I was recording some electric guitar in the studio recently. I recorded the guitar on three tracks simultaneously. We took a feed from the D.I. box on one channel, and then placed two microphones on the guitar amplifier speaker, a Shure-SM57 dynamic mic and an AKG-C414 condenser microphone.
When I came to listen to the Pro Tools session a day later I noticed that the guitar sounded particularly ‘tinny’ and ‘thin’ without the D.I. signal. I knew there was an issue because when I soloed each of the guitar tracks separately it sounded nice and full but as soon as the two sounds were played together a lot of that good signal disappeared. So using Pro Tools I zoomed in really far so I could see that actual shape of the wave forms and that is when i noticed the two microphone tracks were almost exactly out of phase. (See Below)
Where one wave form goes up the other goes down, effectively cancelling each other out. When sound wave forms are out of phase with one-another it causes destructive interference and can have a really terrible affect on your audio.
Fixing The Problem
This is a problem that I should have noticed when I was recording the guitar but at the time I was concentrating so much on playing that I wasn’t truly listening to the quality of the signal. It may be due to the spacing between microphones on the amp. If one microphone is further away from the amp than other microphone the sound will reach the the second mic at a slightly delayed time. On this occasion however due to the fact that the signals are almost exactly 180 degrees out of phase I think it is more likely an issue with the mixing desk or cables that I used. To correct this problem in Pro Tools I used the ‘Invert’ function to flip the polarity of one of the microphone signals. This feature is synonymous with most DAWs. Below you can see the corrected waveform.
This is an example of the guitar tracks played together so you can hear what the phase cancellation does to the sound. Throughout the clip the polarity of one of the tracks is inverted back and forth so you can compare and hear what i think is a vast improvement.
*Other than the inverting of one of the channels the audio has had no editing or mixing applied