Mind Stretchers
On Thu, 21 Jun 2012 04:07:18 -0700, Gary Eickmeier wrote
(in article ):
"Audio Empire" wrote in message
...
On Wed, 20 Jun 2012 16:55:04 -0700, Gary Eickmeier wrote
(in article ):
In other words, at frequencies above 800 Hz a time difference can result
in
a phase difference at low frequencies, but at higher freqs phase makes no
sense. In other well-known literature such as Blauert, the statement
about
level vs time of arrival differences is that the signal will be slammed
completely right or left with level difference of 30 dB or a time
difference
of 630 microseconds to 1 ms.
This MIGHT be true if we're dealing with sine waves, but neither a musical
instrument nor a musical ensemble produces sine waves. Also, the frequecy
doesn't matter unless we are talking about two or more sine wave signals
of
the same frequency, but music is a complex waveform and the ear/brain's
interaction with that waveform is not well understood at best.
Stan Lip****z, in his famous article "Stereo Microphone Techniques - Are the
Purists Wrong?" (JAES 1986 September) puts it like this:
"When listening live, the signals at the two eardrums differ in time of
arrival, level, and spectral content, and these differences depend on the
source position. The time of arrival difference is due to the physical
spacing of our ears and cannot exceed about 630 microseconds in real life.
This corresponds to a path length difference of about 210 mm, and this in
turn represents a half wavelength at a frequency of around 800 Hz. It thus
follows that for frequencies below about 800 Hz there is an unambiguous
phase relationship between the two ear signals and the source direction, the
ear nearer the source having the leading phase. This is what I shall call
the low frequency regime. This phase difference is frequency dependent; in
fact it varies linearly with frequency since it represents a pure time
delay. At frequencies above about 800 Hz the interaural phase shift can
exceed 180 degrees, and so the ability (on periodic steady state signals) to
discern which ear's signal is leading and which is lagging is lost. So,
clearly, interaural phase relationships would appear to be useful cues only
at low freuencies."
I note also that with AE's favorite recording technique of coincindent
miking, or Blumlein stereo, there is NO time of arrival or phase
difference
between channels.
That's ridiculous. phase differences start at the SOURCE of the wavefront,
not at the destination (in this case, the microphones, where they already
exist). No two sound sources are ever completely in phase at the same
time.
In fact one instrument will produce a complex wave-form that has many
different phase relationships occurring at once within it's own sound.
Now you're being silly. Phase differences between what and what? Two sound
sources in phase? If they are two completely different sources, there is no
such thing as phase relationships between them. Phase relationships within
the sound of a single instrument? What in blazes are you talking about?
This isn't worth discussing. If you don't understand sound and how it works
well enough to grasp the concept of the phase difference between fundamental
and harmonics, or between one instrument and another playing together, then
there is no use me trying to explain it.
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