Klas,
The information to do so is out there, on Raimund's site for example.
I've been using an excel spreadsheet I put together several years ago to do=
what you are describing. I mentioned on the list at the time but the contr=
ibution was dismissed as worthless by at least one of the list guru's.
As it's a while since I did this I'm a bit foggy on the conversions, but b=
asically what I did was to calculate the output noise based on self-noise a=
nd sensitivity using this calculation:
MicOutputNoise =3D20*LOG(((Sensitivity/1000)/0.775))-94+Self_noise
I'm not a math guru so dB units(U or A) are possibly wrong but this gives a=
virtually identical result to the RANE table that everyone refers to.
Then convert the dB figure to a voltage:
MicOutputNoise_uV =3D0.775*10^(MicOutputNoise/20)
PreampEIN_uV =3D0.775*10^(PreampEIN/20)
"According to Raimund's site "It is important to note that both noise compo=
nents add geometrically (RMS)." The illustration on the same page (http://w=
ww.avisoft.com/tutorial_mic_recorder.htm) shows this means finding the valu=
e of the hypotenuse (c) using a^2+b^2=3Dc^2. So take the square root of th=
e summed squares of the two voltages to find "c":
SystemNoise_uV =3DSQRT(MicOutputNoise_uV^2+PreampEIN_uV^2)
Then convert the result back to dB
SystemNoise_dB=3D20*LOG(TotalSystemNoise_uV/0.775,10)
The difference between the SystemNoise and Preamp EIN in dB indicates how m=
uch the system noise floor is degraded by the preamp.
Using your 10dB(A) and 10mV/Pa example the calculated MicOutputNoise is aro=
und -121.79 dB(A).
The two most common rules of thumb are either 6dB or 10dB margin between Mi=
c Output Noise and Preamp EIN.
For 6dB margin the minimum acceptable preamp EIN is -127.8dBu
This gives a System Noise figure of -120.82dB(A), which is 0.97dB(A) higher=
than the Noise Output of the Mic.
For 10dB margin the minimum acceptable preamp EIN is -131.8dBu
This gives a System Noise figure of -121.37dB(A), which is 0.41dB(A) higher=
than the Noise Output of the Mic.
If you look at a combination like MKH20 and SD702, the margin between EIN =
and Output Noise is around 15dB, and System Noise is only 0.15dB higher tha=
n the Mic Output.
This illustrates, as Raimund observes, that ALL preamps add some noise. Th=
e goal is to limit the noise increase to an acceptable minimum.
The other complication is that A weight doesn't accurately measure the kind=
of electronic noise we hear in recording gear. Short term repetitive noise=
eludes the weighting algorithm for example.
I doubt that really helps with your request for a three-dee representation =
but that is my understanding of the interaction. Someone with a deeper unde=
rstanding of the math might be able to explain better//more clearly?
cheers
Paul
On 19/12/2012, at 9:37 AM, Klas Strandberg <> wrote:
> Dan, many of the topics that go in circles here, and has for years,
> concern the relationship between 3 parameters:
> 1/ Input noise (Measured EIN??)
> 2/ Microphone output voltage (measured mV/Pa??)
> 3/ Microphone noise (measured dB(A)??)
>
> I wish that someone with a good enough mathematical skill would make
> a 3D diagram describing these relationships, perhaps also a computer
> program where you fill in 2 of the parameters and get the third.
>
> A IRL example: If I want to buy a microphone with a self noise of 10
> dB(A) and a output of 10mV/Pa - how good (EIN) must the recorder
> perform, not to add noise?
>
> I don't think such a formula would be used a lot in such IRL matters,
> but I do think that a 3D diagram would have a great value to help
> understanding.
> (I am well aware of that such a diagram would disregard impedance and
> a few other factors, I still think it would have a great teaching value.)
>
> Or is there already such a diagram somewhere? Link?
> I have never seen any.
>
> Klas.
>
>
> At 07:07 2012-12-17, you wrote:
>>> Still, I think the "unclarity" resembles the debate decades ago, when
>>> there was a lot of talk about impedance matching, preamps and
>>> transformers. Some combinations between dynamic mic=B4s, input
>>> transformers and transistors worked, others did not, and noone
>>> actually knew why, even though "everybody" said they did.
>>
>>
>> The frequency response and output level of a mic will change
>> depending on what kind of load it's driving. An understanding of the
>> history of audio connections might help to explain current practice.
>>
>> The original audio craft was telephony. The talker's mic had to
>> produce enough power to drive the receiver at the other end, with no
>> amplification involved. Experience taught that impedance matching
>> carried the maximum power from one point to another. Pro audio
>> adopted the practices of telephony. Thus line outputs had 600 ohm
>> impedances, and inputs 600 ohms too, "matching" and "terminating."
>>
>> Very long audio lines, meaning miles, still use terminating
>> impedances to prevent the signal from being reflected back. Very
>> high frequency lines, like video and digital audio, use terminating
>> impedances even on short cables, for the same reason.
>>
>> Where there was a "bus" that had to drive several loads, the concept
>> of "bridging" developed, tapping the voltage off a line without
>> loading it. Whereas a terminating input was 600 ohms, a bridging
>> input was perhaps 15 Kohms.
>>
>> A generation later, it was acknowledged that maximum voltage
>> transfer was a better goal than maximum power transfer, and all
>> connections gradually became bridging. In this practice, it's best
>> to have the lowest possible impedance source and a high impedance load.
>>
>> A practical professional microphone circuit has a source impedance
>> of from 50 to 200 ohms, and a load (preamp input impedance) of 2000
>> ohms or more. The factor of ten between source and load insures that
>> the voltage of the source is minimally reduced by the load, i.e.
>> less than 1 dB of loading effect. I dare say all mics sound their
>> best when bridged.
>>
>> PIP mics are a higher impedance source than the balanced
>> professional mics. They are typically 2 to 3 K ohms. That means
>> ideally input impedances of 20 or 30 K ohms. But there are practical
>> limitations to how high the input impedance of a preamp can be
>> without increasing noise, and I imagine that they are generally
>> lower than that. Perhaps someone has measured the actual input
>> impedances of some PIP mic preamp inputs.
>>
>> -Dan
>>
>> ------------------------------------
>>
>> "While a picture is worth a thousand words, a
>> sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause=
.
>>
>> Yahoo! Groups Links
>>
>>
>>
> Telinga Microphones, Botarbo,
> S-748 96 Tobo, Sweden.
> Phone & fax int + 295 310 01
> email:
> website: www.telinga.com
>
>
>
>
>
> ------------------------------------
>
> "While a picture is worth a thousand words, a
> sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
>
> Yahoo! Groups Links
>
>
>
|