Bottom Snare Mic

fazil said:
Phase "invertion" or "flipping" is not the signal upside down. Phase invertion DOES NOT exist. You should understand the differance between phase and polarity.
It seems like a stupid matter of terminology but it's not ...
Click to expand...
Actually, in this case it kind of is. "Phase inversion" not only exists, but it is a very common principle in physics. In this case, phase inversion in the sound pressure yields an inversion in electrical polarity. Or put the other way, an inversion of electrical polarity yields an inversion of phase in sound pressure.

The distinction is that "polarity" usually does not become involved as a technical term untill the sound waves are transduced into electrical current, whereas "phase" can be used to describe properties and events in any physical medium. While sound waves can have "positive" pressure and "negative" pressure as related to the air pressure of an otherwise silent room, these pressure waves are usually not said to have a "polarity". Yet they do cause things such as "phase cancellations" and "phase reenforcements" and the like.

(And, BTW, as a very hair-splitting sidebar, out of curiosity I did some research before coming back to this thread, and I was wrong about one thing; indeed phase shifts can be applied and measured to irregular waveforms like audio recordings. The math is not limited to simple periodic shapes like sine waves or square waves. But to introduce that here is just complicating matters even worse than they already are ;).)

The important thing that needs to be understood here is the action that is taking place. The inverted phase/polarity between a top-miked drum signal and a bottom-miked drum signal is due to the "mirroring" of the sound pressures on one side of the drum skin vs. the other. When the air pressure goes "positive" (becomes denser) on one side of the skin, it more or less simultaneously goes "negative" (becomes thinner) on the other side and vice versa. Their "phases are inverted." This is not the same thing as a time shift or delay that throws the two signals "out of phase". The actual mechanism is what's different, and understand that mechanism is what's needed to avoid the confusion between "phase shift" and "phase inversion" (or, once in the electrical realm, "polarity inversion.")

End of my Mr. Whoopie impersination... :)

G.
 
newbie question, when you don't have a phase switch on a mic (sm57 doesn't right?) is this normally somehting a mixer or record can do easily (say recording software)?

Thanks,
Daav.
 
daav said:
newbie question, when you don't have a phase switch on a mic (sm57 doesn't right?) is this normally somehting a mixer or record can do easily (say recording software)?

Thanks,
Daav.
Click to expand...
It depends on the mixer hardware or DAW sofware you have. Many of the larger/"better" studio mixers have a phase/polarity inverter button on each channel strip, as do many of the higher-end software mixers. But not all mixers or software apps have them.

G.
 
Thanks Glen, I figured it would go like that, but wanted to make sure i wasn't missing hte opportunity to spend $500 on a high quality phase inverter. ;)

Daav
 
What about if i were to record a snare with 2 mics and instead of flipping the phase/polarity nudging the bottom mis wave forward until it was as close to in phase as it could be?
Would that play with the sound and still cause phasing? I did it and didnt notice any phasing although there could be some there. I just dont know.
Anyway here is a sample of the bounced snare after nudging.

Track: "SNARE"
http://www.soundclick.com/bands/3/archamedies_music.htm
 
In the case of a snare, it isn't a timing problem. if you have one mic on top and the other mic on the bottom, when you hit the drum, you are pushing the head away from the top mic and toward the bottom mic. That is why they are out of phase (reversed in polarity)

When you have a direct bass track and a mic cabinet bass track, the phase issues are cause by timing differences. Not so with the snare.
 
SouthSIDE Glen said:
Actually, in this case it kind of is. "Phase inversion" not only exists, but it is a very common principle in physics. In this case, phase inversion in the sound pressure yields an inversion in electrical polarity. Or put the other way, an inversion of electrical polarity yields an inversion of phase in sound pressure.

The distinction is that "polarity" usually does not become involved as a technical term untill the sound waves are transduced into electrical current, whereas "phase" can be used to describe properties and events in any physical medium. While sound waves can have "positive" pressure and "negative" pressure as related to the air pressure of an otherwise silent room, these pressure waves are usually not said to have a "polarity". Yet they do cause things such as "phase cancellations" and "phase reenforcements" and the like.

(And, BTW, as a very hair-splitting sidebar, out of curiosity I did some research before coming back to this thread, and I was wrong about one thing; indeed phase shifts can be applied and measured to irregular waveforms like audio recordings. The math is not limited to simple periodic shapes like sine waves or square waves. But to introduce that here is just complicating matters even worse than they already are .)

