> No. Noise-removing algorithms damage the sound irreparably. There
> are of use in forensics, and in constructing mixes for film, where
> destroying the ambience of a particular clip doesn't matter because
> another ambience is being mixed in.
I'm going to partially disagree with Dan on this. I would distinguish
between "noise removal" and "noise reduction" in particular hiss
reduction. Wideband noise removal destroys recordings, but I use hiss
reduction.
> The way to avoid hiss is to get the mics closer to the source, or
> use quieter mics.
Sometimes it's very quite here, and on woodland recordings, I
can hear Sennheiser hiss and that's on top rank mics.
With the introduction of digital, it has become the norm to remove all
background sounds before and after a music recording. No longer do you
hear the audience hush anticipating the conductor raising the baton,
and on studio recordings, the musicians apparently do not breathe.
This ruins the recording for me by dehumanising it.
An early and still common form of noise removal is a noise gate or
expander (reverse compressor), which drops the level where it falls to
30dB or 25dB below peak, producing a nastypumping effect. Often it
makes a voice track less audible, but again it has become the fashion.
I will now make out my argument for noise *reduction*, especially hiss
reduction which should retain the feel of the rest of the recording
and remain inaudible. It was Audacity 1.3 which made this possible for
me.
What the misnamed Audacity "Noise Removal" does is to use a sample you
provide to subtract the level of the sample sound frequencies by
whatever you choose. I normally reduce by 6dB, and never more than
Message: 9dB.
Subject:
What you need to avoid is audible pumping and generating artifacts.
Try making a copy of broadband noise at a low bit rate MP3, and you
will hear the burbling or tinkling of digital artifacts.
The following is the hiss reduction I sometimes use. Sticking to HF
hiss, any artifacts get lost as low level hiss. Noise reduction does
reduce
reverberation but there is less reverb at high frequencies anyway due
to absorption,
Audacity 1.3
Effects - Noise Removal. (Used for noise reduction)
1) Do any filtering like bass roll-off first. The less you
have in the lower frequen4cies the better. Filtering out the
HF hiss will lose valuable harmonics which are preserved
with hiss reduction.
2) Select the quietist part of the recording which should
contain only background noise, If there are odd tweets or
calls in the sample, trim them out. You need about a second
or so as a minimum sample.
3) With a stereo track, check the levels and if they differ
by more than 2dB, adjust so these are equal on the sample.
4) Filter the sample with a high pass Equalization filter like:
<curve name=3D"HPF4000-3-6-9hiss">
<point f=3D"135.000000000000" d=3D"-35.000000000000"/>
<point f=3D"250.000000000000" d=3D"-27.000000000000"/>
<point f=3D"500.000000000000" d=3D"-18.000000000000"/>
<point f=3D"1000.000000000000" d=3D"-9.000000000000"/>
<point f=3D"2000.000000000000" d=3D"-3.000000000000"/>
<point f=3D"4000.000000000001" d=3D"0.000000000000"/>
<point f=3D"22049.000000000011" d=3D"0.000000000000"/>
</curve>
If you are not into editing
\Roaming\Audacity\EQCurves.xml
then just copy the figures into Equalization.
Note the rounded-off curve which avoids "ringing". If you don;t edit
5) Effect - Noise Removal - Get Noise Profile
6) Ctrl-Z-Z-Z back to 1) after any bass roll-off and reapply any
stereo level adjustment.
7) Effect - Noise Removal
Noise Reduction 6 (max 9)
Frequency smoothing 500 (reduces LF artifacts)
Attack/decay time 1 sec (not critical)
OK
8) restore original stereo levels if moved.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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