A question about phase

R

RecNewbie

New member
Hi,
I've recorded an orchestra with soloists using a far-away Zoom h4n plus a Rode Nt2-a in cardioid mode (about 1.5m from the soloists, space was cramped), and was thinking about phase.

Sound travels at about 343m/s in normal conditions, right?
So if the mics were 10m distant the direct sound would travel from one to another (suppose they're in line with the sound source) in about 0.29...s (10m / 343m/s)
A frequency with that wavelength is 1 / 0.29...s = 34.30Hz
If that distance is to be half-wavelength (completely out of phase) I get a 68.60Hz frequency, which is where I get the most cancellation.

Is my thinking straight? Can I use such calculations (instead of those "3m", "double the distance from the sound source" etc. rules of thumb) to reckon if I'm cool or not?
That is, of course, without taking reflected sound into account.
Thx in advance.
 
Well, you also get maximum cancellation of the frequency that has gone through 1.5 wavelengths. Also 2.5, 3.5, 4.5...all the way up. Why they call it a comb filter. With real world signals there comes a point where the signal at the close mic has "moved on" and is different enough from the far mic to cancel less, but the point remains.

Ultimately it comes down to the relative levels between the two mics in the mix.
 
It really is about relative volume. You can never get mics at two different distances to be in phase with each other. but if the sound in the distant mic is at least 9db quieter than the sound in the closer one, the cancellation wont be noticable.

You can do all the calculations in the world, but it whill only ever tell you what will get cancelled at that distance. There will be no distance where there is no cancellation at all, so it is really about relative volume and not phase.
 
Farview said:
...but if the sound in the distant mic is at least 9db quieter than the sound in the closer one, the cancellation wont be noticable...
Click to expand...
We're talking here about the direct sound from the source, not necessarily the whole signal. Also, it doesn't really matter which one is louder, as long as one is louder enough.
 
Typing on my phone, didnt want to get too wordy. no, it doesnt matter which one is louder, I just didnt feel like typing every possible variation of the setup.
 
Phase, in this context, is a timing issue. since the two mics are a different distance from the source, they will never be lined up time-wise. The time difference (distance) between the mics will determine the frequency that gets cancelled. Change the distance, change the frequency. The only time two mics will be in phase is if they are equidistant from the source.

As was pointed out, the math involved only pertains to direct sound. Reflections and ambience also factor into it.
 
Farview said:
The only time two mics will be in phase is if they are equidistant from the source.
Click to expand...

...which is an impossibility in the case of an orchestra since it is not a punctiform sound source...
I guess each section, even each single instrument will get different frequencies cancelled, so it looks to me that there are just too many variables in order to make any quick, meaningful calculations...
 
But you dont record orchestras in mono, so once you pan the stereo pair, there is no cancellation because the mic signals are not interacting.

You can get decent mono compatibility by using an XY pair. You can get perfect compatibility by using an MS setup.

In the case if orchestras, if there are close and distant mics, the distant ones are generally not as loud in the mix, so cancellation isnt a problem.
 
Farview said:
But you dont record orchestras in mono, so once you pan the stereo pair, there is no cancellation because the mic signals are not interacting.

You can get decent mono compatibility by using an XY pair. You can get perfect compatibility by using an MS setup.

In the case if orchestras, if there are close and distant mics, the distant ones are generally not as loud in the mix, so cancellation isnt a problem.
Click to expand...

In this case I recorded in XY and mixed a single accent mic (soloists) to both channels (slightly panned to the left to match real-life & the video), hence my question.
I also kept the volume of that single mic quite high, even though it's lower than the other.
 
Aha! Spot mics. These are the things that always cause trouble but sometimes people assume problems where there aren't any. When I was a young teenager, through a friend of the family I worked (well coiled a few cables) at a recording of a big band playing Glenn Miller music. Stereo was very new really, and spaced mics were how it was done, and very oddly a spaced band too, left right and central. There was almost total separation between brass and woodwind very odd! The doo-Wa stuff sounded great. I've always worked on the principal that out of phase issues with two mic recording setups didn't spoil things but opened up the sound field. It rarely caused mono cancellation due to differences in levels, as has been said. If spot mics are used to capture instruments just lost in the overall balance then as the main mic component has so little of the lost instrument in it, there isn't cancellation in the traditional sense. The clash that many people don't like when spit mics are used seems to be because the source is so dry because of closeness, so I put some artificial reverb on these mics which seems to work for me.
 
rob aylestone said:
Aha! Spot mics. These are the things that always cause trouble but sometimes people assume problems where there aren't any. When I was a young teenager, through a friend of the family I worked (well coiled a few cables) at a recording of a big band playing Glenn Miller music. Stereo was very new really, and spaced mics were how it was done, and very oddly a spaced band too, left right and central. There was almost total separation between brass and woodwind very odd! The doo-Wa stuff sounded great. I've always worked on the principal that out of phase issues with two mic recording setups didn't spoil things but opened up the sound field. It rarely caused mono cancellation due to differences in levels, as has been said. If spot mics are used to capture instruments just lost in the overall balance then as the main mic component has so little of the lost instrument in it, there isn't cancellation in the traditional sense. The clash that many people don't like when spit mics are used seems to be because the source is so dry because of closeness, so I put some artificial reverb on these mics which seems to work for me.
Click to expand...

