>cmursic wrote:
>> Thank you, Walter, for a very erudite explanation. As I suspected,
>> such measurements are beyond my means. But I'll try the
>> quick "pillow" method you describe with my Rode NT4 and see what I
>> get.
>
>It would be a waste of money to set all that up unless you were a
>manufacturer or such like. In the case of the NT4, the specs include
>what they got.
>
>The pillow method will get you most all you really need. Once you have a
>mic the published specs get to be a lot less important than what you
>find just using it.
>
>I'll be interested to know what sort of sound the NT4's self noise is.
>That's something it's very hard to get info on.
>
>Walt
>
>
>
Hi cmursic--
Great explanation Walt! It's going to the FieldMicList archive!
To accelerate what you can learn about self noise, get at least one
other mic with a known noise rating, two more are even better. Go
outside when the ambience sound level is low like in the middle of a
windless night (~30-40 dB here in Milwaukee) and record just the air
for about 15 minutes each. Set mic record gain at max because the
noise introduced by the preamp is constant and hi to full gain is
when the noise problems really show. Transfer them all to digital
files, listen to them back to back and perhaps use "find peak," to
measure the relative saturation levels. Some mics are a lot hotter
and this is good to note. Then make copies of the files, pick a
section from each when the sound level is the lowest and crop and
normalize this section in all three files to -20dB and play them back
on your normal, reference monitor speakers at comfortable room level.
You'll have no trouble hearing differences. The broadband width
"air"also reveals the transparency-- how much you can hear into the
space.
from earlier in the string, Walt wrote:
>A desirable sound for self noise
>would be a smooth hiss, but some mics sputter or crackle. With a smooth
>hiss, the self noise can actually be audible and not upset the listener.
Very true. Narrow bands right around 8K are pretty common. Use a
narrow peak or notch parametric equalizer to find these spots in all
three files. These can be removed effectively with successive very
narrow notches-- avoid a fat single notch if the actual noise is in a
narrow band (~200-500Hz wide). The sputtering/uneven stuff from some
mics falls down to 3K and is quite hard to remove without really
making the recording sound dull. The Rode NT3's I've used have some
sputtering noises in this range and they're rated at 16dB(A)*. I've
seen some mics with curious narrow noise bands as low as 4700 Hz.
I'm quite sure self noise affects transparency in the low range too
when field saturation levels in the recordings are low (~<6%). Noise
in the low range makes its hard to balance the acoustic space
realistically, especially in relation to individual sounds in the
lower mid range like the human voice. Lots of folks roll-off the
bottom end but the space pretty much disappears and its an important
part of the field experience. I recently found that I could
significantly boost the 125-600 Hz range in a recording made in a
quiet location to make a tinny frog chorus sound more natural and
less harsh on the ears. The boost added warm "air" to recordings
when made with mics in the 16dB(A) self noise range, and worked
really well with some mkh-20 recordings too (10dBA). But when I
tried the boost with recordings made with my Crown PZM's (23dBA), it
made instant grunge.
The hiss does jump out first, but if you compare clarity at other
frequencies, you may notice this too.
Rob D.
* The sensitivty of the Rode NT3 is 12dB(A), about 4dB less than
some other mics with the same self noise rating I regularly use.
Whenever the record gain is high, its easy to hear this difference in
sensitivity because its translates into 4dB? of additional noise when
the sound is played back at final mix level.
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