#1
I've heard that when mic'ing both the top and bottom head of a drum (usually snare) you have to invert the phase of the bottom head. Can anyone tell me why, like more along the lines of physics. I understand that when a wave (such a sound) reflects back, it comes back in an inverted signal leaving the reflected wave out of phase with the original. Can anyone maybe explain how the sound wave may be inverted by the drum?
#2
Ok you know what a wave looks like right?

well if you got a track of audio, duplicated it, it doubles in volume. Why? Because the 2 waves are going up together and complimenting each other. When one goes down and one goes up, it effectively kills the sound. The speakers can't play it because if you think about it when the waveform goes up the speaker pushes out... if you have a sound thats telling it to do it at exactly the same time... just doesnt work.
The reason you need to invert the phase is because both mics share similar characteristics your going to be cutting out the snare sound completely etc etc.

The reason it may get out of phase is because its when the wave hits the microphone its receiving the sound at a different point of the wave.

If its partially out of phase it'll just sound weird.

Make sense? It does in my head but finding the words is a bit hard :P
Last edited by doommaker at Jan 20, 2009,
#3
If you are using a single mic on a single drum, there is no way, for all practical purposes, that it will be out of phase.

The problem comes when using more than one mic. If those sound waves hit each mic at the wrong time, when you play the end result back, one wave formation will be positive and the other will be negative, resulting in phase cancellation. The likelihood of phase cancellation is very high if you have one mic an inch from the top of the snare and the other mic almost equally close.

IMHO, unless you have the tracks and equipment to spare, putting two mics on a snare is a bit excessive. But, whatever floats your boat as they say.

It gets a little more complicated in a typical drum mic setup where, among other mics, you'll have a mic on the snare and two overheads - all three mics will be picking up the snare. You could get phasing between any combination of them. Sometimes (often times) even moving one of the mics a couple of inches is all that is needed to change the relative distance between them such that the sound waves won't overlap negatively on playback.

CT
Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.
#4
Quote by doommaker

well if you got a track of audio, duplicated it, it doubles in volume.


The rest of your post is right on. This, though it seems intuitive, doesn't sit right with me. I think it increases the volume by 1.5x , but I can't find a ready source on that. I'll keep looking.

CT
Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.
#6
For some reason 1.5x sounds right to me too. I understand the whole waves cancelling out thing but how the waves line up so that they are completely out of phase (i.e. the crest of one wave during the through of another).
[If those sound waves hit each mic at the wrong time, when you play the end result back, one wave formation will be positive and the other will be negative, resulting in phase cancellation.]
The waves might not necessarily match positive and negetive perfectly though right, ( like completely out of phase) and if so, would an "invert phase" button do the trick? (the one that looks like a circle with a line through it). Or would a better way to go about fixing it be zooming in and dragging one of the tracks a little so that their waves match up?

...And im guessing the only accountable reason for out of phase recording is due to the wave reaching one mic first as opposed to a phase inversion do to reflection?...

Sorry if i got the quote thing wrong (which i did), its my first time using it.
#7
its simple, the mics are on two opposite sides of a vibrating object, so they are inherently out of phase with each other. when the drum vibrates "down" the top mic sees it as a "down" but the bottom mic sees this as an "up", when the drum vibrates "up" the top mic sees it as an "up" movement but the bottom mic sees it as a "down" movement.

to illustrate, take a plastic ruler or stiff cardboard and "wobble" it back and over between your two hands. its not reflection, its the simple fact that the two sides of the object are vibrating opposite to each other.