Absolute Phase: Fact or Fallacy?

Natural sounds produce different waveshapes during their positive and negative phases, and playback-system polarity reversal often changes the reproduced sound. Does this mean our ears are phase-responsive, or is there something else here we've been overlooking?

There has been much discussion recently among perfectionists about the importance of what is called "absolute phase" in sound reproduction (footnote 1). Basically, the contention has been that, since many musical sounds are asymmetrical (having different waveforms during positive and negative phases), it is important that a system make the proper distinctions between positive (compression) and negative (rarefaction) phases in playback.

There is no disputing the asymmetrical waveforms. Oscilloscope traces clearly show the upper and lower portions of instrumental waveforms to be quite different (fig.1), and it is obvious that the initial strike of a kettle-drum head will cause a rarefaction wave while the impact transients of clapped hands during applause will cause compression waves. But what has never been conclusively answered is the question: Do our ears perceive these polarity differences? If they don't, then absolute phase reversal in music reproduction should cause no audible change in the sound. If it does, then it seems reasonable to assume that our ears are indeed responsive to polarity, and it becomes important to ensure that a reproducing system maintain the original-sound polarities, reproducing compression waves as compressions and rarefaction waves as rarefactions (fig.2). (Absolute system phasing must not be confused with relative phasing, fig.3.)

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Fig.1 The waveform of a violin tone.

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Fig.2 When overall absolute phasing is incorrect, air compression and rarefaction phases are inverted.

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Fig.3 The distinctions between relative and absolute system phasing.

Many audiophiles who have taken the trouble to experiment with absolute phasing (by reversing the polarity of both pairs of speaker leads) have reported that there is indeed an audible difference, and that in many cases, one system polarity sounds "better" than the other. The conclusion, of course, has been that our ears do respond to polarity, ergo absolute phase is a significant audio-system parameter. There is mounting evidence to suggest that this is not the case.

The differences that are observed when system polarity is reversed—changes in the apparent distance, depth, and "aliveness" of most instruments—have been reported with a disturbing lack of consistency. Some have claimed to hear "dramatic" differences, others whose hearing acuity seems no less well developed have reported very little change or, in a number of cases, no consistent change at all (ie, one polarity sounds right sometimes, the other sounds right other times). Ignoring for the moment the fact that one listener's subtlety is another's "dramatic difference," it would seem that this lack of consistency in the observation of something that ostensibly affects the middle range, where everyone's hearing tends to be most receptive to small changes, must cast some doubts as to whether in fact polarity changes are causing the audible changes.

Note also that there has been no effort made, in any quarter, to actually verify whether what sounds like the correct absolute phase does in fact correspond to the original sound-wave polarity. The "correct" polarity is cited as that which brings the sound forward—a dubious contention when we consider the fact that a common failing of most "perfectionist" systems is a tendency to back sounds away from the listener. (This is called richness and depth.)

Our suspicions that something might be amiss here were brought to a head when we started conducting listening tests on DB Systems' DB-7 phase-reverse box. Those initial auditions were done with the Berning TF-10 preamplifier, the Infinity HCA power amplifier, and Infinity's RS-4.5 speaker system in single-amplifier (non-biamped) mode. We fully expected the usual audible difference—not what we would call dramatic, but markedly audible, nonetheless. Instead, we were hard-pressed to hear any difference at all, from any recording we tried (including discs and original master tapes that had been miked for maximum phase coherence). And in most cases, the polarity which sounded "more correct" at one moment sounded less so the next. The best we could manage on blind switching was a bland statistical 50%, which is another way of saying we could not really distinguish at all between one polarity condition and the other.

The clear implication was that our ears were simply not responding to the polarity reversals. Yet, on previous occasions, we had heard obvious changes when the connections to both speakers were reversed. So, we removed the DB-7 and tried reversing the speaker connections. Still no change.

Then, on a hunch, we tried substituting a cheap-and-dirty pair of compact speakers for the Infinity 4.5s. And Lo, there was an obvious difference when we reversed their polarity. And would you believe, one position sounded consistently better than the other! So we tried some other transducers we had on hand, including headphones that ranged from excellent to decidedly mediocre. The results were almost perfectly consistent with what we felt we knew about those transducers: The better they were, the less difference was noted when polarity was reversed!

