Originally Web posted 8 July 1996.
Content last modified Saturday, 9 July 2016 .
External links last verified Wednesday, 25 February 2009.

Polarity Matters

Sonic Purity —Independent Broadcast Engineer— formerly of Albany, CA (formerly with KALX-FM Berkeley)

In this age of advanced technology (or at least so it seems to most of us at the present), many of us assume mundane factors such as polarity have long ago been standardized, or found to be universally inconsequential. Well, i’m here to testify that it ain’t so…

Absolute Polarity (or phase) and the Freeman Effect

Does polarity matter? If, at an instant of time, the sound wave at the microphone is pushing in on the diaphragm, is it essential to accurate reproduction that the transducer at the other end also push, or may it suck at that instant?

Although often taken as a given by individuals who find sonic nirvana by marking up CDs with green pens, freezing them, using vinyl whenever possible, and preferring almost any tube equipment to almost anything solid-state, some “more mainstream” authorities, such as acoustic consultant Chips Davis and recording studio maven Stephen St. Croix ("Sooner Or Later", MIX magazine, May 1991, p.19ff) maintain the same conclusion of the relevance of polarity, with Mr. St. Croix going as far as to say, “It does matter—always, under all conditions, except for really useless recordings”.

Despite this admonition, critical listening at several of Mr. Davis’ SBE presentations, and other attempts from time to time, this writer cannot claim to hear “absolute phase” differences on reproduced music, at this point[1]. However, absolute polarity when listening to one’s own voice “live” over headphones is as easily audible as one speaker of a stereo pair being flipped 180° in phase, and can be at least as annoying!

Enter Freeman

How is listening to one’s own voice as one is speaking different from hearing a reproduction? Bone conduction!, as pointed out by KIIS C.E. Michael Callaghan in his excellent article "The Freeman Effect" (Radio World 1 April 1987, p. 12. There have been at least two followup letters to the editor in RW over the last several years). Quick summary: one’s voice via bone conduction is (by definition) “in phase” (noninverting), so if a simultaneously audible electronic[2] signal path is present, an overall polarity inversion in this electronic path produces a relative phase difference centered around 180° (ignoring goofy phase shifters) between the electronic and bone-conduction paths. This, as usual, introduces all the concomitant comb filtering and other “exciting” phase effects one usually gets under these circumstances. The solution? Flip the polarity somewhere in electronicsland to ensure no overall polarity inversion.

If you haven’t yet personally experienced this phenomenon, don’t take anyone’s word for it: set up the cleanest, most accurate mic->headphone chain easily available. If there isn’t already some piece of equipment in the chain which switches polarity (a mic preamp or, oh say, an Auditronics A-210 console, whose headphone E.Q. In/Out switch also switches polarity[3]), all the trade press articles on the Freeman Effect i’ve seen to date recommend changing the wiring or installing a switch at the mic end of the chain, since the signal here is balanced, and headphones almost never are. Set levels, and speak as an announcer normally would, then flip the polarity and repeat.

Expect to hear yourself in “good tone” in one position (noninverting), and some combination of nasalness, “head stuffiness”, “phasey” effects, and level drop in the other position (inverting). Audibility depends heavily upon headphone and mic quality: the more accurate the transducers, the easier it is to hear the difference. I found it quite difficult to decide which position was “correct” or “better” with cheap headphones and/or microphone in terms of sound quality. But no matter how poor the accuracy of reproduction, i was always able to find level nulls when the electronics inverted (where my voice would sound as though it had dropped to nothing in the headphones) by moving my head-to-mic distance, but could produce no such nulls with noninverting electronics.

So just wire the mics so they sound good, or put in a DPDT switch… simple, right? Weeeelllll…

Are There No Standards?— The Tin Can Analogy

If one takes it as a given that absolute polarity needs to be maintained from microphone to listener[4] in order to stay competitive soundwise at the Turn of the Millennium, flipping polarity at the mic (especially with a switch) or anywhere in line to the transmitter rapidly loses desirability. Such gymnastics would be better reserved for the headphone path, yet it is often more difficult to flip at this end, esp. without adding a whole inverting gain stage. Are there not standards promulgated to relieve a dilemma such as this?

