Microphone phasing and El Musico's wisdom...
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Posted on Jul 01, 2010 03:24 pm
Quincysan
http://www.unitedmusicians.info
Contributor Since: Nov 11, 2007
I caught myself de-railing a "recording bass" thread and thought it would be best to start a new one about microphone phasing.
El Musico said this:
I haven't used the phase scope much and didn't always have the luxury of a device like that so I learned to phase align mics this way:
What I do is find ideal placement for the first mic. When it sounds great I mute it and move on to the 2nd. I place the 2nd mic where it sounds good, keeping the 3-to-1 rule for micing the same source, where the 2nd mic is 3 times the distance of the 1st mic to source. If I'm placing it on a different speaker of the same cabinet then it's considered a different source, acoustically speaking, so the 3-to-1 doesn't really apply but you can still have phasing issues due to it being the same program material, so after i find the spot where my second mic sounds good, I unmute the first mic (i'm only listening to the mics using headphones in the tracking room) and reverse the polarity so that the 1st and 2nd mics have opposite polarity. At this time i krank up the amp so that it's self noise is the only thing i hear in the cans (be sure nobody touches the guitar cuz it will blow your ears). I begin to fine-tune the placement of the 2nd mic by moving it slightly so that I can hear the effects of it's position against the 1st mic. You will hear a degree of comb filtering (swooshing, additions and nulls) as you move it around. Find the spot where the sound is nulled out the most, that's the spot where the 2 mic's signals cancel eachother out the most. Then return the 1st mics polarity back and you should have both mic's signals in phase.
Return the amp volume back down to ideal recording level and let the guitarist go at it. Record a bit of that and zoom in on the waveform to be sure it looks in phase.
Some people just throw mics up and record then align the tracks afterward... not recommended, especially when you consider the fact that you can have phase issues with even just 1 mic, meaning, the direct signal you want recorded being influenced by reflections off the floor and the walls of the room. If I just record 2 mics i threw in front of a source, record them and moved the audio region so that it's "phase aligned" I have just thrown the room and any other nuances recorded with that mic out of what is physically possible. For example, if mic 1 sounds good where it is and mic 2 looks as if it's 5ms behind you would shift the region 5ms to be aligned with mic 1, you would be moving everything it recorded (direct signal and any reflections) up 5ms. If you were to move the 2nd mic itself to be aligned you would notice that the closer to the source the mic is moved, the further away from reflections it would be, the wall doesn't get closer to the source too... So how could you move up the direct signal 5ms, and move the reflections back 5ms if the material is already recorded? You can't.
So there's a ton of wave theory mixed up in there BUT with all that said, if it sounds good then it IS good.
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Quincysanhttp://www.unitedmusicians.infoContributor
Since: Nov 11, 2007
Jul 01, 2010 03:25 pm Holy crap! An explanation that involves using my EARS! Thanks! I've got a big live session coming up with this avant garde nu-jazz group...this will be the technique to practice before that session.
I've definitely been throwing up mics and correcting afterward. Excellent point about reflections preventing me from getting a natural sound by moving waveforms around. Looking forward to hear what the results are when I follow this method...
How do you check phasing when you're close mic'ing drums or an ambient source?
Another group I've been recording has decided they don't like overdubbing; they want to record their rock set live in my drum room. They don't really like close mic'ing and did I mention they hate overdubbing? I really enjoy their music so I'd like to accommodate them, but I imagine this phasing stuff will get hairy when I start putting up mics to capture the room. Got any suggestions for recording a full band in a relatively open drum room? I have to figure out a scenario to get the drummer, aux percussionist, three guitarists, and a bassist recorded all at once. Four of these members sing while playing, including the drummer.
Quincysanhttp://www.unitedmusicians.infoContributor
Since: Nov 11, 2007
Jul 01, 2010 05:08 pm Ok so here's the basic layout of my tracking space...
The only door on this floor is at my roomate's room, everything else is wide open. The openness sounds great when I'm tracking drums without anything else going on, but I question whether or not I'll be able to get a whole band shoved into a space like this.
What about this scenario:
If I were to use acoustic treatments at the opening between the kitchen and the hall to the backdoor, I could put a guitar amp in the room mate's closet, I could stack some acoustic treatment in front of the closet door and put another amp in the main part of his room. I could shut the door to his room, throw some acoustic treatment up at the door, and put yet another guitar amp in the "Hall to backdoor" area.
At this point I could get all the vocalists in my outdoor garage with their instruments, headphones, and vocal mic's. The garage is open enough and I'll lay down something soft to keep the concrete floor from getting too wacky with its reflections.
Since I'm working with 10 inputs, I'll be recording drums and the auxiliary percussion in the same room, very sparsely...so let's see.
API 312: OH Left (LDC)
API 312: OH Right (LDC)
PMix 1: Snare
PMix 2: Kick
PMix 3: Guitar 1
PMix 4: Guitar 2
PMix 5: Guitar 3
PMix 6: Bass
PMix 7: Tight XY matched SDC's on vocalists LEFT
PMix 8: Tight XY matched SDC's on vocalists RIGHT
The bassist will record direct and I'll re-amp his bass later...
