Mind Stretchers
On Wed, 20 Jun 2012 16:55:04 -0700, Gary Eickmeier wrote
(in article ):
"Sebastian Kaliszewski" wrote in message
...
Gary Eickmeier wrote:
There are no "subtle phase differences" beyond about 700 Hz, at which
point the wavelength becomes too small for the perception of any phase
effects, and pure time difference takes over.
This does not agree with things I have read. Please provide source for
that information...
Sorry, thought this was well known. The best reference I found is in the AES
Anthology of Stereophonic Techniques, James Moir, "Stereophonic
Reproduction," Audio Engineering, 1952 October, pp. 26 - 28. I quote in
part:
"These differences justify further discussion. The reason for the difference
in time of arrival at the two ears is evident and requires no further
exlanation, but the question immediately arises as to which part of the
sound-wave cycle is accepted by the ear as determining the time of arrival
at that ear. On an impulsive sound having a steep wave front it may be
assumed that the arrival of the wave front is recognized, but on a
repetitive waveform there is difficulty in understanding just how the ear
recognizes the difference between successive cycles with identical waveform.
A high frequency wave passing from right to left will have several cycles
pass the right ear before the first cycle reaches the left ear, and the
right ear may not know just how many cycles have passed at the instant the
first cycle reaches the left ear. This rather suggests that there may be
difficulty in fixing the position of a high frequency source having a
frequency such that more than half to one cycle of the wave can be
accomodated in the space between the ears. Taking the velocity of sound as
33,000 cm/sec and the ear spacing as 21 cm, it might be expected that
frequencies above 800 cps (half wave = 21 cm) might present difficulties in
location and it is worth noting that this is found to be the case in
practice."
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.
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.
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