Hi Rich,
My explanation was just a simple approach and I don't have much time to dea=
l
with all the details of shotgun microphones at the moment...
> The pattern of the slits in the side of a shotgun I do not believe
> makes much difference.
Yes, it might be true that the pattern of the slits itself is not the most
important thing.
> For sounds comming from the side the sound is arriving at multiple
> times based on the many slits. The longer the tube the better chance
> that these multiple paths will be cancelled out before it reaches the
> pickup element at its base. The multiple slits makes the sound from
> the side interfere with itself more often and be cancelled out. That
> is why the long tube on a shotgun mic is called an interference tube.
Yes, that's correct.
I read an article about microphones by Neumann (they also manufacture shot
gun microphones). The say, that there is another layer of fine gauze inside
the tube. That gauze prevents resonance and decreases the effective length
of the tube at higher frequencies (by increasing the acoustic resistance).
This is important, because the tube must be effectively shorter for higher
frequencies in order to get similar polar patterns for all frequencies.
Another important design detail in shotgun microphones is that they in fact
combine two different principles. For high frequencies (wavelengths shorter
than the tube) it uses the interference tube (relying on the delays and
phase cancellations along the tube). Due to it's limited length, the tube i=
s
not effective for lower frequencies. Therefore, a special pressure gradient
capsule (something like a figure of eight or cardioid element) is
responsible for the directionality at lower frequencies.
As we can see, a shotgun microphone is a very complex thing.
Regards,
Raimund
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