Mic Phase

[tobywan]

Hi Guys,

Just wanting to record a drum kit with 2x mics on the snare. One top and one bottom.

I am going to use my 2 SM57's for this (all I have) but I keep seeing / reading things that say I have to have either mic out of phase with the other. I can understand the principal as to why this has to be..etc BUT how do you do it? Is there something in Pro Tools (button ?!?!) that you click to change phase?

Sorry for sounding like a noob but I have never used 2 mics on a snare before and thought I would give it a go. I just want to try and do it right

Cheers
Toby

[Silent Sound]

There should be an "invert" option somewhere under the audio effects, or whatever it's called.

[MikeFFG]

Originally Posted by Silent Sound

There should be an "invert" option somewhere under the audio effects, or whatever it's called.

ya invert under audiosuite plugins will do it. You can also insert an RTAS such as the digirack EQ or the trim plugin, and hit the phase button. It is a circle with a line through it.

[CJ Mastering]

Originally Posted by tobywan

Hi Guys,

Just wanting to record a drum kit with 2x mics on the snare. One top and one bottom.

I am going to use my 2 SM57's for this (all I have) but I keep seeing / reading things that say I have to have either mic out of phase with the other. I can understand the principal as to why this has to be..etc BUT how do you do it? Is there something in Pro Tools (button ?!?!) that you click to change phase?

Sorry for sounding like a noob but I have never used 2 mics on a snare before and thought I would give it a go. I just want to try and do it right

Cheers
Toby

Why not try moving the position of the mic or mic's so that they are in phase. This would be my first move.
an inch here or there can make a difference in phase.
CJ

[Silent Sound]

Thanks MikeFFG. I'm tired and don't have it in front of me so I couldn't remember the exact menu name.

You can also zoom in super tight on the waveform, and nudge it one way or another to line up the phase so that the two waves rise and fall at the same time. This is usually easier to do with a single source, like a guitar amp, as some of the bleed from the other drums and cymbals might make it a bit harder. But if your snare drum has a good crack to it, it shouldn't be too bad.

Lastly, a lot of preamps have an invert button on them... but I'm guessing yours doesn't or you would have gone for that first. Lots of good options here for you. Any or all could work for you, but it's good to know them all. Especially moving the two mics around. Sometimes you can use phase cancellation to your advantage (kinda like an EQ).

[Eric Pederson]

Originally Posted by CJ Mastering

Why not try moving the position of the mic or mic's so that they are in phase. This would be my first move.
an inch here or there can make a difference in phase.
CJ

Top and bottom mics are naturally out of phase with eachother due to the top mic picking up a "-pull" and the bottom mic picking up a "+push" depending on how the cables are wired tutt


Lol just joking, tell me if I am wrong here

[mikethedrummer]

Originally Posted by Eric Pederson

Top and bottom mics are naturally out of phase with eachother due to the top mic picking up a "-pull" and the bottom mic picking up a "+push" depending on how the cables are wired tutt


Lol just joking, tell me if I am wrong here

No you were right. Think about it. One mic is facing towards the ceiling, one is facing towards the floor. The sound hits both of the mics dead on, so both diaphragms collapse inwards towards the back of the mic. So one goes up, the other goes down. In a perfect world they could cancel out completely and no sound would be heard.

So to the OP, just keep the physics in mind. There will always be phase issues when using multiple mics. ALWAYS. The idea is to get as few as possible. This would bring up room treatment but that's a whole novel I don't want to get into.

For a bottom snare mic, just get the sound you like from the snares and try to get the mic within 1" of the bottom head.
For the top snare mic, get it more than 3" above the head and aim for the point where the stick hits. Phase problems are unavoidable, but here we are taking into account something called the 3:1 rule, i.e, if a mic is 1' away from an object, the second mic should be at least 3' away.

[Blast9]

Just to clarify - you mean *Polarity* rather than *phase* thumbsup

[tobywan]

WOW, loads of good advice. There is indeed an invert function in the audio suite menu...probably should learn the software first

I like the idea of the 3:1 rule as well. I might start having a play around with positions and phase (Polarity) of the mics.

Cheers guys

Toby

[theblue1]

Originally Posted by Blast9

Just to clarify - you mean *Polarity* rather than *phase* thumbsup

Right. You reverse polarity in order to get the two signals from the opposite sides of the drum head to reinforce each other instead of (potentially) partially canceling each other out.

By contrast, when CJ Mastering suggests moving the mic to change the relative distances from the drum head of the mics, he is talking about changing the relative phase relationships of the two signals, vis a vis the sound emanating from the head.

But phase is a term strictly relative to the frequency of a wave. Since the cycle period of every frequency is different, those phase relationships are strictly related to the frequency of any given wave component of the aggregate sound. And that is something a lot of folks around here either don't get -- or simply are too lazy to address correctly.


With a single skin drum, by moving one mic far enough away from the struck skin so that the signal reaching the mic is now delayed by precisely 1/2 the wavelength of the fundamental, we achieve a change in that phase relationship equivalent -- in a sense -- to reversing the polarity -- but only at that fundamental. Other frequencies will have varying amounts of cancellation or reinforcement when the two signals are summed, often leading to the familiar 'comb filter' effect.


So, under that latter scenario (moving one mic), if we consider the fundamental of the drum to be our primary concern with regard to phase (assumptions are often dangerous in audio) and the fundamental to be 800 Hz (to pick a number I've heard a few times, though, of course, the fundamental tone of a drum, and the concentration of energy at a specific frequency depends in large part on how -- and how well -- it's tuned. (Info on snare drum physics: The Snare Drum)

Here's a (simple) wavelength calculator (it assumes 'standard' values for temperature and altitude/air pressure): Wavelength

From that we get a ballpark figure of ~17 inches for the WL of our 800 Hz fundamental. So, to change the phase of that signal in such a way as to invert an 800 Hz tone 180 degrees, you would move that mic ~ 8.5 inches farther (or nearer) the drumhead, vis a vis the other mic.


