Two waves have to be 180' degrees apart AND be of same amplitude to cancel each other out. With slight variances in frequency response of mics, and the fact that low frequencies lose less power when propagating the same distance, it would be hard to calculate with certainty. You'll be able to get the places which are theoretically nodes and antinodes for specific frequencies, but I assume that would be about it... uneven frequency response and the fact that the condensers would pick up a lot of room could mess with the expected results somewhat.

Agreed. But now that I have some numbers, I'm gonna conduct a little experiment. I just want to see if it's noticeable.

From what I understand (I remember reading this somewhere, but the equation is easy enough), when stereo recording, to use mic distance to move everything (well, a target frequency anyway) into phase, the microphones have to be the same distance from the source within less than 1/3 of an inch. That moves phase cancelation high enough into the bandwidth that it's less noticeable. Of course, that doesn't cover every frequency, and of course, how in the hell do you get a guitar player to sit that still???? :)

Shaneperc, the equation is really very, very simple. Sound travel at about 1130 feet per second, depending on temperature and altitude. If you look at a 1hz wavelength it is 1130ft long according to this. Thus 1130hz has about a 1ft wavelength. Each wavelength represents 360 degrees of phase. One complete cycle.(hertz)
With micing, flying overhead speaker arrays(live sound), speaker design and acoustics, this is the basic, basic math. If an 1130hz wavelength is 1 foot long-360 degrees, then 1130hz @ 6 inches would be 180 degrees. Phase canellation, if the wavelengths are equal in amplitude as falken2 pointed out. However if they are not EXACTLY the same amplitude, then cancellation will still occur, in proportion to the difference in amplitude. You can figure that out from the source.(see the inverse square law.)
There are also variying degrees of phase, and thus variying amounts of canellation which are not related to amplitude. 1130hz at 3 inches is 90* and so on.
If you already know this, then I apologize for expounding upon it so much. I think it's important for everyone to have a firm grasp on this concept. I am just trying to get fancy with it, and see if anyone has any tricks centered around it.

cheers.

If you already know this, then I apologize for expounding upon it so much

No problem! I was just commenting (please excuse my lack of anything else to do. :) )

So, it almost sounds like you're experimenting with changing the timbre by using phase adjustment. If signals are in phase, but different amplitudes, then the combination will just be a that signal at a different amplitude. But, if the signals are slightly out of phase (not exactly 180 or 360) and different amplitudes, then the timbre starts to change. Kind of like what happens with off-axis coloration. Is this what you're getting at?

ummmmm, arent we forgetting something... surfaces need to be taken into the equation. relative walls/whatever will have some impact on phase relationship (especially on drum oh's)...

i didnt read all the posts so excuse me if it was covered.

Hey, you know, I just realized that walls and other surfaces could affect phase, especially on drum overheads.

I didn't really read this whole thread so excuse me if this has been brought up.

Shaneperc, this is pretty much where I'm going. I was just trying to see if anyone else had tried this on drums or different instruments and had neat sounds as a result. I also wanted to know if anyone has found danger areas with this. I have used this on clean guitar tracks(electric) in order to give them a more acoustic feel. Seemed to work pretty good.

Creative soundblaster: You are cautioning me about reflections. My drum room is kind of small, but I have foam absorption panels on the walls on either side of the kit, I am of the impression that any reflected sound is so small I can't hear it off the panels. With respect to bass, the corners of the room are filled with loose foam, up each wall and along the roof. The walls in this room are unparalell, and the roof is treated as well. The only reflective surfaces in the room are the window (6ftx3ft) and the cedar paneling, which accounts for about 30% of one wall. The window is nearly 10 ft away from the nearest mic, and the cedar is 4-6ft away. sound from the kit would have to travel twice this distance to reverberate into the mic. With this in mind, I don't see how it can be that much of a concern. If I am mistaken, then please educate me, as I am always eager to learn.

P.S. I have not had the room tested, but it sounds good. If I had to guess an rta I would say .3-4 overall with the 500hz and down range being the least lively.

somehow i dont think fukin2 likes me very much...

in regards to the surface interaction i just meant only true results (if possible) could only come from inside an anachoic (however its spelt) chamber...
the main thing of concern i would think might be the roof. u mentioned the room was small so i gather the cieling is low? thats gonna be a problem on overheads with phase.

