Here is a question recently received by RecordProducer.com. Clearly it isn't mother-tongue English, but the meaning is clear…
“Hi! My question is about issues with out of fase recordings. I've heard that if the first belly of the wave is pointing down, the signal is out of fase. But I sometimes can't define were is the first belly of the wave in graphic. Is that true that an Out of fase bass or kick drum would always sound unnatural and it needs to allways be reversed to work correctly?”
I quite like that expression, “the belly of the wave”. We would normally call it the trough, but belly gives it an intriguing image.
Let's look at what happens when the kick drum is 'kicked'. By the way, we call it kick drum rather than bass drum because we can then abbreviate it as 'kick'. 'Bass' is another instrument entirely.
The action is this…
- The drummer presses his or her foot on the pedal
- The beater pushes the batter head outwards
- Air pressure inside the drum pushes the resonant head (if there is one) outwards
- The resulting wave front travels through the air and pushes the listener's eardrum (or the microphone diaphragm) inwards
- If reproduced by a loudspeaker, and correct phasing is preserved throughout the signal chain, the loudspeaker diaphragm pushes outwards, just like the heads of the drum
But what would happen if the phase were reversed? Wouldn't that suck our eardrums outwards?
Well you can easily try it. Take a recording of a kick drum and invert it. Listen to the result. Or you can imagine it – what does the kick drum sound like from behind the drum, as the drummer would hear it?
Well in practice, it doesn't sound much different. And this tells us a lot about the ear's response to phase – it doesn't much care.
For many signals you can invert and uninvert the waveform as much as you like and you will hear hardly any difference. Indeed, if you try an A/B test on a friend (or they try it on you!), you will see that it is often hard to be sure.
It does make sense of course to preserve phase throughout the recording chain because there is simply no reason to do otherwise. And mixing the phases of two or more signals is a recipe for likely disaster.
But do we ever hear of an engineer or producer inverting the phase of a single signal because they prefer the way it sounds?
How about eagle-eared Audio Masterclass readers? How do you feel about phase? Can you hear the difference?
P.S. In common usage, 'out of phase' is used to mean 'inverted polarity'. Since there isn't a cat in hell's chance of Audio Masterclass getting everyone to use the terminology correctly, we'll just go with the masses. When the context is clear there is rarely any confusion.