Posted by: "Donald Berk"
>
> Thanks, Danny. Is there a discussion somewhere on shotgun vs.
> parabolic mics?
That's practically a religious war sometimes.
Shotgun mics are a mic inside a interference tube to narrow it's field
of pickup. The idea being to attenuate unwanted sounds from directions
other than your subject. A shotgun mic does not provide any extra gain
to pick things up at distance. So, to get distance you amplify their
output considerably. Unfortunately you amplify the mic's self noise,
input circuit noise and so on right along with your wanted sound signal.
This means that to use a shotgun it must be a very high end design for
nature recording.
Parabolic mics are a mic or mics set at the focus of a parabolic
reflector. The reflector provides gain before the sound get's to the
mic. ie it amplifies the actual sound wave before it's converted to an
electronic signal. This is "free" amplification that does not increase
the system's electronic noise. It also means that it's not as necessary
to use the highest end mics in a parabolic for the same signal quality
as a shotgun.
The Telenga Stereo, for instance, uses 4 relatively inexpensive capsules
on each channel, positioned as boundary mics with the central barrier.
Thus gaining not only the parabolic's contribution but a bit of boost
from being boundary mics as well.
The gain of a Parabolic covers all frequencies, but increases with
increasing frequency. This somewhat compensates for the loss of sound
with distance that occurs from high frequencies downward. (why distant
thunder is a low frequency sound). There has been considerable
confusion about parabolics in this regard, sound does not behave like
radio waves and many try to apply electromagnetic radiation theory to
parabolics working with sound. Do not be surprised to find a lot of
conflicting literature on parabolics.
One design feature of the parabolic reflector is that the reflection off
it sets up an interference wave out in front of the dish. A microphone
near that will experience a band of attenuation in fairly low
frequencies. So for good response you want the parabolic's focus to be
at or inside the front plane of the dish. The farther in the better up
to about 1/4 the dish depth, but practicality of design generally limits
most to just a little inside the dish.
The parabolic dish is also a directional system like the shotgun mics,
but a much narrower pickup. Shotguns rarely get tighter than about 60
degrees, parabolics sometimes are as tight as a few degrees.
You might also want to read the white papers at:
http://www.telinga.com/
Walt
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