I'm relatively new to recording outdoor ambient sounds, but am very intrigu=
ed by it. I have done plenty of audio recording (amateur), but also have a=
background in astroimaging. I have been trying to solve the noise floor p=
roblem with microphones, as it seems to bug everyone to some degree. It ju=
st seems difficult and expensive to get rid of it. So that got me thinking=
.
For you audio engineers out there, please tell me if this is solid thinking=
.
When I take an astrophotograph, our exposure times are very long. The chip=
suffers from electronic noise, not unlike mic noise. To get rid of it, we=
"calibrate" our images. One of the things we do is to take a "dark frame"=
of equal length, at the same temperature, and same degree of thermoelectri=
c cooling of the chip. Then we mathematically subtract the noise from the =
image and the image improves dramatically.
As I was cycling through the countryside today it struck me that might be p=
ossible with microphone noise. So I tried it. I did some more recording o=
f my test subject (for now), the pond at our farm. I got a clip of a bird =
chirping (my wife thinks it's a warbler, I'm not an expert on bird calls, s=
o and ID would be appreciated).
I then recorded a 10 second clip with the microphones at the same gain in a=
s quiet conditions as I could find in my study (full of books and very quie=
t). I used that as my "noise floor sample".
Then I imported the audio into Audacity (for simplicity) and first amplifie=
d the bird call sample a fixed amount (19db in this case). I then amplifie=
d the noise floor sample by the same amount. I then did a high pass filter=
to get rid of a distant train passing (troublesome buggers).
Then I copied the clip with and without the noise reduction. In audacity, =
you first take a "noise sample reference", then apply that reference to the=
redording. Obviously, I used the noise sample and applied it to the recor=
ding of the bird. The first half of his mp3 is without noise reduction, ju=
st the 19db amplification and hipass filter. The second half is with noise=
reduction.
I think it worked.
If there is a better way of doing this, let me know.
The equipment used here is a "matched" pair of Rode NT1-A's with foam wind =
guards, into a Sound Devices 702. The gain was set at 61.8db both for the =
recording and for the noise sample as I felt this needed to be exactly the =
same for both.
Here is the comparison sample:
http://m57.net/downloads/sfp/warblertest.mp3
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