If you have a workshop with a lathe, finding the centre and equator of the
sphere is relatively easy (at least to the accuracy this would need) if the
lathe jaws are large enough to take the sphere, a little bit less easy if a
lathe ain't available, just a drill press. With a lathe, just mount the
sphere in the chuck, which will centre it, put your drill of choice in the
drill chuck on your tailstock, start the lathe and drill plumb through the
centre. Now take the sphere out and pass a suitable bolt through the hole
and use this to mount the sphere back in the lathe. Now any suitable tool
can be fitted in the lathe toolpost and moved slowly up it just touches the
outermost part of the sphere, which gives you the equator. Of course if you
have a lathe, you might as well just turn up the sphere yourself (not sure
how well I could do that without CNC, as I haven't used a lathe in well
over three decades!). With just a drill press it's a bit harder. Fix a
block of (preferably hard) wood on the drill bed. Chose a large diameter
bit (say something like half the diameter of the sphere) and drill a hole
in the block of wood, large enough for the sphere to sit comfortably on.
Replace the large bit with a smaller one that is long enough to go through
the sphere, sit the sphere on the hole in the block of wood and drill
through as before. Mount the sphere on a bolt, put it in the drill, set it
going slowly and mark the equator. Great care will need to be taken at all
stages to stop things shifting. Frankly, though, these days I'm more
inclined to get it 3-d fabbed
(http://www.fablabmanchester.org/p20/Dimension-1200es-Series-3D-Printer.html)
Dave
On Jun 21 2011, umashankar mantravadi wrote:
dear eric the way i marked out my holes is this: draw three great
circles, 120 degrees apart. the point they crosss at is the first
microphone. you then mark 240 degrees on each great circle and drill for
the other three. i drilled the holes through the sphere so the wires
could be threaded out( i was using panasonic 6 mm capsules). never
completed the project. umashankar
i have published my poems. read (or buy) at
http://stores.lulu.com/umashankar
> Date: Mon, 20 Jun 2011 11:00:32 -0700
From: [email protected]
To: [email protected]
Subject: Re: [Sursound] B format mic using omnis?
> building/using a soundfield type mic using omni's?
If life gives you 4060s, then make lemonade. I mean, a B-format
microphone!
The problem gets a lot easier if you resign yourself to making
something that will have good utility as opposed to making something
optimal.
I would make a microphone array using a spherical baffle. One can find
quite a variety of wood spheres. Here in the US there are spheres made
of Birch with diameters of 1-1/2", 2", .... Wood is great because it's
cheap, easy to drill, and if you make a mistake you just grab another
one.
If the user doesn't intend to make use of height, I'd want to make the
array a horizontal-only one, primarily because the drilling is a lot
easier! It's difficult enough to find the equator of a sphere without
having to find the vertices of a tetrahedron inscribed in the sphere!
The choice of diameter is tough, because as previous respondents
pointed out there is a tradeoff between SNR and bandwidth. As you know,
the 1st order patterns will be derived by subtracting the outputs of 2
or more of the capsules, and that means that the response will have to
be equalized by apply an LF boost below a critical frequency determined
by the diameter of the sphere. For an open array this is straightforward
but for a spherical baffle you need to include the diffraction of the
sphere. I can calculate this, but not on the back of an envelope. The
spherical diffraction gives an effective gain of 6 dB and this is worth
going before because the self noise of the 4060s is about 23 dBA as I
recall, which is good enough to be useful but not so generous as to
allow one to easily throw it away. So what we would like to do is to
have that critical frequency be somewhere near the frequency at which
the ear is most sensitive to mic hiss - about 2 to 7 kHz. And typical
usable sphere sizes just happen to do that. This means that the array
will only work well up to about 10 kHz, but then that is true of a
traditional SF microphone too!
The construction may be just a little bit difficult. It turns out to be
difficult to find the center of a sphere once you have it in hand. You
will really need to use a drill press to drill the holes. Routing the
microphones into the sphere will also be difficult, depending on how the
end of the microphone cables are connectorized. It may turn out that you
will want to drill a large hole in the sphere at a direction not
populated by microphone capsules, and use that hole for entry of the
microphones and to route them each into their respective holes.
Having done this before, I can give you a bit more specific info if you
contact me off-list.
Eric Benjamin
----- Original Message ----
From: Dave Malham <[email protected]>
To: Surround Sound discussion group <[email protected]>
Sent: Mon, June 20, 2011 3:54:38 AM
Subject: [Sursound] B format mic using omnis?
May seem a strange question, but anyone ever had any experience of
building/using a soundfield type mic using omni's? I have been asked by
one of the artists featured on The Morning Line if there's anything he
could do with his collection of 4 DPA's (4060-bm's). Not something I'd
ever really though about before, but as Angelo's B format hydrophone
uses omni's ... (http://www.angelofarina.it/Public/UAM-2011/)
Dave
-- These are my own views and may or may not be shared by my employer
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