Last month’s column on polarity prompted a number of comments from readers, including a few horror stories. One reader related that he once worked for a manufacturer that produced a set of interface cards that used an A/D chip inadvertently designed with an internal polarity inversion. An end-user found the flaw, necessitating a small recall. In another case, a major studio’s staff discovered that an early version of a multi-hundredthousand-dollar console had an absolute polarity inversion on the outputs. Inserting a couple of simple polarity flippers reportedly “made a huge difference — especially when soloing a kick drum.”
An acquaintance that formerly worked at SSL, Wayne Kirkwood, wrote with interesting perspectives from his experience:
“A number of years ago on the Prodigy-Pro forum, I posted two ‘identical’ sound files and asked the readership if they heard any difference in the two. Most responses were that they were different — different mic or EQ was the most common statement — with one regular poster and experienced engineer going so far to ask if they were the same take. The difference, which I’m sure you’ve already figured out, was polarity.
“Some instruments, due to their waveforms, just sound ‘right’ or very wrong — to me at least — depending on whether they’re upside down or right side up. I’ve found that I almost always chose positive pressure to the ear. As you know, whether an inverted electrical waveform appears right or wrong is a virtual coin-toss once the chain is broken and the signal passes through an inverting link. One sure-fire way that I’ve found to check the polarity of a mic is to simply speak or sing ‘ah’ or ‘ee.’ The glottal pulse our vocal cords produce always generates positive pressure and will have positive asymmetry. Once verified, this mic can be used to confirm ‘up’ and ‘down’ for the rest of the chain all the way to the speaker. I learned this in my work in speech recognition and cochlear implants. It’s a great tool to detect fundamental pitch (ƒ0) in speech. Sometime around 1993, I built a ‘meter’ which would check electrical, and with a known polarity mic, acoustic polarity. My thought was engineers could use it to check both electrical and transducer polarity throughout the chain making sure that ‘up stayed up’ all the way to the master. Although there are ‘poppers’ to do this designed for sound reinforcement setup, I proposed a meter to be used on actual material much the way the typical console phase meter indicates (or tries to indicate) relative phase. The polarity meter I designed would indicate not only absolute polarity on signals which have asymmetry but also the degree of asymmetry. Signals which had high even-order harmonics, and thus a higher degree of asymmetry, would drive the bargraph higher.”
This is interesting fodder for thought, and confirmation that maintaining absolute polarity is worthy of validation in your signal chain.