[Top] [All Lists]

Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "Rich Peet" richpeet
Date: Sat Sep 30, 2006 5:08 pm (PDT)
As you may recall I use a 32" dish with the focal point being close to
the rim.

Do not worry so much about standing waves.
If cavity resonance is noticable to you than you can easily dampen
that.  My windscreens dampen it well.

Ideal in my eyes is setting the focus at the rim or just outside.
On occation I do use a mic above the dish as a noise cancellation
device and if standing waves were a large problem with my dish I would
hear it.

More often I use two close mics without a barrier. One is on target
and one is off. I am not a fan of ever having the target partly out of
the focus. Because I am recording with two mics and hearing one that
is always out of focus I understand focus fairly well. Using my two
mic setup a Song Sparrow can appear to be moving as it gives its song
when the ideal focus is between the two mics yet closer to one.

The focal length a dish is designed for does change the flavor of the
recorded sound. Randy Little who did the math on that, I recall, also
ended up recommending a 1:1 ratio of focus to depth.  Klas does not
agree and that is simply another flavor.

Rich Peet

--- In  "John Hartog"
<> wrote:
> I must be nuts to bring this up again, but I still have some burning
> unanswered questions -so here is one.
> Regarding the focal point (and I know it is not really a point), what
> is the ideal distance inside the rim.  I am hearing different opinions.
> One from a Cornell page suggests if the mic is positioned inside the
> dish there would be a distortion from cavity resonance (or something
> like that) off the sides of the dish.
> Conversely, a few of this group suggest a standing wave at the rim
> opening would cause distortion, so the mic is best put inside the dish
> somewhere below the level of the rim.
> Is the standing wave only exactly at the rim level or does it extend
> somewhat into and out-of the dish?  If so how far each way?  Would the
> optimal position of of the focus be at a certain ratio of the dish
> depth?
> If anyone can bring a little more light to this problem or steer me in
> the right direction, I will be sincerely grateful.
> John Hartog

<Prev in Thread] Current Thread [Next in Thread>

The University of NSW School of Computer and Engineering takes no responsibility for the contents of this archive. It is purely a compilation of material sent by many people to the naturerecordists mailing list. It has not been checked for accuracy nor its content verified in any way. If you wish to get material removed from the archive or have other queries about the archive e-mail Andrew Taylor at this address: andrewt@cse.unsw.EDU.AU