> What exactly is the thermal noise level of the air ( I would suppose it w=
ould vary with temperature) and how is it measured and defined?
Greg,
It is the noise generated by vibrating air molecules at normal temperatures=
.
It is about the zero phons level, but what makes that difficult to define i=
s
that the level rises with frequency at 6dB per octave. This is why weightin=
g
is used. Weighting curves go through zero at 1kHz, and mic hiss gets
annoying at higher frequencies which is a problem when defining noise.
A problem with A-weighting is that this is based on a 40phon human hearing=
response, not zero phons. A more accurate noise weighing is the ITU-R 468
noise weighting which is better matched to the ear at noise levels but it
peaks at around +12dB which gives "poorer" figures for manufacturer's sales=
blurb.
Quality mic manufacturers often give much "poorer" figures than those given=
for cheap mics because they are measuring something different. I've seen
excess noise figures based on the mic capsule being disconnected. They
sometimes get away with this by calling it "self noise".
The lowest possible excess noise mic figure is 3dBs caused by air vibrating=
on the rear of the mic diaphragm as well. The record mic excess noise
claimed by Bruel & Kjaer is 5dB, but for practical mic figures look at
manufacturers like Sennheiser.
Thermal noise varies with temperature above 0Kelvin or -273C, but in
practice this variation is small at normal temperatures. Temperature also
varies preamp thermal noise so this mostly cancels out.
> Then how can the air 'noise level' be specified relative to the standard =
for used for microphones?
It can be calculated theoretically, but unless a manufacturer defines the
base level it uses and the weighting curve used, they can make this up.
Sometimes 0phons is used as a base level with no justification. You can't
make comparisons using different methods of defining the noise measurement=
method.
> Is it equivalent (nominally) to a mic with a self-noise level of 1 dB or =
-3.2 dB or what?
There is no standard specification for "self noise". Some mics can get hiss=
y
when hot. Disconnecting a capsule is meaningless as you want to know the
noise level in its useable state. If you connect a passive capacitor in
place of the mic capsule, that also has its own thermal noise, but if you
leave it open circuit, nothing is comparable. Excess noise over the thermal=
noise level of the air and over the whole recording system, and weighted, i=
s
what counts.
If a spec is too good to be true, ask why. :-)
With a very quiet background or under a pile of bedclothes, you can
sometimes see, on a spectrogram, a rising level in the higher kilohertz. If=
you are lucky and with an excellent mic, this is the fundamental thermal ai=
r
noise rising at 6dB per octave. If you can see this in your recordings, you=
are doing well.
David Brinicombe
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