phase.... huh?

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fire.is.real

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hi,

i'm kind of new to home recording, although i have spent a lot of time in professional studios with my band. i have overheard much discussion about the "phase" of a mic. could someone explain to me what it means when mics are "in phase" or "out of phase" and what does that do to the _perceived_ sound?

thanks,
nick
 
When 2 waves are mirrors of each other they are - out of phase
When 2 waves are identical of each other they are- in phase.

Out of phase - your signal might be canceld or thin or harsh or hollow.
In phase - " " will be doubled in energy.

2 mics receiving the same signal at different times due to the distance between them might cause phase problems. Some times different partials of the wave will be in phase while others will be out of phase (due to the delay in time they reach the mics). This will result in some Frequencies to be boosted (in phase),
and others to be cut (out of phase).
 
This is kinda hard to explain without pictures, so I tried looking for web-sites that explained it, and the best one I could find was this:
http://www.britannica.com/bcom/eb/article/2/0,5716,61092+1+59593,00.html?query=phase
And its, ehm, a LITTLE BIT technical...

So I'll try anyway:

Sound is waves. Like this:
Code: 
 _   _   _   
/ \_/ \_/ \_

If you have two waves they can either be completely in phase:

Code: 
 _   _   _   
/ \_/ \_/ \_
 _   _   _   
/ \_/ \_/ \_

Or they can be completely out of phase, like this:

Code: 
 _   _   _   
/ \_/ \_/ \_
   _   _   _
\_/ \_/ \_/

Or it could be just a little bit out of phase.

If you add the two in phase waves, you'll get a wave that is twice the size of the original waves, but if you add the two out of phase waves you'll get silence.

Since sound usually is made up of loads of different frequencies, the effect you usually get is that some frequencies are made stronger (because they are in phase) and some frequencies get cancelled (because they are out of phase). This effect is usually called "comb filter" since the frequency response you get is a lot of peaks and dips, like somebody had run a comb through your sound.

Explaining how this sounds in a descriptive way I leave to somebody else. I've heard people call it "thin" or "tinny" which I guess is an OK description, but I'm not very good at translating sounds to words.

Makes sense?
 
thanks...and now....

actually, both of your replies helped my understanding greatly. i have some background in physics, i just wanted to know what that translated into for mics/recording... ya dig?

so what you're saying is that to make sure that mics are in phase, one must ensure that they are equidistant from the sound source so that they pick up sound waves at the same time? or did i completely miss the application of waves to microphones!??

thanks
nick
 
You won't hear alot of profound cancelation by moving mics around a speaker cabinet...so don't worry so much bout that. It's an ear thing. Move it till you like the way it sounds. On the other hand, miking the front and back of a cabinet with mikes in phase is usually a bad idea. Waves coming off the back of a open speaker are 180 degrees out of phase with waves from front. On the occasions that I mike both sides of an open back guitar amp, I have a mic cable that is wired out of phase...not having a mixer with phase switching (yet).

Keep in mind, sometimes phase cancellation is a cool thing...running stereo synth pads out of phase lends a degree of breadth that can sometimes be useful. Experiment with phasing by wiring a mic cable out of phase, and miking a set of speakers running source material. You just might stumble into some cool techniques.
 
Re: thanks...and now....

fire.is.real said:
i just wanted to know what that translated into for mics/recording... ya dig?

so what you're saying is that to make sure that mics are in phase, one must ensure that they are equidistant from the sound source so that they pick up sound waves at the same time? or did i completely miss the application of waves to microphones!??
thanks
nick
Click to expand...
The problem arises with more then 1 version of the signal are combined.
For example more then 1 mic near a source.

When 2 or more signals are combined out of phase, they will strengthen or cancell depending on several factors.
1. Reletive volume
2. How far out of phase
3. harmonic content

Here is a couple of ways to help aviod phase problems

1. Use the least possible mics needed (close all mics not needed
at the time you record/play as well)

2. There is a rule called 3:1 - Place all mics (other then the main one)
three times the distance then the main mic to the source.
Example mic to trumpet is "3" ?, then the rest shoud be at a distance
of "9".

3. Use a stereo pair of mics that are placed very close together like an "X-Y"

4. Pan hard left and right with a stereo pair lessens the phase problem

5. Use mics patterns to help minimize leakage to each other.

6. put a distance between the mics. When you double the distance the
volume drops by 6dB -helping leakage problems.

7. Move mics and listen. sometimes just a reposition can solve it by bringing
problematic harmonics into place.

8. Flip the phase switch on your mixer (leave this as a last solution)

9. If using a delay on guitar for example, this can cause problems as well.
Listen to your mix in mono and adjust delay times to slove the problem.
 
thanks

thanks,

i think i've got it. i was just wondering because i'm going to be recording a drumset with two overheads... i just wanted to see what was up.

thanks everyone,
nick
 
Well with two mics it's really simple. Listen to how it sounds with the mics connected to the pres in phase with each other. Reverse the phase of one mic at the pre-amp and if the volume goes down dramatically then they were in phase. If it goes up they were out of phase.
With more mics involved- Shailats suggestions are right on the money. Suggestion # 7 in particular.
 
Hey, if your recording a drumset with 2 mics for an overhead try some of these techniques........

http://www.dpamicrophones.com/

goto "Microphone University"

then goto "Stereo Techinques" and there will be lots of different setups for ya..... a popular one is the XY technique....

Have fun,

Sabith
 
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