Typically the drummer and the aux percussionist share the same stage space so cramming behind the drum throne shouldn't be a stretch for these guys, they practice that way.
I mention using acoustic treatment, and when I say acoustic treatment I'm thinking more along the lines of two pieces of sheetrock sandwiching some cheap insulation or something. I'd like to be able to afford some really quality GIK treatments but even raw 702 fiberglass is a little pricey for me right now. Sheetrock and the most dense/cheap insulation I can find is going to have to work. Hell I'll probably have to return the crap after I'm done with it, hah.
SO...all that leads me back to phasing. I don't think I can change polarity on my Projectmix pre-amps, so I'll have to test mic positions using the API...no big deal. Should I even worry about phasing though since the goal is to isolate the different instruments?
I guess my primary phasing concern will be the LDC's in concert with my AKG D112 kick mic and an SM57 on the snare; but I'd like to hear if there are other phasing concerns in this sort of live set up.
el musicoFreeleance Producer/Engineer/GtrMember
Since: Aug 11, 2002
Jul 01, 2010 05:24 pm I was thinking about doing this too, since I really didnt want to hijack a thread answering your questions...but my wisdom? haha... opinionated for sure!
Well, when it comes to acoustic sets... sheesh, i could write a book here... use the 3-to-1 rule, which i'll reiterate, is when you are micing the same source, after you place your first mic and it sounds great, the distance between that mic and the source equals X. If the 2nd mic you add isn't going to be at the very same point as mic 1 (diaphrams level at X) then the placement of the 2nd mic needs to be 3X from the 1st mic. So if close miking a source 6" away your 2nd mic will be 18" away from the first mic to prevent phasing issues.
So, being that an acoustic drum set is made up of several instruments it can get overwhelming to think about. Just break it down and when close miking consider each piece as an individual source, and consider using mics for their polar patterns and keep in mind how each mic rejects sound and from which direction to reduce bleed from other sources in the proximity.
This is my basic drum setup:
I double mic the top of the snare (mic diaphrams level), add a bottom mic underneath to capture the snares (reverse polarity), I have a kick in mic for the beater slap (inside near head), a kick out at the port for low end punch, close mic each tom, hi-hat mic pointed down at the ridge, spot mic each cymbal/ride/crash, Overheads Left and Right (spaced pair, snare centered in image), and Room mics either XY or MS config. Also, through experimentation, we also developed a miking method we call TMBD (The Mic Behind the Drummer) which is a large diaphragm condenser just behind the drummer about 2 feet from ground, and about 4 feet away (which is 1/4 wavelength for the lowest frequency we want to accentuate about 60 hz). It adds a nice touch of low end power to the kit once mixed in.
Yeah, it's a lot... but it pays off in the mix. Oh and yes this is usually on a Pro Tools HD 3 Accel system, so I have plenty of inputs available.
Well, about your next session, Q, it's hard to say, given that you're probably limited on track count and they are picky. I would set them up as if they were playing a live venue and treat it as such, being sure to capture DI's of the Gtrs and Bass so that you can re-amp later, without having them replay their performance and to add quality and reduce bleed in the tracks. It's overdubbing for sure, but your point to make with them is that it's not like they are playing it again and sacrificing the vibe, just tweaking the tone of their already groovy live performance. So mic the drums/perc with your best, mic the room, mic the cabinets with sm57's for scratch (maybe to keep), record DI's, and add vocal mics. ReAmp the guitars and bass cabs later with your best and prepare for overdubbing Vocals just in case ;)
el musicoFreeleance Producer/Engineer/GtrMember
Since: Aug 11, 2002
Jul 01, 2010 05:26 pm I must have been typing while you were replying... i'll read what you have and get back to you with ideas
oh and about inverting polarity on certain mics... if you dont have a button, just add an EQ 3 to the channel and click on the phase button on the input area in the plugin
el musicoFreeleance Producer/Engineer/GtrMember
Since: Aug 11, 2002
Jul 03, 2010 01:55 am So Q, given your space I would obviously have the drummer and aux perc in the main room there and have the musicians/vocals within eye sight of the drummer but keep the amps as far away as possible, like the carpeted room then re-amp later. This should provide the best isolation for the drums, and if the guy's voices get into the "room sound" of the overheads, no biggie, they should be masked in the mix.
As far as your acoustic isolation, the sheetrock idea, for your purpose i don't like it personally, although it's good for building a wall :) Walls are for sound isolation between rooms/outside and they add reflections. Generally speaking, studio owners put a lot of money into making their walls as invisible as possible through use of absorption, diffraction and diffusion. Put any money you have into absorption without adding reflections like bass traps and gobos for tuning the area you are recording in as you need it. I like having gobos since I can move them around the room and tune the area i need, like around room mics if they are near a wall either to isolate them from the reflected signal or the direct signal, other sound source, whatever.
