Tobiah wrote:
It would seem that a dynamic mic has no self-noise, is that so?
Yet to get the same gain as a condenser one has to crank the gain
up much more on the dynamic, raising the noise level. Is it ever
the case that when recording soft sounds, one could get a lower
noise recording with a dynamic? Would it take a very expensive
preamp to achieve this?
There are two sources of noise in a moving coil microphone:
1) The resistance of the coil itself at room temperature generates
"Johnson Noise" due to movement of the electrons. Microphones with
stronger magnetic fields will have better sensitivity and will give a
better signal-to-thermal-noise ratio, so the sensitivity figures are
important when considering this type of noise.
2) The impact of the air molecules on the diaphragm (Brownian Motion)
will give shot noise. Each time the area of the diaphragm is doubled,
the wanted signal, which is in phase across the whole diaphragm, should
increase by 6dB, but the shot noise is not in phase across the
diaphragm, so it only increases by 3dB. For every doubling of the
diaphragm area there is theoretically an improvement of 3dB in the
signal to shot noise ratio. So a large diaphragm is important when
considering this type of noise.
In a well-designed microphone, these two sources of noise are
approximately equal; there is no point in spending a lot of design
effort decreasing one of them if the other one is going to predominate.
The noise of the pre-amp should be negligible when compared with these
two sources of microphone noise. Most modern pre-amps will achieve this
as long as they are correctly matched to extract the maximum power from
the microphone.
--
~ Adrian Tuddenham ~
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