Aww. I'm sorry if my eccentric sense of humor has made your panties tie up in a knot. ;P

No offense was meant.

Soundblaster:

Originally posted by creative soundblaster:

the main thing of concern i would think might be the roof. u mentioned the room was small so i gather the cieling is low? thats gonna be a problem on overheads with phase.

Originally posted by Steve@speedRiverStudios:
With respect to bass, the corners of the room are filled with loose foam, up each wall and along the roof. The walls in this room are unparalell, and the roof is treated as well. The only reflective surfaces in the room are the window (6ftx3ft) and the cedar paneling,

There's your answer. I think we can consider it more or less anaechoic (sp?)... The drum set itself would probably cause more changes in phase due to reflected waves than the surrounding surfaces.

I agree the roof might be a problem, Tell me what you think. The ceiling sits at 7.5 ft.(fromm floor to the first layer) The building is constructed with 2x10 joists, which we have filled with r-20(pink). Beneath that we have a sheet of 58 drywall, and two mor 38 sheets. Beneath that are sheets of egg carton shaped matress foam glued to a hard rubber mat backing. Beneath that is a grid of thin lattice, filled with greenery. Beneath that is the drum kit.

Ok folks.. play nice.

The concept of intentionally making something out of phase just weirds my morning out! :D I come from the old school where everything needed to be mono compatible. I think this is still valid today. My 2 cents. Kurt

Kurt, would you pleas explain to me the mechanics of phas in a recoring and mono compatability? I am assuming that if there is a left right difference in phase, than when you mono the mix, that difference would cancell out it's counterpart, thus effecting the mix? Could this not be manipulated to your advantage if that was the case?

Steve,
Yes, that is what happens. If things are out of phase, when they are collapsed to mono, the volume changes or in worst case the element completely disappears. IMO this is not a good thing and I can’t see as how that could be used to benefit. I want the balance of my mixes to be the same in mono or stereo.. Kurt

Thanks, dude. :c:

Maybe my little experiment will have an unintended benefit. I will now look into these differences in the context of stereo imaging. With my previous example of phase relationships from mic to mic, this could effect the attack, or resonance of each tom. Keeping in mind things like proximity effect, spl's, and off axis response, keeping a constant distance from each drum when close micing, or changing the distance of the overheads, from the snare, and using the angle of the mic to change the tone. For a different style say, metal to pop, one could have a standard distance for close mics in relationship to the overheads. Or vice-versa.

This is just an idea. I haven't really thought it out yet. I'm off to another studio soon, and I'll work it out when I get back. Hope I don't sound like an idiot.

I think it COULD be made to work if the sampling was done to a mono track, then re-imaged later. It would cause a comb boosting that could bring out a series of wanted frequencies to make a certain track stick out better in a mix. The peeps over at Acoustics probably could say this with a lot more certainty, but I think that objects reverbate at frequencies which are multiples of each other very well (guitar strings and harmonics, for example), and it could lead to boosting of those specific frequencies should they reflect sound waves.

That could be what's being emulated here.

With everything I do I think visually, not sonically. And in my mind I see this very similar to a parametric eq. The varying degrees of phase being akin to the q, and the 180* diff being the center freq. This is how it sounded to me when I did a clean guitar with two mics. I just move the far mic until I got cancellatio at around 600hz. Sounded good. Didn't notice mono probs. Who knows? I'd better get to work soon or I'm gonna start thinking some more! :D

Originally posted by Steve@speedRiverStudios:
With everything I do I think visually, not sonically. :D

there's your problem.
you guys are speaking in very simplistic theoretical terms. It's a very multidemensional problem. Sound leaves our theoretical point-source in all directions in a 3-demensional space, bouncing and being absorbed in countless ways and mutlple frequencise...coming back to our two mics with a complexity that makes the simple straight line (ray) measurements/hypothesis almost ludicrous......
...an the BEST part is you already have the raw parallel processor that beats any computer, chart or theory...YOUR EARS.

