How do you reverse the phase on a mic?

[Kellie's Dad]

I've heard a bit about phase cancellation, like when you use 2 mics, say one in front of a guitar amp and one from behind. How does one go about reversing the phase? Also, I heard at one time that you would not encounter a phase cancelletion problem if the 2 mics are of the exact same make and model, true?

 

[Cave Dweller]

the easiest way to do it is to reverse the hot and cold wires on one end of an XLR mic cable... these should be pins 2 and 3... just re-solder them and label the cable as "phase reverse". if you are micing the front and back of an amp, or top and bottom of a drum, or opposite sides of just about anything, no matter what mics you use

 

[Creepy]

While double micing something like described usually requires phase reversal of one of the mics, it should be noted too that the distance from the source has something to do with phase. Often, when I mic the bottom of the snare drum, either the top or bottom mic is farther way from the drum than the other. Let's say that the bottom mic is twice the distance from the head as the top mic, phase reversing the mic would actually cause phase cancellation. So, I am just saying that you need to listen to the mic combination to hear if phase needs to be corrected on one of the mics. Also, reversing phase only cures phase problems that are 180 degrees out of phase. Your mic may only be 30 degrees out of phase with each other. Reversing the phase on one of them may cause more phase problems in the this case than you already have. As with most things, moving the mics around is the best solution to try first, then resort to other fixes if problems persist.

Most microphone preamps have a Phase button or switch on them. It may just be a symbol of a 0 with a / through it.

The way to check phase is to only listen to the two tracks together, panned center in the monitors. Reverse the phase on one of the mics and listen to what happens. Usually one or both things might happen. You will either lose a lot of low end in the sound, or, the sound will sort of seems to circle around your head when you are in a perfect triangle with the monitors.

Like I said earlier, switching the phase on one of the mics may not solve your problem, in the case that the mics are only slightly out of phase.

There is an old rule in mic placement called the 3:1 rule. A mic you don't want picking up a sound should be at least 3 times farther away from that source than the mic you are picking up the sound with. This is something to remember too.

I am a Creep!

 

[matty_boy]

If you are recording on a p.c., you can get a plugin that reverses phase as well. Analogx makes one.

 

[hauT]

Do the phase cancellation happen when you record or when you play the sound? If it happens only while playing, then it also should help to move one of these tracks 1 or 2 ms forward (or backward). Only if you record on a PC and multitrack. Wouldn't it be the same result as placing a mic farther away?

 

[Creepy]

Phase cancellation is a physical thing. In this sense, it is the result of two mics picking up the same sound, but at different places of the actual sound wave. When it is played back, you will have the same frequencies almost in time with each other, but not quite! We are talking a couple ms or less. Basically, on one mic, the sound is on the + side of the cycle while on the other side of the mic, the same frequency is on the - side of the cycle. Complete phase cancellation is when they on exactly opposit sides of nuetral from each other.

Moving the tracks around in an editor might help, but it can also have unpredictable results in the actual sound of the audio. To make a long story short, check your phase in mono. You will find that usually, it is only certain frequencies that are out of phase with each other, and this could be used creatively as well as being destructive to the audio. You basically have to use your ears to tell. If the two mics are going to be panned hard left and right, then phase takes on a whole new meaning as opposed to using to mics to create a deeper sound in mono per se. In the hard left and right panning deal, the audio would seem to sort of swirl around your head and would tend to confuse the listener as the exact location of things in the stereo field. In the case of creating depth on an instrument with two mics picking up the sound, you would be panning them in the same exact spot in the mix, so you only need to consider that the audio will be deficient in certain frequency ranges. This could be either good of bad. Depends on what you want to hear really.

When it is all said and done, like most things in recording, you should strive to do things correctly while tracking instead of trying to fix things in the mix. Moving tracks around in the time line of the song in an editor just isn't the same as good mic placement while tracking. With the mics in the right spot, you have to factor in the sound of the room in that spot into the overall sound. You will not have that same effect by moving the track forwards or backwards. I am not saying that moving the track around won't sound good, it will just sound different than moving the mic because moving a mic away from the sound source increases the indirect to direct sound at the mic, meaning you hear more of the room. While moving the mic closer to the source increases the direct to indirect sound at the mic. Some of the phase cancellation at the mic may be due to reflections of nearby walls rather than the direct audio being out of phase between the two mics. So, like I said earlier, the different effects altogether. Which is better is totally up to the person listening and making the decision about the sound while recording.

I am a Creep!