[opendtv] Re: Microphone for Parabolid Reflector
- From: "Hunold, Ken" <KRH@xxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 20 Jan 2012 11:25:40 -0500
The reflector looks like a parabola, but you'd have to measure and plot
it to be sure. If you really just substituted a mic "capsule" instead
of the entire mic, you will get a different sound. You would have to
modify the mount, perhaps substituting a mic clip placed so that the
axis of the mic and the axis of the parab are in line. A mic clip might
also give you enough fore-and-aft adjustment to place the phase center
of the mic at the focal point. The location of the phase center of the
mic is a parameter that is not often listed in the spec sheet.
Covering the cancellation ports changes the response pattern of the mic,
so the sound will be different. The results are best determined by
trial and error.
Because the reflector is not very "deep," you get a lot of sound from
the "back" of the reflector. After all, that's the direction the mic is
pointing! Don't fail to consider sound coming through the reflector, or
more correctly, sound that makes the reflector vibrate. However, once
you make the reflector deeper and cover the outside with a material to
prevent sound from vibrating the reflector, you've got quite a monster
to hold/mount.
There are always tradeoffs...
Ken
From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx]
On Behalf Of dan.grimes@xxxxxxxx
Sent: Tuesday, January 17, 2012 12:58 PM
To: opendtv@xxxxxxxxxxxxx
Subject: [opendtv] Re: Microphone for Parabolid Reflector
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
Other related posts:
