Strictly speaking, it is not really true that one cannot hear absolute
polarity (phase if you must). This is definitely audible though not in as
conspicuous as a way as one might expect a priori.
The thing is that polarity reversal is not interpreted as directional
information.
Rather, it is perceived as a change in the perceived character of the
sound.
In listening in reality, the sound behind is also distinguished
from the sound in front by a change in character of the sound
but that is caused by how the sound comes around the head and into the
ears (the ears are not front to back symmetric!). And this is not the
same perceived change as polarity reversal.
In Ambisonics the sound behind sounds as if it were behind
because it is behind(largely). Of course in Blumlein stereo, all
sound is physically in front and the sound from behind sounds in
front but with the character of a polarity reversed sound(which
is not all that much different from un-reversed and in any case
the reversal is not interpreted as behind)
Remember the stereo system where speakers were also
put behind but EQ ed as to sound in front, the result
being a better stability of stereo?(G Finsterle did this,
called something like AVS or ASV or the like?)
Robert
On Sat, 26 May 2012, Augustine Leudar wrote:
brilliant thankyou J?rn,
so with two capsules pointing in opposite directions Im kind of faking
a figure of 8 with 1 capsules by artificially phase inverting one
channel - which would happen naturally with a one capsule figure of 8,
I have more questions - but I think ill give you guys a break for a
bit and do some more reading before I come back to mither - but thanks
again for all the info (=;
we cannot hear absolute phase, so if you are listening to a figure of
eight microphone, there is no way to tell whether the sound you're
hearing was coming into the (positive) frontal lobe or the (inverted)
rear lobe. hence, the only information we can get from a lonely fig8 is
"it's obviously not coming from the side".
now if you add an omni to the equation, all of a sudden you have the
possibilty to discern between the two lobes of the fig8: one is in-phase
with the omni, the other isn't.
thus, the 0th order component helps you resolve the ambiguities of all
the 1st order signals.
btw, if you move to second order, the situation is the same: with a
cloverleaf directivity, you only know "it's either coming from
front-or-back, or from left-or-right". only with the first-order
information, the ambiguity can be resolved.
W also has a subtle role in distance coding, but its main job is really
to help us make sense of the fig8s.
to wrap up, three real fig8s won't ever give you ambisonics. but if you
obtain the fig8s by subtracting two back-to-back cardioids, you can also
add them, which then gives you a nice omnidirectional component. note
that you will have to attenuate the resulting omni signal according to
the number of capsules you used.
best,
j?rn
--
J?rn Nettingsmeier
Lortzingstr. 11, 45128 Essen, Tel. +49 177 7937487
Meister f?r Veranstaltungstechnik (B?hne/Studio)
Tonmeister VDT
http://stackingdwarves.net
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