Actually -131 dBm. I guess that i should a calculator for such simple
calculations as -174+43
.
The noise level depends on the resistance of the transducer, the figure
you have given appears to correspond to 15Kohms. Most microphones are
in the range 50 to 600 ohms with thermal noise levels of -167 to -156
dBm (BW=20Kc/s, Tamb=20C)
Both the (loaded) noise voltage and noise current depends on the
source resistance, but the noise power is simply P=kTB regardless of
source impedance.[/QUOTE]
Thats OK if you express the noise level in dB relative to 1 milliwatt,
which gives the same figure regardless of the resistance. If you use a
50-ohm microphone and express the noise level in dBm, the calculation
becomes much more complicated because dBm refers to 1 milliwatt into 600
ohms and is a voltage-based measurement.
I think you have to calculate the voltage across 50 ohms which
corresponds to the noise power, then work out what power that would give
if it were across a 600-ohm resistor. Then that power is compared to 1
milliwatt using a decibel scale.
The LF noise is the biggest problem as far as measurement is concerned,
[...]
At least in older op-amps, the 1/f noise level dropped to the white
noise (constant noise density) levels somewhere between 1-10 kHz.
I don't know if it is still true, but it used to be said that Nuvistors
were better than any semiconductor device for low LF noise. They were
particularly useful for the front ends of electroencephalograph machines
and for low jitter V.F.O.s. at R.F.