Aaah thanks for this informative input, a new perspective indeed...
That (the component of single instruments in the various mics is something I never thought of...)
Edit must spread rep before giving it to you... anyway thanks again!

Tootler said:
Jargon alert.
Couldn't you just have said "point source"? It's a perfectly acceptable term.
Click to expand...

11 posts and you join a discussion only to make a nuisance of yourself? wow, that makes you think...
Besides:
1. "punctiform sound source" is perfectly acceptable as well and I find it understandable and clear
2. I'm not a native English speaker so apologies if I don't mix up "its" and "it's", "their" and "they're" (unlike many native speakers), and use a correct sentence (if somewhat technical) in the right place.
Best regards.
 
What I do with spot micfs is I use the main mics for the overall sound and just push the spot mic just to the point where it accentuates the instrument, but doesn't separate it from the overall. But I supposed that is a matter of what you are trying to accomplish.

If you know the distance between the spot mic and the XY pair, you can nudge the spot mic track later by the appropriate amount approx. 1ms/ft. But as long as it isn't really messing with the tone of the instrument, you really don't have a problem.
 
RecNewbie said:
..11 posts and you join a discussion only to make a nuisance of yourself? wow, that makes you think...
Besides:
1. "punctiform sound source" is perfectly acceptable as well and I find it understandable and clear
2. I'm not a native English speaker so apologies if I don't mix up "its" and "it's", "their" and "they're" (unlike many native speakers), and use a correct sentence (if somewhat technical) in the right place.
Best regards.
Click to expand...
'24' post and...
I think this should all work out in the wash of things yes?
 
Farview said:
What I do with spot micfs is I use the main mics for the overall sound and just push the spot mic just to the point where it accentuates the instrument, but doesn't separate it from the overall. But I supposed that is a matter of what you are trying to accomplish. ..
Click to expand...
I've always been intrigued with all the possible variations one can play with, in size' and placement of pieces in a mix. Being just a casual listener of orchestral music as an example, one thing that to me often sticks out is the spot mics, they seem say 'this is out of place' in the mix on many recordings.
 
mixsit said:
I've always been intrigued with all the possible variations one can play with, in size' and placement of pieces in a mix. Being just a casual listener of orchestral music as an example, one thing that to me often sticks out is the spot mics, they seem say 'this is out of place' in the mix on many recordings.
Click to expand...
That's unavoidable because all the the instruments sound much different up close than they do far away. A really loud violin hitting the xy pair will sound very different than a close miked violin mixed with the stereo pair.

It's something you run into live as well, when there is a solo that needs help with a spot mic. It becomes obvious that the sound of the orchestra is coming from one place and the sound of the solo is coming from the PA plown 20 feet above the stage.
 
mixsit said:
'24' post and...
I think this should all work out in the wash of things yes?
Click to expand...

At least mine were 24 polite and, in some cases, even helpful posts (I try to be a giver when I can).
Sorry if you find my retort inappropriate, but I do believe that I have the right to reply to someone who's obviously trolling (or just had a bad hair day).
It always works out one way or another ;)

Farview said:
That's unavoidable because all the the instruments sound much different up close than they do far away. A really loud violin hitting the xy pair will sound very different than a close miked violin mixed with the stereo pair.

It's something you run into live as well, when there is a solo that needs help with a spot mic. It becomes obvious that the sound of the orchestra is coming from one place and the sound of the solo is coming from the PA plown 20 feet above the stage.
Click to expand...

Well, I think I struck a balance because the "accent mic" was quite far from the soloists (1-2m, because of the cramped space).
Plus, we were in a church and the stereo pair was also a little too far (20/20 hindsight obviously).
I couldn't make tests in there because I had to play so it was all in the dark.
Being so far back, there is a noticeable delta between the dryness of the NT2-a & the (huge) reverb of the stereo pair, just a little bit more than I would have liked.

I tried mixing the two tracks again, this time inverting the polarity of the mono accent mic during playback: there was a slight change in the overall timbre but nothing dramatic, so I guess I'm ok.
Next time I will probably do things differently & put the stereo pair nearer (especially in a church...), but all in all I like the end result, so... :)
 
The added benefit of putting the stereo pair closer is the stereo image becomes wider, along with the reduced room sound making the spot mic not stick out quite so much.

You'll get it next time.
 
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