> I remember coming across a conversation a while back (but cannot locate i=
t again) about the =EF=BF=BDreal world=EF=BF=BD self noise of the BP4025 be=
ing much lower than Audio Technica quotes in the specs (14dB). Certainly, m=
y own experience with this mic is that I never hear any sign of noise, even=
at high gain.
Jamie,
If you can't hear mic noise, then your system is not optimised. As I've
written before, mic noise plus the thermal noise of the air should be the
highest audible noise in any system used for recording quiet sounds.
What makes noise difficult to pin down is the sensitivity with frequency of=
the ear at low levels and standard weightings do not give a good account of=
this. The best is ITU-R 468 and derivatives of this which match low sound
level hearing. The often quoted "A" weighting is based on much louder sound=
s
at 40dB SPL which is where many wildlife sounds occur. Noise levels have to=
be well below this where the hearing curves are different.
Just for the record, a mic noise figure of 14dB means that the weighted mic=
noise is 14dB higher than the thermal noise of air molecules. The loweat
possible figure is 3dB as air molecules are bouncing off both sides of the=
mic diaphragm. The quietist mic is claimed by B&K, the sound level
specialists, at 5dB excess noise, and I wouldn't believe any other claim fo=
r
a similar mic noise level.
The final check is with your ears, having raised the playback level well
above normal. Noise will also show up on a spectrograph. Comparing the mic=
with a dummy input of about 150 ohms is the key test. I sometimes double
check HF noise by slowing down a sample to a quarter speed as my old ears
start to fade out at 11 to 12 KHz. If the mic doesn't introduce hiss, then=
the system is not optimised. End of.
David Brinicombe
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