Hi David,
I had sort of figured that out sitting here thinking about it. So EIN is no=
t the way to talk about it. But I do note that when I engage the -20 dB sen=
sitivity switch on my H2, the H2 noise level drops by 20 dB. If I didn't ha=
ve an external preamp, the signal would also drop by 20 dB. but with an ext=
ernal preamp providing the gain, now the recorder noise has dropped but the=
signal level is still high.
What I am still unsure about is exactly how to quantify this. How to calcul=
ate the amount of noise at the end of the chain when you have microphone ou=
tput noise, preamp 1 EIN and preamp 2 EIN.
I mean, if one just uses the EIN for each preamp in the calculation, there =
appears to be no advantage to using an external preamp with lower noise and=
feeding it into a recorder preamp with higher noise. Yet I can see by prac=
tical application, that if I feed a quieter preamp into my Zoom H2, and I e=
ngage the -20 dB pad on the H2, I get a much better result than if I feed t=
he same mic directly into the H2 without the -20 dB pad.
It appears to me that the numbers give an accurate prediction of the result=
if I pretend that the EIN of the H2 is now -120 dB instead of being -100 d=
B.
But you seem to be saying that is wrong, so what is the correct way to calc=
ulate the total noise at the end of this chain?
Am I making any sense?
John
--- In "Avocet" wrote:
>
> > For example, if your H4n has an EIN of -107 (at full gain), and
> > adding your preamp to the chain allows you to reduce the gain on the
> > H4n by 35 dB, does that mean the EIN of the H4n has now been reduced
> > to -142 dB? I'm really not sure I am understanding this correctly!
>
> John,
>
> No. The Equivalent Input Noise takes any gain setting into account.
> Another way to define EIN is the input level which would increase the
> audible noise by 3 dB (twice the energy level)
>
> The measured EIN should stay constant whatever the gain settings, but
> if not, the lowest EIN should be that measured at high gain. If EIN
> goes up with gain, there is extra noise in the recorder. This sounds
> odd but you subtract the gain, so the EIN should stay the same.
>
> Another way of looking at this is putting a 10dB attenuator on the
> input. The noise level will be the same, but relative to the overall
> gain the EIN will be 10dB higher.
>
> The EIN of a line input could be quite high, but that would normally
> be swamped by the higher input level going into the line input. What
> the Telinga preamp does with PIP inputs is to swamp any recorder EIN
> which can be quite high with "affordable" recorders.
>
> What makes EIN difficult to pin down is different noise frequency
> profiles, for instance adding LF noise to HF noise. That is why
> weighting is used to make measurementrs more standard.
>
> Hope this helps.
>
> David
>
> David Brinicombe
> North Devon, UK
> Cogito cogito ergo cogito sum - Ambrose Bierce
>
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