The phrase absolute phase, seems to be one that is widely misunderstood. The term phase is most often used to describe the orientation of the + and - terminals of the speaker and amplifier. This relationship is more accurately defined by the term polarity. In other words, a system whose + and - amplifier terminals are connected to the speakers' + and - input terminals, is properly connected from the standpoint of polarity, but not necessarily phase. It is true that reversing the polarity of one speaker will put that channel out of phase with the other, but the polarity is reversed. The following text will help to explain the concept more completely.
To better understand how phase and polarity are related, let us examine the very beginning of the process that brings music into our homes - the recording session. Musical instruments create sound by modulating the air, producing sound "waves." These sound waves are actually compressions and rarefactions of the air molecules. The pressure changes impinge upon our eardrums and are interpreted by our brain into the sounds we hear. A microphone, like our eardrums, senses these same pressure changes, converting them into electrical impulses which are fed to the recording equipment.
When the leading edge of a musical waveform (compression) strikes the microphone diaphragm, a positive voltage is created at the microphone output. Similarly, as the waveform passes, its trailing edge (rarefaction) produces a negative voltage output from the microphone. Simple, right? No, unfortunately. For in between the microphone and your ears lie a vast array of electrical "obstacles," each capable of skewing phase to some degree.
Ideally, after all the mixing, signal processing, mastering and cutting have been completed, that same positive output voltage from the microphone will produce a positive (forward) motion of the loudspeaker diaphragm. Often it does not, and therein lies the rub.
The making of records is a very complex process involving a bewildering quantity of electronic equipment, each possessing the potential to manipulate phase. The microphone, cables, mixing console, tape machines and the myriad of signal processors (delay, echo, reverb, equalizers, limiters etc., etc.) could each be wired out of polarity with respect to one another. In addition, smaller phase errors (several degrees here and there) may occur within the electronics of any of these components. Finally, your stereo system may perform tricks of its own to further complicate the issue. Many tube preamps for instance, invert polarity. Often this occurs only in the phono section, which puts the turntable out of polarity with the line sources. So, to correct this, we must reverse the + and - leads at either amplifier or speaker. This would be done for both channels. But how do we know which orientation is correct? A trial and error approach is really the only way that we can determine proper orientation.
Ofcourse, inverting polarity at the amplifier terminals will change the polarity for all sources. But this may not be desired if, as described earlier, your preamp inverts phase of the phono section only, then inverting the polarity at the speaker terminals corrects the problem for the turntable, but inverts polarity for all other sources. In a system where only the turntable had its polarity inverted (because of a phono preamp), you might choose to reverse the leads at the rear of the phono cartridge.
What happens, sonically speaking, when our system is out of absolute polarity? Most listeners experience a constricted soundstage with a pinched center-image and reduced lateral spread. In phase, the soundstage will open-up, giving a more rounded feel to the center image, and offer a more correct presentation of depth.
To test for proper polarity, choose a recording with which you are quite familiar. I prefer those with simple instrumentation rather than more complex instrumentation, thus allowing the listener to concentrate on just a few instruments or voices. Listen for the characteristics outlined above as you make the switch between connections. Choose the one that sounds more correct, to your ear.
Unfortunately, every recording is different. It is possible that the first selection you play will sound best one way, while the next piece sounds better reversed. All I can say here is that you should set the phase for a few of your favorite recordings and hope that setting will be right for most your recordings.
A "trick" devised by David Fletcher of Sumiko may help you in determining whether a particular recording is out of polarity. Sit a few feet in front of one of your speakers. Listen to see if you can readily hear the other speaker. If you can, the system is in-polarity with the recording. If you cannot, then the polarity is inverted. This test relies upon a phenomenon known as the Haas Effect, and seems to be somewhat dependent on the room and type of loudspeakers employed.
I have had varying results with the "Fletcher Test." Apparently, it depends rather heavily upon the recording used. The recording must be phase coherent to begin with (which usually suggests a minimalist recording technique using only a pair of microphones). Complex multi-miked affairs are likely to give ambiguous results.
Returning a system to correct polarity can result in significant improvements. Experiment with the process a bit to familiarize yourself with the effects, and you will readily be able to recognize and correct an inverted polarity condition.