> > I agree; the 5 dB figure may apply to urban interior noise, which
> > has a lot of traffic rumble.
>
> The noise figures which we are discussing as "excess noise" are
> relative to the fundamental thermal noise of the air. This rises at
> the rate of 3dB per octave so, in comparison, low frequency noise is
> much greater than low frequency thermal noise. Fortunately our ears
> are less sensitive to LF at low levels.
But as long as we look at the EIN specifications of the preamp, we don't ha=
ve to care about the microphone and the thermal noise of the air!
Keep in mind that the EIN figure is just a noise VOLTAGE that is measured i=
n volts or dBu and it alone has nothing to do with sound levels, phones or =
the thermal movement of air molecules.
> This is why weighting is important in all noise measurements. I
> haven't concentrated on the source of the 5dB figure but this is
> probabably about right with A weighting.
I did that and when I apply an A-weighting filter to a typical band-limited=
(20 Hz to 20 kHz) noise floor of a good preamp (again, please note that th=
ere is still no air involved at this stage), then I get only a difference o=
f about 2 to 3 dB!
This difference can also be found in the specifications of the SoundDevices=
recorders:
http://www.sounddevices.com/products/702/specs/
Equivalent Input Noise:
Mic: -130 dBu max (-132 dBV), 150 ohm source, A-weighted filter
Mic: -128 dBu max (-130 dBV), 150 ohm source, 20 Hz=9620 kHz BW flat filter=
, gain fully up
I agree that the spectral properties of the microphone self noise would be =
a completely different story. Bye the way, some microphones exhibit a self =
noise spectrum that is even different from what you described above. Look f=
or instance the Sennheiser MKH series that is adopted to the human hearing =
curve (they exhibit an increased floor at frequencies above 10 kHz). Severa=
l years ago I created a chart that illustrates this:
http://www.avisoft.com/test/noisefloors.pdf
When recording in the field, then we would also experience other effects su=
ch as the atmospheric attenuation of the sound depending of the signal freq=
uencies and distance of the sound sources. That's the reason why we often h=
ave that nasty low-frequency traffic rumble. But I don't think that these e=
ffects should be taken into account for judging the noise performance of a =
microphone/recorder combination.
Regards,
raimund
> Going back to a related subject, adding noise at -4dB to a signal at
> 0dB raises the level by 1dB, which is just about what we could detect
> on a meter. However to the ear, adding hiss to a rumble will be very
> noticeable and this is only one instance when the theory breaks down.
>
> David
>
> David Brinicombe
> North Devon, UK
> Cogito cogito ergo cogito sum - Ambrose Bierce
>
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