As a starting point, let us revisit a well-known, early standard audio system:

tin cans with string

In this system, as normally utilized, a pressure wave (positive sound pressure) impinging upon the bottom of the “sending” can pushes this diaphragm inward, slackening tension on the string. The reduction in tension allows the “receiving” can diaphragm to push out, reproducing the original pressure wave. If one defines positive string polarity as a slackening of string tension originating at the sending end as noninverting, then the overall tin can setup is noninverting, and reproduces a pressure wave at the receiver when one hits the transmitter, just as though the listener weren’t using a transmission system. Nicely intuitive and straightforward.

Do existing electroacoustic and electronic standards conform? Let’s start at the microphone. Quoting Glen Ballou’s presentation of EIA-221-A (formerly RS-221-A) in the Handbook For Sound Engineers— The New Audio Cyclopedia (section 13.15.3, p. 383-384), “The positive or in-phase terminal is that terminal that has a positive potential and a phase angle less than 90° with respect to a positive sound pressure at the front of the diaphragm”. Wiring all XLR or other connectors consistently (hopefully all XLRs pin 2 hi, pursuant to AES-14 1992, EIA-297, et al.) will thus present a positive potential to the mic preamp. So far, so good.

Though i could unearth no standards for polarity in amplifier stages, the overwhelming majority of professional broadcast and recent high-quality home audio amplification equipment measured maintains noninversion from input to output, at a minimum along the primary signal path (tape decks and such are more dicey). Noninversion at each stage is intuitively appealing in that any number of stages may be used with no change in system polarity. Let us assume for the sake of this discussion that the overall electronic path to the monitor speaker terminals and headphone jack is consistently noninverting, and that this is proper and desirable, especially since this definition would correspond with the “Tin Can Standard”.

Though i have yet to run across a standard to this effect, every loudspeaker driver i have measured equipped with a factory terminal polarity indication produces a pressure wave when fed with a positive potential, in theory re-creating the pressure wave which originally impinged upon the microphone diaphragm—once again Tin Can compliant! One would expect to find the same with headphones….

Steeveee! My Head’s Out Of Phase!

So i proclaimed to Steve Hawes, KALX Chief Engineer, in early August 1994. We had just finished installing a new Auditronics A-210 console in the Production Studio, and i was performing final function checks. Having been diligent about maintaining correct wiring polarity (including all XLRs pin 2 hi) throughout the room, i was amazed to hear my voice clearly “messed up” in my AKG K-240 headphones, speaking over the Sennheiser MD-421U mic. Similar precautions had been undertaken in the Air Studio during its rewiring in 1989, yet with the same model mic, and the same pair of headphones, there was no problem in Air.

Just as i was about to decimate the integrity of the XLR on the MD-421U, based upon mouth popping test indications of pin 3 hi, Steve handed me a most interesting box, containing a 45Ω intercom speaker with foam stuffed between the frame and the rear of the cone, a 9V battery, 1000µF 20V electrolytic capacitor, SPDT lever switch, and BNC[5] connector. Using this Cosky Phaser (details to follow), i was able to more accurately test the 421, and found it did indeed already conform to the pin 2 hi standard. The rest of the system from mic pins to headphone jack clearly measured noninverting (headphone E.Q. Out). My K-240s had been a reliable reference for over a decade. I was stumped, and was beginning to think maybe bone conduction could be inverting!

Things Are Not Always As They Seem…

Further investigation revealed the answers: the Air studio mic cables were wired pin 3 hi, and the K-240s produced rarefaction with a positive potential applied (in other words they sucked when they should have been blowing)—definitely not Tin Can compliant. Was this a common headphone standard?

Measurements of 14 pairs of headphones representing 7 manufacturers and all quality levels from recording studio mainstay to painful revealed the majority push air with positive excitation, though two manufacturers preferred their products inhale with positive excitation. Sigh.