Put on a pair of headphones....LISTEN two the intereaction of your two mics in real time.
pan them up the middle (mono) to focus more on frequency related phase issues...and for more advanced analysis...listen in stereo (each mic paned to a seperate ear)..then you can hear how phase affecst the stereo image...
...of course it takes more than just the ear/brain to get usefull information...you also have to have the right "software"...i.e. the experiance to know what your hearing and what does it mean.

close your eyes and listen. (after your done reading this)

Steve@speedRiverStudios !!
You're to smart for your own good :c:

Recorder man: You are right. when it comes down to the nitty gritty, it's all about how it sounds. You have brought a valid point and are telling me this probably wouldn't work. Does that include a room which has been treated acoustically?

You say that visual thinking is my problem. When I speak of "thinking visually" it is my way of understanding the workings of what is going on. I may think visually in order to imagine the three dimensional soundspace in which I will create in a mix. I may think visually when placing a mic.(what will this mic see, pointing this way.)

When it comes to this issue surrounding phase, I DO realise the complexity of sound in a room. I don't mean to over simplify things, but I do believe that this overly simple math will shed some light on the situation.

Don't you make sure that your snare and kick are the same distance from your overheads when micing in stereo?
If you two-mic something don't you throw the phase switch to see which sounds better?

I am simply trying to better undertstand, and more so, to deliberately predict this before I even place the mic. If this isn't possible, if it can be learned with experience, then surely there are constants which can be understood.
When a sound starts at the source, it's greatest amplitude is before it comes into contact with an object. Once it hits that object I'm sure you know that it's amplitude will diminish proportionately, depending on what it hits. In my situation, sound would have to travel almost twice the distance (reflected), and at that, bouncing off an absorber, before hitting a mic. Meanwhile the direct(and very loud sound) would travel to the mic, from the source, way hotter than the room.
I would think that issues pertaining to the sound of the room, although important, would not be worth looking at, as the amplitude of such sounds would be so small, it would not greatly effect this equation.
With regards to sounds that would not be effected by the absorber, they are way out of the frequency range I am considering, as are the harmonics that may go along with them.

Do you have any multi-mic techniques you have had success with? :cool:

Originally posted by Steve@speedRiverStudios:

I would think that issues pertaining to the sound of the room, although important, would not be worth looking at, as the amplitude of such sounds would be so small, it would not greatly effect this equation.

Not true.
The room is a prime contributor to the sound you get anytime you throw up even one mic.
So mauch so that it makes or break mic'd recordings, and is the 2nd most important factor after the quality of the source itself(musician/instrument).
Give me a good musician/intrument in the right spot of the right space (live/dead ,big/small , postion in space relative to liev end - dead end ect) and '57's over great gear in a bad sounding space.

I'm about bottom end...it's the easiest to hear phase problems with, and if you focus ion getting the bottm right, the rest will follow.

Perhaps I should clarify. Obviously the sound of the room will be quite audible in a room mic, like an over head. What I'm talking about here is the difference between a close mic.(a few inches) and a room mic. The distance if the close mic being the issue, and 90% of these issues would be above 1500 hz.
Bottom end gives the human ear almost no perception of the source directionally, and mid-high sounds articulate every aspect of your soundspace. They are also imprtant with, pick strokes on guitars, timbrance of drums, vocal tone and sibilance, 101 other useful things to record.

Not to say that phase issues are unimporant, but a 100hz wavelength is big enough that mic placement wont change a phase issue.

Do you feel the mic on a floor tom @ 1.25" is going to pick up enogh of the room to create a problem? :)

Originally posted by Steve@speedRiverStudios:
I would think that issues pertaining to the sound of the room, although important, would not be worth looking at, as the amplitude of such sounds would be so small, it would not greatly effect this equation.

The room is extremely important. It will either help get the result you want or it will get in the way. Never met a neutral room though.

With all due respect, I would stop thinking of it as an equation. An equation implies there is a given, and in recording there are no givens. The same mic/preamp combo that thrills my soul one day may sound horrible the next.

I know some pretty "rocket scientist" engineers/designers here in Nashville (for gear you would know) that could do those phase calcs in their head on the fly I'm sure. But when it comes down to it they do what every other great engineer does.... listen.

In my experience, I can attribute some of my best assets, to experimentation. I can also attribute some of my greatest wastes of time to this. I do think there is something useful that will come of this experiment, but if not then that is one less thing not to think about.
Among other things I have mentioned, knowledge of this might help you to zero in on which mic is problematic, faster than just guessing and then using your ears.

I did't mean to suggest at any point that ears should not be used, just that some additional info could help you get your ears what they want a little faster.