That and the other post should cover things for you, let me know if I overlooked anything or have further questions. Have fun. Let me know how the session turns out
Quincysanhttp://www.unitedmusicians.infoContributor
Since: Nov 11, 2007
Jul 06, 2010 05:38 pm Your phasing guess and check with headphones scenario seemed to work pretty well when mic'ing a bass cab last weekend, thanks! One thing I noticed is that engaging the polarity reversal on one mic didn't seem to effect the swooshing sound I heard, but I was able to find a null spot.
Hmm, our approach to recording is very different. I'm confident in my methods and should have just asked the question I meant to ask without all the imagery and show, sorry. I was sort of drafting the gameplan for this session instead of staying on topic, oops. That said:
How do you diagnose phasing issues on drum mics? If you spot mic every cymbal then you're sure to get plenty of phasing issues. How do you correct them?
To me it seems nearly impossible to correct phasing on a kit where every cymbal is spot mic'd. I understand what you said about each drum being a different source, but we know that if you were to solo a drum mic that's not the case in application.
el musicoFreeleance Producer/Engineer/GtrMember
Since: Aug 11, 2002
Jul 06, 2010 07:09 pm Q, the swooshing sound is definitely more apparent in a distorted guitar track, especially in the high frequencies.
"How do you diagnose phasing issues on drum mics?" Depends... where do you hear the phase cancelation? You have to hear it to diagnose it. That's kinda like visiting a Doctor and asking what is wrong with you. Where does it hurt? If I hear something funny, then I mute/solo tracks to narrow it down, i try reversing polarity on the tracks and looking at waveforms to see if I can hear/see an issue and then address the mic positions.
"To me it seems nearly impossible to correct phasing on a kit where every cymbal is spot mic'd. I understand what you said about each drum being a different source, but we know that if you were to solo a drum mic that's not the case in application"
It seems to me that you have it in your brain that spot miking cymbals is problematic, IMHO it's no worse or better then miking toms, in fact, toms are closer together in most instances. Keep the 3-to-1 rule in use along with the mic's natural rejection/polar patterns, and pan each track to it's proper place in the image you should be fine. Of course, if you solo the mic you'll hear bleed, but the power/amplitude of the secondary sound shouldn't be enough to cause any noticeable phase issues (if you have properly isolated the primary source and reduced bleed to your best).
Miking a drum kit can be a daunting task but you shouldn't over complicate it. Hell, we aren't even addressing any issues with the acoustics of the recording space that can color even the best mic technique... but this is what I think about when miking drums or anything else for that matter. I'm not a sought after professional with gold and platinum records on the wall (yet?) just trying to explain what I learned in school, on my own, and have been applying in the studio everyday.
Quincysanhttp://www.unitedmusicians.infoContributor
Since: Nov 11, 2007
Jul 06, 2010 07:16 pm I never hear phase cancellation. I hear a flanging/phasery effect and I don't know how to correct it because when I solo 2 microphones; I hear the effect, and when I solo another two micophones, I hear the effect, and when I solo one microphone, I do not hear the effect. I can't correct one pair without effecting the others, so how do I get rid of the flanging effect if I can't correct each mic pair as a unit? I've only experienced this issue when recording drums and when I did, I moved my overhead regions to match up with my close mics. But like you said before, this isn't a natural way to correct cancellation.
Granted, the effect isn't always apparent but when it is I'd like to trace down the trouble spot vs randomly moving mics and drums.
"...Of course, if you solo the mic you'll hear bleed, but the power/amplitude of the secondary sound shouldn't be enough to cause any noticeable phase issues (if you have properly isolated the primary source and reduced bleed to your best)."
Isn't this like saying "if you're have a phasing problem, you probably shouldn't be having a phasing problem."
el musicoFreeleance Producer/Engineer/GtrMember
Since: Aug 11, 2002
Jul 06, 2010 07:48 pm Ok, what you're hearing is comb filtering, which is phase cancelation, and sounds like flanging/phaser. Which 2 mics do it? Are they miking the same source or is it like the cymbals mic is conflicting with the tom mic that also picks up some of the same cymbal?
"Isn't this like saying "if you're have a phasing problem, you probably shouldn't be having a phasing problem."
Haha... no but I like it ;)... I must have misunderstood the context in which you made the statement I was referring to.
Quincysanhttp://www.unitedmusicians.infoContributor
Since: Nov 11, 2007
Jul 06, 2010 08:05 pm I think what we're getting at here is that the only way we know how to check for phase cancellation problems before tracking is to listen to two mics and get the flangey/swoopy/comby stuff nulled as best as possible then move on to the next mic pair, checking those mics for phase issues with the two that are already in place before moving on, etc etc. until all mics are placed. That sounds like a time consuming process to me...but it makes sense that this is the only way to isolate a phase problem with my ears, and I am satisfied with that answer...thanks for enlightening me.
I wonder if cranking pre-amp gain would give us enough signal on the drum mics to mimic the thing you had me do on the bass cab?? I'd guess it would, but I guess wrong more often than not...