Ever fascinated, i surveyed a representative cross-section of manufacturers of headphones commonly found in broadcast and recording studios. Representatives of Koss, beyerdynamic, and Stax report no knowledge of a promulgated standard, but all their products (with the exception of an older Stax model) are claimed to comply with the pressure wave upon positive excitation “tin can” standard, which is each of these firms’ internal working standard. Uwe Sattle, Technical Manager at Sennheiser recalls an AES or IEC standard to the effect of outward reproducer diaphragm motion with positive excitation, and notes that Sennheiser headphones conform to this. Most commendable perseverance on the part of Gary Henderson in the Sony Pro Audio division (thanks, Gary!) finally provided an answer from Sony’s Sustaining Engineering Group in Japan: all Sony Headphone and speaker diaphragms move toward the user’s ear when excited by a positive-going waveform (“tin can” standard). David Rohn of Harman/JBL Professional, representing AKG products, reports that AKG headphones are consistently wired to rarefy the air when positive excitation is applied, and that the Austrian parent company explains that this is part of their design philosophy.

Indeed, of the 4 pairs of headphones which rarefy when positively excited (inverted polarity per the tin can standard), three were AKG (early-mid ’90’s K-240M, 1982 K-240, 1983 K-340). The fourth was a pair of Telex 610-2 monophonic “language lab” headphones.

Is There A Headphone Polarity Standard?

After having spent most of the first half of 1995 in hot pursuit, what to my wondering eyes should appear, but EIA RS-331 (December 1966), which proudly proclaims (in part) “The following polarization is standard: The individual phones shall be polarized so that diaphragms move toward the listener’s ear when a positive potential is applied to tip or ring, with negative return to the barrel” [sleeve]. It’s intuitive, and matches the “Tin Can Standard". Seemingly, RS-331 has been lost to the ages, at least in the eyes of some headphone manufacturers. Then again, it proudly proclaims Tip=right and Ring=left, which was blown away by the Japanese/European? de facto (EIAJ?) Tip=left, Ring=right standard in the early ’70’s (everyone has agreed on this since about 1973, best i can tell. If you still use older American equipment wired to RS-331, you may wish to verify channel assignment…).

On the subject of polarity standards, SMPTE Recommended Practice RP 134-1994, Polarity for Analog Audio Magnetic Recording and Reproduction, has tons of useful tidbits for keeping polarity straight from mic to speaker. Besides reconfirming the Tin Can Standard for mic and speaker and XLR pin 2 hi, it defines a “positive magnetization is the same direction of magnetic flux flow as that observed in a bar magnet where the flux flows out of the north pole and into the south pole”, and that this positive flux should be in the direction of physical movement of the magnetic medium. Best of all, it describes an easy way to measure this with a half-wave rectified sinewave (or other asymmetrical signal) and a piece of wire!

So Where Does This Leave Us?

Again assuming one abides by keeping pressure waves noninverted at least as far as the tower (or the recording medium du jour), the fundamental choices are:

  1. Ensure all the facility’s headphones are consistent in polarity (hopefully RS-331 compliant), and that all headphone signal handling is complementary (preferably noninverting), or
  2. Install headphone polarity inversion and switching circuitry (if none exists) and teach everyone what it is, and how to use it.

I personally feel this is no tempest in a teapot, but has everything to do with getting the best from your talent. Having worked closely for 15 years with many individuals hearing themselves electronically for the first time, and based upon my own early D.J./production experiences, let me assure you that self-esteem in regards to the sound of one’s voice is critical not only to the immediate performance, but to the decision to even pursue this line of endeavor in the first place. I feel it is especially critical for any broadcast/recording organization which considers itself a training facility for our future generations to expend the effort to maintain correct polarity from every mic to every headphone, and hereby encourage adoption of the polarization standard in RS-331 (same as Tin Can standard) by all equipment manufacturers and users.

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Anyone having information on additional polarity standards, comments, criticisms, questions should e-mail the author.

[1] I believe it’s important, and run my life accordingly, but i still haven’t heard it.

[2] actually electronic/electroacoustic.

[3] In = inverted. There by design, or by accident??

[4] Don’t gripe too heavily about polarity standards at the listening end. 13 years of home audio repair & testing revealed only one or two post-1975 tuners or receivers whose tuner sections differed in polarity from an arbitrary bench reference unit. Almost no power/integrated amps invert.

[5] During updating of this article to utilize HTML <abbr> and <acronym> tags in March 2005, i was surprised to learn that there does not seem to be one agreed-upon definition of the abbreviation “BNC”. Some of the popular options are Bayonet Neill-Concelman, Baby Neill-Concelman, and Bayonet Nut Coupling. As is often the case, one can find the current thoughts on this in the appropriate Wikipedia article.

Next: Easy Polarity Measurement

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