--- In Rob Danielson <>
wrote:
>Part of our difference in whether mic sensitivity is a factor could
>be our usual recording environments/goals. When isolating a bird
call
>with a mkh-60 or parabola, the noise introduced by sensitivity is
>usually not audible in the sound file. The saturation of=A0 the
>sustained body of the call might be 20-40% with percussive peaks in
>the 60%-100% range. To contrast, in the middle of the night in a
>rural setting, with an MP-2 cranked max and mkh-20's (10dB[A]
noise),
>the noise floor of the forest will produce 6% sound file saturation
>with distant effects reaching ~12%. The later file has to be
boosted
>a great deal digitially to be heard upon playback and is a demanding
>test of noise, from any source, including bit depth and resolution.
Hi Rob,
I agree, that such quiet ambient recording situations are much more
demanding than recordings of relatively loud calling animals at
close distances. However, I believe, that it does not make much
sense to amplify extremely soft sounds artificially for playback.
You should listen to these recordings also in a quiet environment
(e.g. using headphones) at low sound levels with the playback volume
adjusted not much louder than at the original recording site. You
could improve the subjective quality of a recording, if you placed
the microphone as close as possible to one or two foreground sound
sources (e.g. a calling owl or something else) in order to increase
the peak sound level (and the dynamic range). This would also make
the situation more interesting to the listener. Another trick is to
fade in noisy recording slowly. This would allow our ears getting
used to the noise.
The logarithmic sound level scale is based on the dynamic range of
our human auditory system. 0 dB corresponds to the lowest sounds
that we are able to perceive. A microphone noise floor of 10 or 5 dB
would be very close to our hearing threshold. However, when
considering these few remaining dB's, mother nature still seems to
have the more advanced know how in designing low-noise
omnidirectional 'microphones' than any other human microphone
manufacturer ;-).
As Walt pointed out recently, the absolute sound levels in natural
environments are usually very low. Imagine, you recorded a soft
sound, lets say with a peak amplitude of 40 dB by using a microphone
having a self noise level of 10 dB. When you then playback that
recording at a higher volume in your living room (e.g. 20 dB louder
than at the recording site), the original microphone noise of 10 dB
would be reproduced at 30 dB (10 + 20) and the peak sound level at
60 dB (40 +20). The amplified microphone noise of 30 dB can then be
very annoying. However, if the original sound level at the recording
site were louder (say 60 dB), and you set the playback volume also
to 60 dB (20 dB less gain compared to the first example), so that
you have the same sound level in your living room as at the original
recording site, the microphone noise would be reproduced at 10 dB
only. That soft noise would be nearly inaudible. Therefore, the
subjective quality of a field recording would be better if the
original absolute sound level at the microphone was higher.
This seems to be the case in Evert's recordings he made with the
SoundMan microphones. The ambient sound level in a train is usually
higher than in a quiet natural environment. Additionally, the broad
noise spectrum in a train will mask out the microphone noise (by
psycho-acoustic effects).
MIC DIY mics:
>The MP-2 spec
>says that the input noise is -128 dBV so that the self noise of the
>mic is 12 dB above the noise of the channel. This isn't as big a
>difference as I expected but if you take a noise signal you won't
>hear much difference at all between it and it added to a noise
>source 12 dB lower so that it is safe to say that the mic self
>noise, low as it is, will mask the noise of the MP-2.
It should be noted, that two independent noise sources (the mic
noise and the pre-amp noise) will add geometrically. This means,
that the sound levels of the two noise components (expressed
linearly in Pa) will not simply add. Instead, the resulting sound
level of the two noise components will be less than the sum of the
two. For those who are familiar with this kind of math - it is the
root mean square:
c =3D sqrt (sqr(a) + sqr(b)) where a and b are the two noise
components and c is the resulting noise level.
When comparing mic noise and pre-amp noise, one should also
consider, that the condenser microphones we are talking about
already include some kind of amplifiers. A classic condenser
microphone requires at least one FET transistor or a tube to convert
the high impedance of the condenser element to the low impedance of
the output connector. The Sennheiser MKH series microphones use
instead as special HF circuit to replace the noisier FET transistors
by normal bipolar transistors. Actually, the condenser element
modulates the frequency of an oscillator running at about 2 MHz (if
I remember correctly), which is demodulated subsequently (similar to
a FM radio receiver). All the transistors in that circuit have
similar properties to those employed in a pre-amplifier. Therefore,
the transistors of the microphone represent the very first stage in
a chain of several amplifiers. Generally, the total noise floor of a
compound amplifier is mainly determined by it's first stage (the
microphone in this case).
>Sensitivity will not play a role with the very good mic
>pres, but the pres most field recordists use don't fall into this
>category. MD manufacturer's, for example, don't publish noise specs
>on the mic preamps of their consumer recorders. The mic pres in the
>TASCAM DAP1's=A0 I frequently borrow are not as quiet as the MP-2 and
>they do start to introduce noise when cranked with mkh-20's to my
>ears. I've not seen useful noise specs for the TASCAM either.
Yes, there are differences between the various recording gear.
Especially in small portable consumer recorders, the manufactures do
not use the best available electronic parts. There is a tradeoff
between noise performance, supply current and price in the available
electronic parts.
Regards,
Raimund
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