Phase Cancellation Help?

MattRobertson

MattRobertson

Well-known member
I'm confused on what exactly phase cancellation is, but i'm not totally sure practically how it works. I understand that it happens when two mics that are close to eachother are picking up the same sounds and weird stuff happens. Does anybody have examples of what this sounds like? I have an upcoming shoot where i need to Mic two people on camera in a hotel suite. I've got a DPA 4060 that I love and an Audio Technica AT4073 that is excellent as well, and I understand using a hyper on each of them would make this problem much less apparent yes? They will probably be close by eachother, but for the sake of this shoot, the convenience of using lav's would be very beneficial. We're recording some videos which are going to youtube that will be recorded at different times during the day on this 3 day conference. Wouldn't be practical to hire an audio guy, and I have most all the gear, sans getting a second lav/hyper to match whatever I end up using on one of them. Been wanting to get a 2nd lav/mic to match with 2 person setups anyway.

Just not sure what the best way to do this would be. Would having them on camera 3, 4, 5 feet apart from eachother be too close still?
 
Sound moves as sine waves. Each frequency has a "cycle" (plot the sine wave on a chart: x = time, y = amplitude). One cycle is the time it takes for the wave to go from 0 on the "y" axis to its full positive, down to its full negative, and back to 0. The more cycles per second, the higher the frequency. Cycles are also, in terms of sound and electricity, rated in Hz (Hertz). So 200 cycles per second = 200Hz, which is in the low end of the frequency spectrum (human hearing generally ranges from 20Hz to 20kHz - 20,000Hz - though most people can't hear much over 15kHz).

Now, if sound is collected by two sources at different times, the sine waves will be out of phase, meaning that their 0 points on the y axis will occur at different spots on the x axis. When they are out of phase, they begin to cancel each other out (wherever you have a positive "y" value and a negative "y" value sharing an "x" value... you do the math). If they're 180º out of phase, or starting at the same place but moving in exactly opposite directions, they completely nullify each other (+5 plus -5 = 0, right?).

Phase cancellation sucks the life out of sound, and you end up with a very weak signal with very low amplitude. If you have a near perfect inversion, you'll lose most of your signal and it will sound very, very thin. This cancellation won't happen the same way when using headphones because of the separation of each driver in each ear, but phase cancellation is easy to detect even in headphones when you know what tho listen for... it both sounds and feels like the audio is being sucked into the middle of your head. Also, not all frequencies and sounds are out of phase in the exact same way, so you likely won't end up with absolute silence.

If you have two microphones, and one is closer to the source than the other, the sound will hit one mic before the other... that knocks the sound out of phase between the two mics. How much that phase cancellation affects the signal depends on the distance of the mics from each other, and the balance between the two.

There's a general rule that two mics should be 3x the distance from each other as from the source. So, if you have two lavs and each is 6" from the respective person's mouth, 18" between the two people should help a lot. But for what you're doing, your best bet is to lav them both and record them to separate tracks. Then, in post, you have control over the mix and any phasing that may potentially occur. Phase cancellation won't happen until the two sources are mixed together for playback through the same speaker(s).

Slight adjustments in mix level to favor the audio track of the person talking will also minimize, if not eliminate, phase cancellation. So in your post-production mix, simply mix back and forth between the two as necessary; it doesn't take drastic adjustment, rather only about -6dB difference or so.
 
Oh duh! I knew the answer and didn't even realize it! :D Thanks for the feedback. I'll be going into my Fostex DC-R302 so i'll have 2 channel and it won't be a problem. I mix all my tracks in logic so I can clean up any problems if there are any. Thanks!
 
Blending the two tracks usually works fine - it's the unexpected phase cancellation that wrecks things. Especially when you are recording to mono, or have two tracks panned mono on top of each other. Once you've done that, you are stuffed - especially if you didn't notice. What happens is you get partial cancellation, never complete, because there's often a distance component, so the two waveforms never overlap completely - so the result is a strange sort of 'hollow' sound. Once you have heard it, you will remember it. A bit like the musicians phaser pedals, but instead of sweeping, it stays reasonably constant. Oddly - it also causes problems when people mic up drum kits in live or recorded situations when they use two mics on a snare drum - a popular technique. The sound of the top skin has the crack, while the bottom skin has the rattly snare wires - so miking both up and blending them gives more control over the snare sound, but when the drummer hits the skin, the mic diaphragm moves one way, at the bottom of the drum, the skin also moves the same direction, but as the mic is pointing up, it has a reversed polarity - this gives the strange hollow sound - and the signal from one mic gets reversed by pushing the polarity switch on the mixer channel for one of the mics, restoring the sound. It's only recently people have been renaming this button to polarity, rather than the manual label 'phase'. Nowadays people realise that all the switch does is reverse polarity - to change the phase would mean introducing a delay as phase really is a time effect.
 
Ahhhhh......felt like I was back in school. Good explanation Alex and Paul!!!!!! Couldn't add a thing.

Other than......phase is also a tool that can be used, for the same reason you try to avoid it. When I was trying to remove artifacts from some old recordings in my DAW, I found copying that artifact, flipping it 180 out, and then mixing the 2.....I could remove them quite well, without having "blank spots". I would just play with the phase shift rotation it until the sound was "gone" but there was still something there.

Lots of work, but that was the only option for me at the time.
 
You must log in or register to reply here.
Back
Top