There were two original questions that I made in the beginning, and only one of which have I received a response. The info posted here so far has been great. I wonder if those who have posted would also share some ideas on multi-mic setups?

Experimentation is our numero uno friend as RE's...The thousands of pages written on this subject came about through some original experiment that some bright lad or lass concocted bassed very likely on noticeing such differences as you have, and voila theres another paper on it...

Multi mics in any three-dimensional space is going to depend SOLELY on the aspects of the space. As was stated already by The RecorderMan, first there is the source, and second(only slightly behind) there is the container of the source.Everthing past that is either going to sound the way it does due to sensitivity or its sonic footprint.

When I'm setting mics, I like to get a basic sound which I acheive due to my experience with micing and my knowlege of basic mics and their patterns. Then I like to go uner a set of phones and move things around until there is some sort of nirvanic glue between the pieces.I suppose I could bring my calculator but it would simply be a step that my brain has already completed and my ears have summed up.I do record the positions and a brief description for every micing I do.Cause the old grey memory she aint what she used to be.

I know I havent directly addressed your question but it seems that after a bit, theory becomes a bit burdensome and actual useage and knowledge gained through trial and error,builds an inner confidence that makes for a quality RE.

Kevin: Well, there are no givens, but in this case, there are a few basic rules that waves follow - the reverbations picked up simply cannot have more power than the direct sound (unless the mic is facing away from the source). Assuming a -3dB difference for the reflections compared to the initial sound, we're talking at most, 20', maybe 25' phase shift away from the original signal. (this is worst case if every single reverbation comes in at 90' phase compared to the original - closer to 0' builds the wave, closer to 180' reduces the wave, both with less shift than 90' phase.)

So, although the wave shifts due to reflected sound, it is very possible to calculate which frequencies phase cancellation is likely to cut and boost, within a small window of "maybe"s.

Originally posted by falkon2:
Kevin: Well, there are no givens, but in this case, there are a few basic rules that waves follow - the reverbations picked up simply cannot have more power than the direct sound (unless the mic is facing away from the source). Assuming a -3dB difference for the reflections compared to the initial sound, we're talking at most, 20', maybe 25' phase shift away from the original signal. (this is worst case if every single reverbation comes in at 90' phase compared to the original - closer to 0' builds the wave, closer to 180' reduces the wave, both with less shift than 90' phase.)

So, although the wave shifts due to reflected sound, it is very possible to calculate which frequencies phase cancellation is likely to cut and boost, within a small window of "maybe"s.

In a theoretical open space scenario...maybe...but I'd be done with a couple of listen's/move the mic's...before you got your calculator turned on. period.

You win. I concede!

Well, it seems to me that everyone here seems to think this experiment is a waste of time. When it comes down to it you are probably right. Experience and listening is definitely the best way to setup a recording. I am having fun experimenting with this. I do have a good, fast routine that enables me to sucessfully mic without undesidable phase issues. I am simply trying to gain a more complete understanding of the goings on that I can't see.

I hope that someone here has at least gained something through this discussion, as that is what we are all here for, I know I have.
thanks, guys.

Originally posted by Steve@speedRiverStudios:
Well, it seems to me that everyone here seems to think this experiment is a waste of time. When it comes down to it you are probably right. Experience and listening is definitely the best way to setup a recording. I am having fun experimenting with this. I do have a good, fast routine that enables me to sucessfully mic without undesidable phase issues. I am simply trying to gain a more complete understanding of the goings on that I can't see.

I hope that someone here has at least gained something through this discussion, as that is what we are all here for, I know I have.
thanks, guys.

It isn't and wasn't a waste of time...actually when I was learning/assisting I went down these paths in great legtth...making three demensional mesurements doen to the 1/8" of any and all mics and instruments whenI tracked....using headpones for multi mic placements...buying clickcharst (one I had was so deep it actually had a sinlge 8 1/2 X 11 page for each beat with all note values between a whole note to 128th note, triplets and dotted between 50bpm and 160bpm ...and then the same thing for frames-per-beat), smpte calulator, phase clicker...whatever...I WAS SICK. I made me allergic to phase (they don't have medication for that yet)....and en the end taught me more that ever to throw it all away and really use my ears...still it was a good way to show me the way...so keep experimenting...but always go back to the ear.