The important thing that needs to be understood here is the action that is taking place. The inverted phase/polarity between a top-miked drum signal and a bottom-miked drum signal is due to the "mirroring" of the sound pressures on one side of the drum skin vs. the other. When the air pressure goes "positive" (becomes denser) on one side of the skin, it more or less simultaneously goes "negative" (becomes thinner) on the other side and vice versa. Their "phases are inverted." This is not the same thing as a time shift or delay that throws the two signals "out of phase". The actual mechanism is what's different, and understand that mechanism is what's needed to avoid the confusion between "phase shift" and "phase inversion" (or, once in the electrical realm, "polarity inversion.")

End of my Mr. Whoopie impersination...

G.
What I am trying to say is that phase Flip (shouldn't have said inverstion) and polarity inversion are not exactly the same.
Maybe I should not have used the term phase inversion. Acording to what I understand Phase is a way to express the "time" difference between multiple waves. So a phase shift depends on a given frequency on which the phase shift should be applied.
If you invert polarity you get the inverse (- -> + and + > -).
If you invert phase on each frequency you get something different than the above.

In cases like a snaredrum where you have two opposite mikes as in a mirror (good expression :)) it does mostly work effectively because there is actually no time difference between the two mikes and their source. The problem is that the two drumheads make an opposite movement which will result in cancelling. Ofcourse polarity inversion can be switched in here, because it will electrically convert the movement of the second skin to be the same as the first skin.

Hope this makes sence :) I'm not a audio physician (is that a word?) I'm just trying to make sence out of this mess of information concerning these terms. I hope my basic thoughts are correct so that I'm not confusing people even more.

Glen, thanks for correcting!
I'm a little confused though.
Was I wrong to use the term phase inversion because it's something else or because I'm completely wrong ?

greets, arno
Click to expand...
Click to expand...
 
Fazil,

At the heart of it, I don't think we are really saying anything all that different. I agree with you that "phase shift" and "polarity inversion" are indeed two different things, and that what is happening with the top/bottom drum thing is a mirroring that when miked results in two electrical waveforms of opposite polarity, which is why one needs to "flip" the polarity of one of the signals (either on the mic or later on down the chain) to avoid the two wavesfroms from obliterating each other, and that this "mirroring" has nothing to do with a phase shift caused by a delay in timing. I honestly think that we are just getting caught up in word usage here a bit.

This may or may not help anyone else to clear things up, but I think of it this way: Inverting or flipping polarity in the electrical signal inverts the waveform; makes it a top-to-bottom mirror image of itself. It can also be said that this throws the waveforms exactly out-of-phase with each other in the fact that when you sum them together, you'll get 100% phase cancellation and wind up with a 0 DC flatline. By definition, then, the waveforms are in inverted phase to each other. Under this light, "inverting the polarity" and "inverting the phase" are really just two different ways of saying the same thing.

On an analog mixing board, "inverting the polarity" may be a bit more accurate of a description since that is what the circuitry is actually doing, electrically-speaking; the phase inversion of the waveform is just the end result. In a digital NLE, there is no electrical signal polarity actually being flipped; instead what is happening is the values of the digital bits representing the waveform are being modified to represent an inversion of the phase of the waveform. So again, what is actually happening - what is physically being modified - is entirely different, but the end result of an up/dowm mirroring of the waveform is the end result.

None of which has anything do to with a "time-dependant phase shift", which is another animal alltogether.

G.
 
Absolute phase

Hi, I'm sorry, I really don't want to confuse things but you guys do seem to have this all sorted out (phase = description of air pressure, polarity = description of electrical signals).

What made me wonder about the original post is the fact that no one seems to mind that flipping things until they work (as opposed to reasoning the problem out) could leave you with the kick attack not pushing but pulling the speaker when it is played back. I had heard that some people are sticklers about this even though a lot of people claim it makes no difference in the sound. Any sticklers or nay-sayers out there?
 
I'm not sure that it makes a big difference. I will routinely flip the polarity on the bass track to see if it makes it jell with the kick better. The relationships between instruments always seemed to make more of a difference than the absolute phase of anything.
 
You must log in or register to reply here.
Back
Top