>> [Four 4060s aligned in a tetrahedral pattern will provide the basis for=
>>ambisonic surround ~Bernie]
and Marcus Buick wrote:
> my understanding of ambisonics <>
> the four capsules in the tetrahedral array are not only aligned symmetric=
ally,
> but electronically as well
It is possible to make an Ambisonic recording using omni microphones. At s=
ome
risk of exceeding the technology threshold, I'll elaborate.
The original Soundfield microphone uses four nominally cardioid capsules
assembled in a tetrahedral pattern. The omni output is created by summing =
all
four capsules together, and the three figure-of-eight outputs are realized =
by
subtracting the capsule signals in pairs. For instance, subtracting the tw=
o
back capsules from the two front capsules gives a forward facing figure of=
eight. The electronics does the subtraction, and it also provides compensa=
tion
(equalization) for changes in response due to the size of the array. The r=
esult
is something that performs pretty well over the entire audio range, althoug=
h
both ends of the spectrum are problematic.
It's easy to see that if you have a group of omni microphones that you coul=
d get
a good omni response by simply summing them. What is not so obvious is tha=
t you
can get a figure-of-eight pattern by subtracting them. But they need to be=
close enough together that there aren't multiple wavelengths of sound separ=
ating
pairs of capsules. There is another problem. The Fig8s that are derived in=
this
way roll off at 6 dB/octave below the frequency at which the capsules are
separated by a wavelength. So you end up with a frequency response that lo=
oks
like a haystack; there's no flat region in it. This can be fixed by
equalization. But it takes a lot of EQ. If, for instance, the capsules we=
re
separated by a single wavelength at 10 kHz, and it was desired to have a
response that goes down to 100 Hz, then the system would need to have 40 dB=
of
bass boost. All of that bass boost increases the noise, although mostly in=
the
bass.
That sounds pretty grim. But omni microphones are almost always better in =
SNR
than figure eight or cardioid microphones. Also, they have essentially
unlimited bass response as compared to figure-eight or cardioid microphones=
made
in the ordinary way. Those all roll off below about 150 Hz to 200 Hz, desp=
ite
what the manufacturer's spec sheets may say.
So it ends up being kind of a wash. If you use omni microphones to achieve=
a
figure of eight response then you end up with a Fig8 that rolls off below, =
for
example, 100 Hz and above 10 kHz. But regular figure of eights do that too=
.
And the omni SNR is made worse by the processing, but then regular figure o=
f
eight and cardioid microphones have worse SNR too.
It turns out to be a good idea to put the microphone capsules on the surfac=
e of
a rigid sphere. Imagine something about the size of a baseball, although y=
ou
would probably want to use something easier to work with than a baseball. =
There
is a pretty large technical literature on this subject. I'd say that ther=
e
have been several dozen technical papers describing various aspects of it i=
n the
last five years. Most of them are full of things like Bessel functions and=
Hankel functions that really aren't friendly to the typical nature recordis=
t.
But you don't need to know that stuff to actually build and use a microphon=
e
array.
> I own eight 4060=E2=80=99s, and would gladly reconfigure four of them for=
ambisonic
use
If this is something that you really would like to pursue then I would glad=
ly
help you. But we would probably want to take the discussion off list. You=
would need to be comfortable with the process of drilling holes in somethin=
g to
make the array, and we would need to find a process that allows you to do t=
he EQ
in a way that is easy for you.
Eric
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