Phase Issues?

  • Thread starter Thread starter UnkleSlam
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xtremedb said:
Sorry, I’m not buying it. That explanation may work fine in a textbook, but in the real world, it is *never* that easy. For one thing, there are always placement constraints. On the underside of a snare, sometimes, you’re lucky to fit a mic at all, much less have the liberty to measure how far out of phase the wave is and move the mic accordingly. There are lots of similar situations where this is the case. As I mentioned in my previous post, the stuff you buy from the store will often arrive wired out of phase to begin with. Are you supposed to move the mic all over the place, just because they can’t conform to the iso standard?
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- Be it live, or in a studio, I have never, ever, not found a space to put a mic under a snare, from large condensers to 57's to, now, small clip-ons, there is plenty of space? As a matter of fact, nine out of ten times I use another mic as well in the same general area, to capture the kick hammer attack.
- No, you don't have to move a mic all over the place under a snare to correct phase. The waves of the predomimnant frequencies a snare transmits are relatively short, movement between in- and out of phase is therefore small.
- If you find that stuff you buy arrives wired out of phase, may I suggest you return it? Or, as I have done for the last 35 years, always make your own wiring. Whatever you do, you don't work with faulty equipment.


If you don’t have a phase switch on your console, I admit it makes things a little tougher. If you don’t want to change cables, they build adapters, which are basically metal tubes with out of phase contacts. Just plug it in before the board and listen for the low end. That’s a hell of a lot faster than moving the mics around and guessing. Once you’re at least in the same ballpark, you can work on placement.
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- In my opinion faster is not the way to go about microphone placement. Apart from that - you advocate checking phase, and reversing phase if you find a mic to be out of phase, and THEN work on placement? Why? So you can move them out of phase again?



If the cable was wired out of phase to begin with and you move the mic to correct for this, you may create another problem. For instance, now you might now really be picking up the ride etc. Where has that gotten you? Nowhere.
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-Why do you assume that everyone works with faulty equipment as a matter of cause? If that was the case, ever single post in this forum should end with "it might not work 'coz you've got a good chance your gear is fucked"


BTW reversing the phase on the underside of a snare is *not* done for effect. It is simply good sound engineering. This is a common phase problem, as is reversing the phase on an open back cabinet, when using both front and back mic’ing.
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- How can I explain this one? When I said I often have one in- and one out of phase microphone on a snare drum, I actually ment .... exactly what I said. For effect, for sound, not to correct a phasing problem, because I didn't have a phasing problem, because I place my microphones correctly, and because I use cables that are wired right. Now THAT IS simply good sound engineering.

One further point. This is a site for home recording and today a vast number of homers use DAW's. One of their pleasant features? An editing window. You can see the waveform, you can see if something is in phase or not, you can even ascertain if you have a phasing problem which floats between frequencies. That is if you want to see it, normally your ears should tell if you have a problem or not.

Does that help Brad?
Sjoko (an unfrustrated, not running 'round in circles person with in-phase gear)
 
Nice thread.
Why is mic placement such a dirty little secret?I am absolutely in awe of the golden age engineers who tracked jazz bands and such with a single mic and produced classic tracks.I guess my pet peeve is that technology is being used now as an replacement for this kind of fundamental technique.
I would love to read a mic placement roundtable with sjoko2,ed,bruce,harvey and the other wise guys here at home wreck.

Tom
 
Here's another amusing factoid: xtremedb states that is it impossible by the laws of physics to have phasing issues with only one microphone. Would that that were true- I'd have one hell of a lot fewer gray hairs...

Don't get me wrong- I'm not picking on you, sir. That certainly _is_ true in the electrical domain: you have only one signal to work with, so you're not going to cancel it out on the mix bus. However, in the *acoustic* domain, you can have all manner of phase cancellation problems due to the delayed, reflected signal arriving back at even a single mic. If the delay paths can change, the resulting comb filter can move around a lot, resulting in a _very_ audible and distracting phasing effect. This can be a real issue in vocal recording in unprepared (or underprepared) rooms, and it can drive you nuts. This isn't caused by an electrical signal being reversed in phase: this is entirely an acoustical effect.

