Pedants everywhere (myself included) silently correct people referring to the ‘Phase Button’ but Polarity Invert, that funny button with the O with a line through it is one of the most powerful tools we have in audio. The reason for this is that by inverting a signal you create a precise opposite of the original and if you mix your inverted version back in with the original you will attenuate it until it disappears to silence just as surely as one minus one equals zero. Here are five ways inverting a waveform can lead to brilliant things.

Mid Side

One of the first places people encounter the surprising power of polarity is when they first encounter Mid-Side. Whether recording with a Mid-Side array of a cardioid and a figure 8 mic or using mid side processing, the ability to capture and process the sum and difference between a pair of stereo signals is a revelation to the aspiring engineer when they first encounter it.

Central to the way Mid Side works is the role of polarity. The Sides signal, as captured by a sideways facing fig 8 mic doesn’t produce that smile-inducing width when you push up the S mic’s fader without the polarity being reversed on, by convention, the right hand side. It has to be said that the super-wide, and very mono incompatible, Sides channel is the crowd pleaser but it is the inherent mono-compatibility of the Mid channel which is the really useful part of an MS pair. Leaving mic arrays to one side, the creative possibilities of MS processing is alluring and the ability to apply EQ to the sides for some added sparkle is just the beginning of what can be achieved.

Like so many good ideas, there is nothing remotely new about MS. It’s as old as stereo and is central to how cutting vinyl in stereo works. Have you ever wondered how two channels can be reproduced by one stylus in one groove on a record?

If you’re a Pro Tools user and want to experiment with Mid Side download this Pro Tools session which allows you to convert stereo material to Mid Side for processing and back to stereo. While there are plugins which can do this for you, this session uses the Pro Tools Mixer and Trim plugins to do the necessary routing and polarity inversion.

Multipattern Mics

You need only look at the price difference between a multi pattern condenser mic and its cardioid-only variant to see that multi pattern mics are effectively two mics in a single enclosure. The dual diaphragm arrangement of a multi pattern mic allows any of the first order polar responses from omni, through cardioid, to fig 8 to be created by combining the outputs of these capsules in different amounts and with different polarities.

If you’ve ever wondered exactly how this works, both capsules are cardioid, arranged back-to back facing in opposite directions. If you combine them together you get an omni output. Combined together but with polarity of one diaphragm reversed you get fig 8, to get cardioid you just use one diaphragm. To get the ‘inbetween’ patterns you vary the relative level of the diaphragms.

To see just how powerful this polarity control can be, check out the Austrian Audio OC818 which allows you to output each diaphragm separately, allowing you to design polar patterns which vary with frequency!

Cardioid Sub

However clever our tools become, there are some annoying consequences of physics which conspire to compromise our experience of audio. Take controlling the dispersion of sound from a speaker. We take it for granted that higher frequencies can be controlled by waveguides and are relatively easy to direct in a specific direction. However it has always been a given that bass is more or less omnidirectional. Being able to control and direct deep bass has been something that PA engineers have been able to do successfully, though it has to be said the scale of large events is an advantage when working with low frequencies. Working outdoors on a large system more control of bass can be achieved but this hasn’t until relatively recently been something available to studio monitoring.

Through the use of multiple drivers, and clever use of DSP including manipulation of phase and polarity, the Kii Three studio monitor manages to control the directionality of its bass output, minimising the output to the rear extremely effectively. Kii are extremely effective but aren’t unique in controlling the directivity beyond what is usually achievable using waveguides. Dutch and Dutch incorporate a passive, acoustic cardioid ‘midwoofer’ handling frequencies down to 100Hz but the electronic approach taken by Kii can be extended further and expect to hear more and more about beam steering systems which can deliver audio so specifically that it is possible to deliver different audio to different listeners over loudspeakers. While beam steering involves far more than just manipulating polarity, a cardioid subwoofer can be created simply by placing three subs together with one facing backwards. The backwards facing sub’s polarity is inverted because it is facing in the opposite direction. The combined output is cardioid.

Noise Cancelling

While most consumers are yet to experience beam steering loudspeakers, most people have experience of noise cancelling headphones. Travellers everywhere from long haul flights to trains full of commuters know how effective the best Sony headphones or Apple AirPods are when used to exclude external noise. While it’s become more and more sophisticated, this technology relies on polarity and cancellation to achieve what it does. This technique of cancellation by summing a sound with an inverted version of itself lies behind far more than just getting some peace on a plane.

Line balancing, the technique by which a balanced audio cable carries a ‘hot and a ’cold’ version of the signal with one signal being polarity-inverted at one end and being re-inverted again at the other results in the wanted signal being cancelled out then ‘un-cancelled’ again but any unwanted interference picked up by the cable only being cancelled once, therefore being left inaudible. The same principle lies behind humbucking guitar pickups and for those who need to overdub performers who are monitoring playback over speakers the “Spill Pass’ technique is clever, effective and relies on polarity inversion.

If you’re recording someone who needs to hear their playback over speakers, for example an ensemble where providing enough headphones would be impractical, record two passes. Once with the monitor playback only but with all mics open, and a second time (without changing anything) with the monitor playback and the performers performing. Invert the polarity of the ‘spill pass’ and the monitor spill will be cancelled out on playback leaving just the performance!

Null Test

Lastly there is the Null Test. The final arbiter of truth when answering the question ‘are these two sounds identical?’ If you invert one and they cancel to silence they are the same. No ifs, no buts. Though check this parody article which claims different. And of course you have the ‘Delta’ available so if they don’t cancel perfectly you can hear exactly what the difference is. Try this with a wav and an mp3 of the same audio and you’ll never look at mp3 compression the same again!

Other applications of this use of the null point as significant and identifying something useful are using a polarity flip on one of a pair of mics on a guitar cabinet to help manually time-align them. While we have tools like Sound Radix’s Auto Align to do this automatically, given the choice I’d always do it the old fashioned way and by moving one inverted mic past a second on the same speaker of a cabinet. The deep null you hear as they pass each other shows you just where the diaphragms are exactly aligned.

While this guitar example uses polarity to align signals in time, a similar approach can be used to match the gain of two mics exactly. Place the two mics as close as possible together. Invert one and monitor their summed output on a meter. Adjust one to an appropriate level and then adjust the other to exactly the point where they cancel. This can be easier with a steady tone than with complex signals like music or speech. You’ll see, and hear if you’re monitoring the output, a distinctive narrow dip and the point of cancellation. That is where the gains of the mic and preamp combined are identical. Because of this, this technique is better than using digitally controlled mic preamps because it takes the, possibly not identical, sensitivities of the mics into account as well as the preamps.

I could go on but hopefully you will agree with me that the simple act of turning an audio signal upside down is one of the oldest and most powerful tools available to people working in audio. What other examples are there? Share your thoughts in the comments.