From: daw@cray.com (Dave Wright)
Subject: Re: polarity inversion
Organization: Cray Research, Inc.
Date: 6 Jan 93 16:32:58 CST


First off let us get some terminology correct. Polarity and phase are two
different metrics with different measurement symptoms, different units,
and different audible effects. YES THEY ARE BOTH AUDIBLE ON ANY AVERAGE
SYSTEM. You do not need a high end sytem or a recording studio system to
hear the effect.

First, phase is a Frequency dependent time shift that is measured in
degree's that is computed from the frequency and the amount of time shift.
Phase is extremely important in imaging when the direct sound and
electronic or acoustic reflections (time delay) are combined. When
this happens there results what is called a comb filter. Phase problems
in a loudspeaker crossover can cause frequency variance in the crossover
range. Loudspeaker engineers try to spend time on the xfr to make it
minimum phase and smooth out the response in the crossover region. This is
complicated by the time delay difference betweenthe acoustic center of the
drivers and time delay this spacial difference makes. In fact, our outer
ear (the pinna reflections) intentionally create comb filters that our mid
uses to localize where the sound is comeing from. Phase is measured in a
Bode plot y axis is degree of shift and plot x is the frequency. ANother 
method of ploting is called a Nyquist curve which is essentially a polar
plot. The x axis is resistence and the y plot is capacitance/inductance.
The plot looks like a curly squiqle which is what is traced out by a
rotating vector that goes from 20hz to 20khz. The position along the curl 
is the frequency. A good smooth loudspeaker system will have a controlled
curl that may approach a circle meaning a well behaved speaker.

Polarity is a frequency independant measurement that indicates the
orientation of the energy. If you looked at the sound on a oscilloscope
THe upward direction is positive and a downward direction is negative. On
a bode plot, polarity will cause the entire plot to start off 180 degrees
off for all frequncies. In a Nyquist plot, the curl will rotate in the
opposite direction.

Both are measured differently with different units. THe difference again
is that Polarity is frequency independent and Phase is frequency dependent.
Polarity is sometimes referred to as Absolute Phase meaning the frequency
is absolute or not in metric. It is however a misleading term and should
be avoided.

Now for the effects. Several in the group have described some of the
effects that polarity can have. Recording Engineers have known for a long
time that polarity can have positive and negative effects. I for one was
mixing a song where the vocals completely dropped out by a polarity problem
that caused the sound to completely cancel. Making sure all your audio
equipment is properly polarized is critical. The description about vocals
is very true. Speech is a asymetrical sound with most of it's energy in the
positive domain if looked at on an oscilloscope. I have listened to
polarity switches from a single speaker source and can hear a tonal change.
That does not mean that I know which is positive or negative, just that
there was a difference.

The polarity change can have different effects at different frequencies.
For low frequency sound. If the polarity of one speaker or even both is
reversed, the sound will combine in the room differently. For example, a
100hz tone in a room could generate what are know as standing waves in the
room. You may hear a standing wave in the shower or bathrooms that are very
reflective. In the room there will be hot spots and dead spots when you
move around in the room listening. THe hot spot is a node and the dead spot
is a null. These are created by the sound reflecting off the walls and
combining with the direct sound. If you reverse polarity. These hot/dead
spots will move. In one studio I worked in (EV Sentry V and Crown DC300),
clients often prefered the polarity reversed because it gave them a node
where they were listening. I prefered it as well because then they did not
want to overcompensate for the problem in the mix. As someone mentioned
this is because of the poor design of the room.

	So polarity reversal will move low freq nodes and nulls in a room.
	This would only be audible if you walked the room in both cases. 

	At high frequencies the polarity reversal will affect the nodes and
	nulls in a comb filter affecting different frequencies. This is
	especially audible if one speaker is reverse polarity from the
	other. The two speakers will now combine in the room differently
	and be audible. The original Flanging(phasor) that Hendrix did on
	Electric Ladyland was made more pronounced by a polarity reversal
	of the effect relative to the direct sound. In those days the time
	shift was done with a tape deck, now we have digital delays.

	Also if comb filters and masking the comb filter created by the
	pinna, then imaging is another audible effect. It is easier to
	externalize a properly polarized and phase coherent recording.

Even on stage, polarity can be a big problem. With sound combining all over
the place, comb filters are everywhere. These are what cause feedback.
Feedback can be controlled better if everything is properly polarized.
There are companies that make click boxes to check polarity and keep those
nasty comb filters under control.

If you know what the physical effects are, and how to measure them, you can
learn what to listen for and under what conditions to do the listening
experiment. I don't know anyone that can take a single frequency tone and
tell whether is is in or out of polarity until it is combined with a
reference but with a complex tone that is asymetrical, it is more audible.

Dave Wright,

PS. I don't believe in the audible differences of high end cable that is
not measurably different but polarity is a very audible effect and anyone
on this net can learn to hear it.
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