Here's a quick exercise to try it with: set up a nice cardioid, with a *hard* wall 4 feet away behind the mic in the null of the cardioid (think "bad vocal booth"). Sing something with lots of sibilants, and allow yourself to move around say +- 8 inches WRT the mic as you sing (think "Joe Cocker on a well-controlled day"). Listen to the playback soloed, and listen to the effects of tha slap-echo-driven comb filter as it saws your upper frequencies into ribbons. This is a nontrivial effect, and is one of the reasons that vocal booths are so hard to get right: talk about mic placement being everything...

This affects every type of mic, of course. After all, the polar pattern of the mic is a direct product of controlling the phase of off-axis sounds by both electrical and acoustic means within the mic capsule itself...

This tends to be one underappreciated issue with micing snares from the bottom. Sometimes, the resulting comb filter from the (very powerful!) floor reflection can work in your favor. Sometimes, it just doesn't. And *gawd* help you if the mic stand is loose and the mic drifts down towards the floor during an otherwise keeper take. Talk about _ugly_ combfilter effects: it can sound like the beater head is pulling out of the counterhoop....

Anyway, my point is that the mic doesn't work in a vacuum: we all agree that the relationship of mic to sound source is critical. However, my assertion is that the relationship of the mic to *reflection* sources is equally critical. Seems like it takes a long time to learn to look beyond (or behind) the instrument of interest, or the mic, to see what is really contributing to the overall sound field. This is one of the reasons that it is so damnably hard to duplicate setups: same drum, same mic, moved six inches closer to a wall, may experience a totally different acoustic environment, and sound significantly different. The less well treated the room, the greater this effect becomes- which is why it is so important for homercorders to understand it.

The rule of "Always move the mic around until you're getting the best reproduction" applies. But think about the corollary that seldom gets mentioned: "And always move the sound source around until it can coexist with the room, _before_ spending hours stringing mics!"
 
Couldn't agree more Tom. Unfortunately there are to many people today who disregard mic placement, or even learning about mic placement, in the false belief that they can compensate for this with equipment. Not so. Microphone placement is an art, you have to learn it, feel it.

and skippy - totally right - and that is why it is an art, not just technology or science, its a relationship between technology, sound, acoustics and perception.
 
Okay, I'm way down at the bottom here. When I double mic my voice and my guitar I sometimes get a sound I don't like, usually because the mics are too close together. Is that phasing? When I mute one of the tracks I've recorded and listen to just the other one, very often the spill from the other sound source is not very pretty. Is that phasing?

Also: "One further point. This is a site for home recording and today a vast number of homers use DAW's. One of their pleasant features? An editing window. You can see the waveform, you can see if something is in phase or not, you can even ascertain if you have a phasing problem which floats between frequencies. That is if you want to see it, normally your ears should tell if you have a problem or not."

How can you see phasing in the waveform? I've only been recording a year and half, but I've been looking at a *lot* of waveforms - teach me how to spot phasing in a waveform.
 
Magnify your editing window until the wave is clearly visible.Waves that are in phase will allign their zero points (where they cross the x axis).
Imagine a simple sine wave.It rises smoothly,crests, falls across the x axis and mirrors that shape below before coming up to the zero point to begin again.With two separate waves,most editing software makes it simple to slide the waves into phase allignment.If the phase problem is within a single track,you can at least this way identify the problem visually.The classic out-of phase audible cue is loss of bass.The frequencies just cancell each other out at 180 degrees out of phase.That was how the old Music Minus One records were made a generation ago.A popular recording was made out of phase and everything panned up the middle just vanished (the original karioke I guess).

Tom
 
Skippy… Thanks for pointing that out. I have had similar problems with floor reflections when micing cabinets. It didn’t even occur to me until after I read your post.

Dobro… you would notice phasing in exactly the way sjoko mentioned earlier in the thread. One waveform will be on the upturn, while the other is on the downturn. If they are 180 degrees out of phase, the crests will be evenly interspaced to resemble a figure 8.

For your other question… If you were playing back one channel at a time, the result was probably not phasing… unless you had a reflective surface creating a second image of the source, as Skippy mentioned in his post. It is far more likely you are simply hearing a very colored off axis response from the microphone.

Sjoko… I felt your initial post lacked clarification. Thank you for clearing things up. I don’t think that it was necessary to take a stab at me, but hey… to each his own. If you think you know me well enough to hint that I don’t understand mic technique, or that I use faulty equipment, knock yourself out.

