Let's see. Plastic salad bowls and such like have been used in a pinch,
though finding one big enough would be a little problem. Their main
disadvantage is they are usually a section of a sphere, and have a flat
bottom. Both of these mess up the focus a fair amount, so it's kind of a
case of better than nothing, but not much better. The closer to a
parabolic section you can find the better. The calls you are trying for
are low enough frequency you want to make sure and stay up in reflector
size. Note that the design I showed screws together just clamping the
reflector between the two rug castors, so you can fairly easily change
reflectors.
The foam would be like closed cell foam, the foam used to pack equipment
in cases, even a few styrofoam peanuts would do it, it's not critical.
Wadded up newspaper or paper towel would work but is not very weatherproof.
You only need to put foam in three places, I'd think filling it all
would be overkill:
1-Wedge the mic in the end of the pipe under the end of the end cap with
a little to keep it in place, shown in yellow in the drawing. Just wrap
the mic with enough foam to keep it in place, nothing fancy. Don't cover
the front of the mic. The foam will provide isolation from the pipe if
you don't pack it in any more solid than enough to reliably hold. The
tie tac mic I mentioned is about 1/4" in diameter and 3/4" long and
weighs next to nothing.
2-a small amount of foam at the other end of the perforated pipe to
prevent noises traveling through all the piping and reaching the mic.
You could also plug the handle end with foam or such like to wedge the
mic cord as a sort of strain relief. If this is for strain relief, pack
it pretty dense with the wire in place. Additional strain relief could
be obtained in the perf section by using tie wraps to hold the mic wire
there.
3-I used a bit of foam pipe insulation on the handle to provide some
isolation from handling noise. That's in place in the photo. Any sort of
soft foam that does not make noises when handled will do. Preferably
something that won't soak up water.
The pipe started as a plain piece of gray PVC Electric conduit. I
happened to have some, and it's fairly thick walled. That little piece
of pipe was nearly all the work in building this. You can cut slots in
the pipe with all kinds of tools, or they could be just drilled holes.
It's the access of the sound from the reflector to the mic, so the
bigger the better as long as the piece is still strong enough. I've done
slots like you see in the picture with a drill press and a twist drill
or router bit, takes a little planning to do them that way. But in this
case, I have a mini-mill and cut the slots with that using first a end
mill, then a countersink. They came out prettier than my past jobs doing
this sort of thing. I've used some pretty ugly looking stuff and it
still worked. Just be sure and trim off any fuzzy bits. And use care,
this is the sort of task that can hurt.
My feeling on glue is you would need to glue the pipe for the handle to
the T, enough force there I'd not trust it without glue. Just use the
regular PVC pipe cement. How much of the front structure to glue is a
choice. I'd not be inclined to glue the end cap on or you will be trying
to set the mic in place through the perfs. It all holds pretty firm in
the one I did just by friction.
Note the piece of perfed pipe is going into a adapter fitting that's
threaded, check the drawing on this. You need something with a shoulder
so as to clamp the reflector. And you want to be able to take it apart
if necessary without damaging the reflector.
Also, if you have the tools to do it, the other way on the T that is
just capped could become a tripod mount. Either just drill and tap the
end cap, or glue a 1/4" nut inside the cap to provide more solid
threads. It should stay pretty well if you use epoxy. Of course if doing
that make sure and glue that all to the T.
The easy way to find focus is to simply plug the mic in, set your
recorder to record/pause, and listen with headphones while pointing the
reflector at something calling a ways out there and moving the mic in
and out by hand or rubber banded onto a stick along the centerline until
you find the best reception point. That will be the distance to use. The
Telinga dish this is about 5-6" from the dish along the centerline.
Edmonds lists theirs as 6" though you would want to test that.
The design as is has little wind protection. A simple start might be to
find a thin baby size tube sock that would slip over the pipe covering
down beyond the perfs and rubber band that on. Next layer would be to
build a cover for the entire dish opening, like is made by rycote for
the regular telinga. This is a simple light fleece/polartec type
material that is edged with elastic cord to hold on the edges of the
dish. Also the tie tacs sometimes come with a tiny foam "windscreen" if
so try to keep it on the mic.
Note that designs like this are not fixed in stone, there is a great
deal of latitude for inventive use of material. This design is fairly
fancy compared to a lot of the ones I've seen. But all the parts except
reflector and mic are straight out of Home Depot.
Walt
deer2me22002 wrote:
> Dear Walter
> I am very delighted with your pictures of the homemade
> parabolic mic you made, and am intending to try to build one myself
> to use in the woods next month. I have a few more questions on this
> mic if you would not mind answering them. My first question will
> probably sound very off the wall to you, but i was wondering if using
> a large plastic salad bowl or similar desighn for a reflector? I was
> also wondering what kind of foam you were refering to when you
> metioned using it in your mic, and also would you fill in the whole
> handle and T with this, besides in the cap with the mic, and just
> leave the peice of PVC with the cutout holes empty of this foam? How
> do you go about finding the focus point of the reflector to be?Should
> i cement all the caps , including the mic cap? Also were the holes
> already inthe pvc pipe or did you cut them yourself, if so what did
> you use to do this so neatly? Would the part with the holes in it be
> cemented to the T, or another way? And lastly, how would the mic be
> mounted in the cap with foam?
> Thanking you greatly:
> John E Parks..
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