> Or spaced meaning spaced a couple feet or more?
John,
Spaced mics mean any lateral separation as seen from a distant sound object=
.
A coincident pair of mics are usually mounted one above the other or very
close side by side.
You can get a stereo image of sorts from a pair of Omni mics spaced a coupl=
e
of feet apart, but it gets swamped by out of phase effects from the whole
sound scene. This happens when a sound object, (water splash, bird, etc) ha=
s
a path difference of more than about 17cms between mics. With a mic spacing=
of 2ft or 60cms, the "clean" object width is only 16 degrees wide. anything=
wider than this becomes out of phase.
A coincident pair of fig-8's looking 90 deg apart gives clean forward volum=
e
stereo over an object width of 90 deg. Because the mics are symmetrical,
there is also a clean image over 90 deg from the rear. Between those
directions, the sounds are out of phase and give more of a "reverberation"=
stereo effect.
With fig-8 mics, you not only get a directivity for left and right, but
sources from above and below are reduced as well which can be useful.
That's the theory anyway. :-)
One thing I didn't mention is that time difference stereo only really works=
with headphones. With loudspeakers, volume difference stereo is the major
effect as moving your head alters any time differences between speakers and=
ears.
Some "fun stats":
17 cms corresponds to the wavelength of sound at about 2Khz.
The time difference at the notional head width of 17 cms corresponds to hal=
f
a millisecond stereo time difference.
The ear can easily detect a stereo time difference of 0.11 ms which
corresponds to only 5 samples which I at least find remarkable. Have a
listen using headphones to the attached file which changes by 5 samples at=
five second intervals:
David Brinicombe
|