But it's important to remember that that snare sound is not only comprised of its fundamental pitch -- drums -- and particularly snares -- have a tendency to produce extremely complex waveforms with a lot of different frequency components. There are the myriad of issues revolving around the complex character of the snare drum, particularly the fact that (while many drummers remove the bottom skins from most of the drums in their recording kits) the drum will generally have two skins. At lower frequencies, the skins will tend to move in the same direction. But at the higher mode formed by the enclosed space, the skins will actually be moving apart, making the sound quite complex. And then there is the snare 'spring' itself.

So, in all likelihood, moving one of the snare mics may produce pleasing results in the sum of the signals but it will not be that much like what would be accomplished by having both mics equidistant from the drum head and reversing signal polarity of one of them.



The 3:1 relative distance rule of thumb will help save your sanity. Keep in mind it's a relative guideline ballparking the relative levels of a given signal reaching each mic. (You're basically trying to get the level of a given drum loud enough in its own mic that it will dominate when that mic is summed with other mics, and relative effects of cacellation are minimized.)

When considering phase relationships in complex drum miking scenaria, another sanity-saver is to focus on one drum and its relationship with its mic vis a vis the other mics around the kit at a time.

Since sound radiating in a free space is basically inversely proportional to the square of the distance, we know that if mic X is one unit of measure away from a single point sound source and mic Y is three units away, the level of the signal reaching mic Y will be nine times (3 squared) less than that reaching mic X. (However, there is a lot more chaos there, though, since a drum head is certainly not a single point source.)

[Eric Pederson]

Originally Posted by Blast9

Just to clarify - you mean *Polarity* rather than *phase* thumbsup

Yes Polarity, thank you.

But technically, "in a perfect world" like mike said, arent the top and bottom mics "180 degrees out of phase" with eachother?

I was trying to draw it out like my instructor did for us (years back) and couldnt really figure it out

[theblue1]

Originally Posted by Eric Pederson

Yes Polarity, thank you.

But technically, "in a perfect world" like mike said, arent the top and bottom mics "180 degrees out of phase" with eachother?

I was trying to draw it out like my instructor did for us (years back) and couldnt really figure it out

I suspect I was still writing my post (above) when you asked. Take a look at that.

It's a bit more complex than it might seem from a quick absorption of various rules of thumb.

That's why -- as valuable as rules of thumb are -- it's probably best to understand the underlying principles of physics that fundamentally inform those brief, aphoristic renderings of practical strategies we love to throw around like candy on Hallowe'en.


The issues I outlined above are just some of the things that go into making the misuse of the term phase (when one actually means signal polarity) one of my pet topics.

[Crash]

While all this is good, you need to just play with it. Sometimes you will prefer it in and sometimes 180 out.

[RCM - Ronan]

I solve this problem by never using a bottom mic on the snare.

[Crash]

Originally Posted by RCM - Ronan

I solve this problem by never using a bottom mic on the snare.

I don't do it often either.....

[theblue1]

Originally Posted by RCM - Ronan

I solve this problem by never using a bottom mic on the snare.

Originally Posted by Crash

I don't do it often either.....

Gotta say that I mostly didn't either, in my (fondly remembered but long lost) live kit miking days.

That said, I've been on sessions where guys got pretty good results with it. But I've seen maybe at least as many sessions where the bottom mic got tossed out of the mix early on, which was typically the case when I tried it.

[Roc Mixwell]

Originally Posted by RCM - Ronan

I solve this problem by never using a bottom mic on the snare.

I've always thought curiously why I would ever need MORE of the STRAINER on a snare, as opposed to the amount that is being picked up through every single one of my microphones. I almost NEVER use a bottom snare mike, unless its headed into a EL8 being over compressed and enveloped cleverly. I do often use multi mic technique on the snare with one on the shell, either with a Figure Eight [aiming the Side at the things I don't want to pick up] or with a Moving coil and condenser w/ their capsules as close as possible for two different textures that will have relative time to each and blend with low comb filtering. There are certainly many schools of thought, but its about the type of texture that is required, for the snare, the rest of the kit, and of course the drummer and the music he is drumming. With that said there are some excellent ways to capture more than you need from a snare drum, and using a Moving Coil and Condenser or Ribbon - can help with that. Just make them relative and you'll be able to use them together.

[Eric Pederson]

The blue I appreciate the effort but you made my head hurt and yes I didnt see your post until now.

Another thing is you can always flip the phase till they cancel out good to get a good top/bottom relationship, then flip it back before tracking

[raggedman]

Originally Posted by CJ Mastering

Why not try moving the position of the mic or mic's so that they are in phase. This would be my first move.
an inch here or there can make a difference in phase.
CJ

snare top and snare bottom are both pointed at the respective heads...which means they are facing each other therefore when one is compressing the other is rarefacting. it would be physically impossible to move the mics so that they were in phase and still have them aimed at the drum...thats why we phase flip the BOTTOM mic

[xjake88x]

I love using a bottom snare mic. It really seems to add character, detail, or texture to the snare sound.

I linked 2 images.

The first waveforms are a kick drum. Top is inside and bottom is outside kick mic. The lower track, (outside) was out of phase. Notice how the wave goes down first, then up. The rest of the tracks were up then down. So I have that track's phase invert toggled on.

The second is a snare top & bottom. In this one, the snare top (first track) goes down then up. The snare bottom (second track), goes up then down. When you examine all the other tracks, they are consistent with the polarity of the top snare mic. So I invert the phase of the bottom one.

Hope that makes sense!