An Experiment
Pursuing this further, we then conducted an experiment of dubious scientific value but having interesting consequences nonetheless. We obtained several inexpensive cone-type drivers, un-baffled, propped each one (in turn) on its side (as it would be if mounted in an enclosure), placed a probe mike in front of the cone, and fed a warbled 1kHz sinewave to it, observing its measured output on a VTVM. Then we used fingers to hold the cone a measured distance (1/8") behind, and then in front of, its normal at-rest position. With two of the speakers, the 1kHz level was almost 2dB lower at one excursion extreme than at the other. In other words, they exhibited a nonlinearity which was compressing the signal level more in one excursion direction than in the other (fig.4). And when we got around to listening to those speakers (still un-baffled), the ones which showed the greatest output difference at the two cone positions rewarded us by producing the greatest audible change when their polarity was reversed!

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Fig.4 How excursion nonlinearity compresses half of a symmetrical input.

Why would excursional nonlinearity seem to change the tonal balance of reproduced sound? Probably because, in most asymmetrical signals, one half of the waveform conveys more middle-range energy than the other (fig.5). And when that predominantly-mid-range half-cycle is reproduced by the cone-excursion range which exhibits nonlinear compression, the result is an actual attenuation of those middle-range frequencies.

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Fig.5 One half of this waveform has more middle-range content than the other.

But wouldn't such nonlinearity show up clearly on conventional harmonic-distortion tests? Certainly it would, and almost certainly does. Rarely does any loudspeaker driver measure below 0.5% harmonic distortion at moderate to high listening levels; never to our knowledge has anyone endeavored to pin down the source of the nonlinearities which are causing such relatively high readings. They could stem from asymmetrical nonlinearity.

Summing Up
It would appear then that the necessity for absolute phasing of a system arises in direct proportion to the amount of asymmetrical nonlinearity it introduces, and that the more significant absolute phasing is found to be, the more transducer asymmetry is present. We can, on the other hand, find no evidence to support the contention that our ears can "tell" whether a reproduced sound field is a replica of the original or is inverted in phase. Thus, absolute phase is apparently one of the few audiophile concerns which is actually of no significance, except insofar as it may subjectively improve the sound of systems whose transducers exhibit asymmetrical nonlinearities.

If loudspeakers exhibit this shortcoming, it is reasonable to assume that it afflicts many microphones also, as well—possibly—as circuits incorporating certain configurations of cathode- or emitter-follower outputs. The asymmetrical nonlinearities of these can be expected however to be of many orders of magnitude smaller than those caused by some loudspeakers, so it is pointless to pursue them until the majority of loudspeakers are clean enough to show them up. (For the benefit of those readers who wish to try some of our tests for their own edification, we suggest trying Stax SRX, Infinity ES-1, and Audio Technics TK-33 headphones. If you can tell from these which polarity is which, more than 50% of the time on a blind test (footnote 2), let us know and we'll reconsider our position.



Footnote 1: "Absolute Polarity" is an alternative and more pedantically correct nomenclature.—Ed.

Footnote 2: Track 8 on the first Stereophile Test CD allows listeners to test for themselves if they can detect reversals in absolute polarity.—Ed.

COMMENTS
xp9433's picture

You published a 34 year-old article about absolute polarity, without any up-to-date commentary, research or analysis - WHY?

The implication is that Gordon Holt had it right back in 1980. He didn't! His research was flawed, as anybody would discover if they read the "Wood Effect" by Clark Johnsen, published back then (and still available).

To add insult, after posting this article online you subsequently removed reference to the "leading commentary and reference” on this subject (Clark's publication) from the article!

For a start, Clark makes it clear that to consistently judge absolute polarity you need loudspeakers that have all the drivers connected with the same absolute polarity and thus have a chance of being "phase coherent".

I think you will find the Infinity RS 4.5 didn't meet that criteria! A simple step response measurement would show that some drivers were connected out of absolute phase/polarity to the others.

Back in the 1980's, I had an interest in a manufacturer that made preamplifiers with absolute polarity switching, which added significantly to the cost of production (to do it properly). It wasn't done because it was a fad, but because in testing we could consistently identify the audible difference with the "right speakers". For example, it was impossible to hear differences with the large Shahinian speakers, but easy to hear with locally made Lambert and Canadian made Swan speakers, both of which were phase coherent.

And then of course there is the fact that different amplification often have a reversed absolute phase output. So incorrect absolute polarity/phase becomes a system issue. Reversing the positive and negative leads of both speaker terminals is not the best way to experiment when trying to hear the audibility of absolute phase.

Here is the kicker, we are supposed to rely on the skills of reviewers, most of whom:
1. Don't know about or believe in absolute phase
2. Can't hear the clearly audible differences
3. Insert new items into systems without testing for correct system absolute phase, and thus can make errors in judgement when comparing against their reference system (using their favourite records, which coincidently are usually were recorded with the same absolute phase as each other - another subject).

