>> I also added calculations so that if one knows any two of the
>> parameters one can calculate the third.
>
>> 1) EIN (dBu)
>> 2) Self Noise (dB SPL)
>> 3) Sensitivity (mV/Pa)
>
> Yes, these are comparable but only if the base definitions are the
> same. I have seen different reference levels used by different
> manufacturers which doesn't help with a global comparison of mics.
The references are quoted above. I prefer sensitivity quoted in dBu for 1 Pa,
because then everything is in dB.
> 1) EIN is the sound level entering the mic which would be equivalent
> to the total noises generated internally by the mic. Put another way,
> it is the acoustic sound level which increases the mic output by 3dB
> over the mic noise level. (The dB addition of equal random sounds.)
>
> EIN provides, or should provide, a comparison between the internal
> noise generated by different mics, irrespective of how they are
> connected or the volume setting of a recorder.
Sorry, David, but EIN has nothing to do with microphones. It's the noise of the
mic preamp when terminated by the specified source impedance (150 ohms). The
Rane article follows.
-Dan
EIN. Equivalent Input Noise or Input Referred Noise
What is tested? Equivalent input noise, or input referred noise, is how noise
is spec'd on mixing consoles, standalone mic preamps and other signal
processing units with mic inputs. The problem in measuring mixing consoles (and
all mic preamps) is knowing ahead of time how much gain is going to be used.
The mic stage itself is the dominant noise generator; therefore, the output
noise is almost totally determined by the amount of gain: turn the gain up, and
the output noise goes up accordingly. Thus, the EIN is the amount of noise
added to the input signal. Both are then amplified to obtain the final output
signal.
For example, say your mixer has an EIN of -130 dBu. This means the noise is 130
dB below a reference point of 0.775 volts (0 dBu). If your microphone puts out,
say, -50 dBu under normal conditions, then the S/N at the input to the mic
preamp is 80 dB (i.e., the added noise is 80 dB below the input signal). This
is uniquely determined by the magnitude of the input signal and the EIN. From
here on out, turning up the gain increases both the signal and the noise by the
same amount.
How is it measured? With the gain set for maximum and the input terminated with
the expected source impedance, the output noise is measured with an rms
voltmeter fitted with a bandwidth or weighting filter.
Required Conditions. This is a spec where test conditions are critical. It is
very easy to deceive without them. Since high-gain mic stages greatly amplify
source noise, the terminating input resistance must be stated. Two equally
quiet inputs will measure vastly different if not using the identical input
impedance. The standard source impedance is 150 ohms. As unintuitive as it may
be, a plain resistor, hooked up to nothing, generates noise, and the larger the
resistor value the greater the noise. It is called thermal noise or Johnson
noise (after its discoverer J. B. Johnson, in 1928) and results from the motion
of electron charge of the atoms making up the resistor. All that moving about
is called thermal agitation (caused by heat -- the hotter the resistor, the
noisier).
The input terminating resistor defines the lower limit of noise performance. In
use, a mic stage cannot be quieter than the source. A trick which unscrupulous
manufacturers may use is to spec their mic stage with the input shorted -- a
big no-no, since it does not represent the real performance of the preamp.
The next biggie in spec'ing the EIN of mic stages is bandwidth. This same
thermal noise limit of the input terminating resistance is a strong function of
measurement bandwidth. For example, the noise voltage generated by the standard
150 ohm input resistor, measured over a bandwidth of 20 kHz (and room
temperature) is -131 dBu, i.e., you cannot have an operating mic stage, with a
150 ohm source, quieter than -131 dBu. However, if you use only a 10 kHz
bandwidth, then the noise drops to -134 dBu, a big 3 dB improvement. (For those
paying close attention: it is not 6 dB like you might expect since the
bandwidth is half. It is a square root function, so it is reduced by the square
root of one-half, or 0.707, which is 3 dB less).
Since the measured output noise is such a strong function of bandwidth and
gain, it is recommended to use no weighting filters. They only complicate
comparison among manufacturers. Remember: if a manufacturer's reported EIN
seems too good to be true, look for the details. They may not be lying, only
using favorable conditions to deceive.
Correct: EIN = -130 dBu, 22 kHz BW, max gain, Rs = 150 ohms
Wrong: EIN = -130 dBu
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