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Re: Microphone Noise Floor Problem

Subject: Re: Microphone Noise Floor Problem
From: "Rob Danielson" danielson_audio
Date: Sat Sep 12, 2009 11:52 am ((PDT))
At 4:36 PM +0000 9/12/09, jmccubbinmd wrote:
>  I'm relatively new to recording outdoor ambient sounds, but am very
>intrigued 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 problem with microphones, as it seems to bug
>everyone to some degree. It just 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 thinkin=
g.
>
>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 thermoelectric 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 possible with microphone noise. So I tried it. I did some
>more recording of 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, so and ID would be appreciated).
>
>I then recorded a 10 second clip with the microphones at the same
>gain in as quiet conditions as I could find in my study (full of
>books and very quiet). I used that as my "noise floor sample".
>
>Then I imported the audio into Audacity (for simplicity) and first
>amplified the bird call sample a fixed amount (19db in this case). I
>then amplified 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 recording of the bird. The first half of his mp3
>is without noise reduction, just 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>http://m57.net/downloads/sfp=
/warblertest.mp3


Hi JM--
Fun analogy to think about. Are there some astrophotographers who
insist they are documenting "space" opposed to only identifiable
objects? "Noise" is a very rich construction when I think of it as an
overlap between perceptual and technical limitations.

Here's a QuickTime movie experiment I did with your recording and
noise reduction examples:
http://tinyurl.com/okwpc6

If you have trouble viewing it, I can make an mp3.

Rob D.



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