Stereophile posting of this old article without up-to-date research or current commentary could be seen as an indictment on your editorial policy, Stereophile’s reviewing techniques, and your reviewers’ capabilities. That is, unless you intend to do a follow up.

If so, why don't you involve Clark Johnsen in your testing, to make sure it is done right.

Regards
Frank

audiogeorge's picture

Dear Frank, Editor, and Music-Loving Audiophiles.

FYI regarding Absolute Polarity at: www.AbsolutePolarity.com * www.PolarityGeorge.com

You may respond via email at: AudioGeorge@AudioGeorge,com or call 619-401-9876

Respectfully submitted,

George S. Louis, Esq.
President San Diego Audio Society™ (SDAS)
Perfect Polarity Pundit Chief Polarity Buster of the Polarity Police

corrective_unconscious's picture

"Perfect Polarity Pundit Chief Polarity Buster of the Polarity Police"

"Puerile" comes to mind.

prm1177's picture

Over 20 years ago I attended an AES presentation on absolute phase at Dolby Laboratories. The most revelatory moment was when the presenter told us that the cilia in a human inner ear generates an electrical pulse only when moving in one direction. This means that the inner ear, in effect, rectifies the incoming sound.

I understood then why absolute phase was both audible and controversial. Audibility depended on the source waveform. A inverted waveform with asymmetries would look entirely different than the original after rectification, while a more symmetrical waveform would show less change.

jomondz's picture

Dolby Lake

listentwice's picture

If you have a remote control with a polarity switch (like TacT Millennium) you may learn easily to detect the difference between the polarity options.
Record companies with a history back to mono recording usually keep to the inverted polarity with most if not all their recordings. This works well with a single (mono) speaker because it fills the room with a more diffused reproduction.
Listening with headphones will give a similar impression, the diffused perceived inverted signal reduces the nearness of the signal, inhead-localization.
Only the stereo replay with 2 speakers may ask for a defined well-focussed projection of the phantom sources between the speaker positions. With the correct polarity a tympany will first redraw the speaker cone and not push the woofer cone out when the drummer pushes the diaphragm of the real instrument in.

The eardrum relaxes after a pulse, so a negative (rarefaction) movement will always be followed by a positive - but weaker- excursion. Music with wrong polarity is perceived less dynamic, provides less impact to our perceiption.

The nerves can transport positive pulses only, and the hear cell will be opened anly py positive pressure at the cilia. A relaxation (inactive) period follows.

It takes a while until the fundamental notes provide their positive slope, while their overtones (higher frequencies) have presented several wayves with positive slopes. This easily explains why the inverted polarity appears more diffused and a little disembodied. Treble is perceived earlier, bass later. They do not integrate as well as in nature.

There is no way of manipulating a real direct-to-disc recording that is distributed world-wide.
Obviously the many digital copies pressed to CD may vary over the nations, so a list of record labels with inverted polarity may be taken with a grain of salt.

With some training the differences are easily detected. But mastering engineers seem to have their own opinion what the listener deserves.
Too often an opposite polarity of the real thing.

Alaskagram's picture

First off, I would like to say that I was under the impression that the articles were published in tribute to a departed comrade.Which brings to question your ablity to understand music, equipment yes, art not so much.
Now onto the question of absoulte phase. As an audio technican I have reconed many speakers and O degree phase is not a consideration when mounting voice coils in the gap.There are "resting points" set by the spider, but where the voice coil is centered in the gap is usally determined by desired resonance vs excursion and other factors but not absoulte phase. I would hazard a guess that 0 degree phase would be if the spider rested the center of the voice coil in the center of the magnetic field. I would hazard a guess that it is this you can hear, possibly due to the assemetrical nature of sound waves.Remember there is a mechanical aspect to sound reproduction that is extrmely difficult to model, before you re write thermodynamics check to see if the glue let go on the dust cap.

GeneZ's picture

Absolute phase requires speakers that are phase coherent to truly know what it does. Many speakers have the tweeters and mid-woofers wired out of phase.So, how can audiophiles give an accurate report? I have been using full range speakers. Also a two way speaker with a first order crossover. Reversing the polarity is as plain as day when tested with the needed criterion to detect it. Otherwise? We are only reversing a part of the speakers spectrum at a time with speakers that are not phase coherent. It can never be correct... only what taste finds preferable. This over looked factor is how valuable details in accuracy can get lost over time in the shuffle. When its not tested with the right kind of components needed the test is non productive. Making a difference is not the same thing as making a correction. Once established,the listener can choose for which setting he prefers. Some days I actually like what I am hearing reversing the polarity. Be that as it may. :)

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