122dB mark.
> The relevance of this, is that you need to amplify a mic, and it's noise to use it.  Often quiet mics have low outputs, so need to be amplified more, and the real test of a mic pre's EIN is not at 60dB gain, getting an EIN of better than -124dB at gains around the 20-40dB.
> Hope this helps.
> Kind Regards
> Crispin Herrod-Taylor
> Managing Director, Crookwood
> www.crookwood.com
> Tel: +44 (0)1672 811 649
> Mobile:+44(0)7910 637 634
> Sign up for our great newsletter here! and keep up to date with the audio world
> -----Original Message-----
> From: ProAudio <proaudio-bounces at bach.pgm.com> On Behalf Of Dan Lavry via ProAudio
> Sent: 11 June 2021 20:44
> To: proaudio at bach.pgm.com
> Subject: Re: [ProAudio] Microphones question
> My question is about mic output impedance, in relation to noise:
> Both the mic and the micpre contribute to noise. The micpre generates some noise voltage which can be measured by replacing the mic with a short (0 Ohm). But there is also mipre generated noise current, which is no problem for 0 Ohm, but real mics have some impedance...
> At some point, it was decided to model a mic noise with replacing the mic with 150 Ohm resistor.  I am not proposing to change it, just trying to understand why 150 Ohm.
> The value 150 Ohm makes 1.568nV/sqrtHz (at room temp), so for 20H-20KHz noise voltage of .225uV. Given that we are interested in noise power, we can use the dBu scale to realize that the resistor itself sets a limit on the noise floor at -130.9dBu. But say the impedance is 1K, then we have -122.8dBu.
> I assume that the resistor modeling is a simplification. I would be interested in comments from the mic experts here.
> Thank You
> Dan Lavry
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