[opendtv] Re: Microphone for Parabolid Reflector

  • From: dan.grimes@xxxxxxxx
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 17 Jan 2012 09:58:12 -0800

Thanks, Ken, for your response.

I believe it is a true parabolic mic., a Dan Gibson EPM P650:

http://www.rdsystemscan.com/specifications.html

We are using it in a rather unique application (I think, it might be a
quite common application).  I have mounted it to one of our studio cameras
and it points to whatever the camera points at.   We use it in an
auditorium to get questions from the audience.  We tried lots of other
techniques, including passing around a wireless handheld mic.  Obviously,
the handheld provided the best sound when used properly, but most people
refuse the mic and just yell.   Zone mics often got the sound but with lots
of reverberant energy.  Once we applied the parabolic mic to the camera
that shoots the audience, we found a great compromise with clear
intelligibility and low operator error.

I first replaced the capsule with a mic element out of a Shure SM63 (omni,
dynamic).  The resulting audio had only the high end frequencies, no lows,
and sounded poor.  I couldn't move it in and out to get the perfect focal
point but I believe I was within a millimeter.  It also had a slight echo.
I'm guessing this is from the direct sound adding to the reflected sound,
even though it is about 20 cm total travel difference.

I then installed an Audio Technica AT8528 Cardioid Condenser mic.  It
provided a similar but much more sensitive response.  The highest
frequencies were through the roof!  The negative of the sensitive mic is
that it picked up a lot of local sounds, too.  With the microphone mounted
just above the lens, it picked up the focus and zoom servos loud and clear.
And again, tinty sound.  Adjusting the mic in and out of focus definitely
changed things with a gain peak at the focal point but only in the higher
frequencies.  I would think this is because of it's relatively small size,
less than 50 cm diameter.

So I experimented with closing off the cancelation ports.  This immediately
boosted the low end and gave a much more normal frequency response.
Directionality remained and a there was a strong focal point.
Unfortunately, it also made it very sensitive to local vibrations.  Still,
it is a reasonable trade-off for good sound.  I finally came to an optimal
point between directionality, frequency response, and sensitivity by taping
over only two of the three cancellation ports with the open one pointed
towards the lens.

I think I will try these experiments again, but this time connect it to a
spectrum analyzer to see just what is really changing.  I'm using the mic
in a production today and will see how it works for our application.

Dan

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