If everyone checked their gear for phase or I always worked in the same environment, I’m sure these problems wouldn’t exist. However that is not the case. The statement I made in my last post was that I test all the gear I am using before I begin every project. Makes sense to me… but what do I know? Somehow you turn that around to suggest that I use faulty equipment. Why?

If a mic sounds better with the phase reversed, I’ll work from there. Does that make me a criminal? I don’t think so. I realize faster is not better. If I had lots of free time on my hands and my clients had an unlimited budget, I’d try everything. When you’re working on a deadline, faster is often the *only* way to work.

Brad Gallagher
http://www.just-for-musicians.com/
 
ADDENDUM:

OK… I went back and read your first post. I owe you an apology. Just as I finish railing you for taking my words out of context, I review the beginning of the thread to find I have done the same to you.

When you gig a lot, Monday becomes your weekend. The English language seems slightly harder to comprehend after a beer… or twelve ;-) After a second look, your post makes much more sense. I apologize for my previous rant.

Brad Gallagher
http://www.just-for-musicians.com/
 
Brad, try not to take this all too personal...we've all been dragged over the coals once or twice or ten times as well as dragged others....but like I said keep an open mind....in all of this, i learned something myself....though it makes perfect sense, I would have never thought phase issues come into play with one mic.....
 
Tom, Brad - thanks, I think I'm dealing with off-axis sound after reading your posts.

As for spotting phasing in waveforms: when I zoom in and compare two waveforms, there's no particular pattern. Sometimes the two waves cross the center line at the same moment, but more often not - and when they do, sometimes they're both moving in the same direction, sometimes not. But my life isn't long enough to go along the entire length of every waveform I record and look for occasional phase cancellation between waveforms. :D I still don't see the advantage of the visual representation of phasing.
 
Dobro, there can be a number of advantages, but the use of them really depends on how comfortable you are working with waveforms.
In its most basic form it becomes usefull when you hear you have a problem and need to fix it, in which case listening and looking allows you to identify the location of your problems, and edit them out. As everyone who does it regularly will be able to tell you, after a while you can just look at tracks and find the problems, it takes some getting used to.
Very often a minor adjustment to one track will fix a problem.

A trick I often use, when tracking set-ups with multiple microphones, like drums, percussion, or grand piano, is to test the set-up by sending short bursts of frequencies into a tracking room, which I record. I then use wave form analyses to see if I have a potential problem, where the problem stems from and at what frequency range. This is also extremely useful for deciding on things like baffle placement, as well as instrument placement within a tracking room.


It seems you would really benefit from studying mic placement in a bit more detail. I tried to find some good stuff about it on the web, but found this instead: http://www.recordingeq.com/
Free course for registered people, seems to good not to, they say it includes mic placement
 
I think that should pretty much answer your question, dobro. It is very difficult to look at a complex piece of music and figure out what’s going on. It’s kind of like watching an RTA respond to program material. Real cool to watch, but it doesn’t do you much good ;-) On the other hand, with pink noise or a sine wave, it becomes a valuable resource. If you can find the problems visually, you can eliminate them *before* you begin tracking. That way, you don’t have to sort through a complicated mess later.

I’m not much of a DAW guy (although commercial clients are pushing me that way). If I’m missing the point here, please somebody speak up… I have a lot to learn in this particular area (and as usual, not much time to learn it).

Brad Gallagher
http://www.just-for-musicians.com/
 
Thanks you two - I should have expected it would be more difficult than I thought. :D

Additonally, I would like to take this opportunity to bestow upon the redoubtable Sjoko the highly-prized Cool Link of the Month Award. Well done, that man!

might even be the Cool Link of the Year... let you know in December...
 
You guys are going to give the guy who posted this question diarreah!! Relax!

The answer is simply this:

If you are using 2 mics on your cabinet as you have described, you could expect some amount of phasing difficulties. However, as a recording hobbyist, I doubt you would notice it or give a rats ass because it would be so minor. So, try it both ways and just have fun!! Experimentation is what it's all about.

But the 3 to 1 rule isn't a bad idea. bye
 
Simple test to see if 2 mics are in-phase:

2 mics right next to each other, pointing at music/noise source

cable them into 2 channels

all eq set to 0 and pan set to middle

one mic at a time (with the other mic disconnected or muted):

set the levels trim/fader to give 0 on meters (each mic separately)

connect (un-mute) both mics, the level on the meters will be ~3dB higher if the mics are in-phase and lower if they're out-of phase...


- Wil
 
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