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wjgypsy
New member
I'm sorry but, i STILL do not understand it. For one thing, I don't understand voltige and stuff like that so i'm lost. So what i said about phase in my last post WAS'NT true?
Light
New member
Here is a better version of a 1 Hz sine wave. It is on a graph with voltage on the Y axis, and time on the X axis. When you mix two signals together, you are adding there voltages at each moment in time. If both have a positive voltage, then the voltage at that moment in time will be higher. If one is positive, and the other is negative, the voltage will be lower. If one is 100mV and the other is -100mV, they will cancel out completely. It is simple addition, which is why it is called summing.
I think, Zeke, that one of your problems may be your youth. I do not know what sort of math classes you have taken, but if you have not had at least an introduction to trigonometry, this may be hard to understand. Sine wave functions are a big part of trigonometry, so hopefully you are working hard in your math classes. This stuff is VERY important to understand, and is also very complicated.
BTW, you can not actually hear a 1 Hz sine wave, but it makes for a relatively clear picture.
Alright Skippy, I did the picture, now you make sense of it in words, you seem to be better at that than I am.
Light
"Cowards can never be moral."
M.K. Gandhi
I think, Zeke, that one of your problems may be your youth. I do not know what sort of math classes you have taken, but if you have not had at least an introduction to trigonometry, this may be hard to understand. Sine wave functions are a big part of trigonometry, so hopefully you are working hard in your math classes. This stuff is VERY important to understand, and is also very complicated.
BTW, you can not actually hear a 1 Hz sine wave, but it makes for a relatively clear picture.
Alright Skippy, I did the picture, now you make sense of it in words, you seem to be better at that than I am.
Light
"Cowards can never be moral."
M.K. Gandhi
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littledog
New member
Nice!
Are you selling signed copies suitable for framing?
Are you selling signed copies suitable for framing?
M
mixsit
Well-known member
Yes you're getting close.ZEKE SAYER said:
Another example... Think of voltage as presure, and polarity as direction. An analogy for both could be two people pushing on something. One is pushing up (toward the top side of the wave), the other pushing down with equal force. There is no movement, and the voltage is zero. If they both push the same direction, they go twice as far and the signals add.
Wayne
Light
New member
littledog said:Nice!
Are you selling signed copies suitable for framing?
10 minuites on VectorWorks, and I was done. Nothing much to it, but if you want to frame them, go for it.
Light
"Cowards can never be moral."
M.K. Gandhi
skippy
New member
Good job with the diagrams, Light- thanks!
Let's see if I can come up with some more words to help with this. We all know that sound waves are really just a series of disturbances in the pressure of the air: the air is compressed (positive pressure), and then rarefied (negative pressure), as the waves pass any given point in space. Let's take an identical pair of SM57s. And just for argument, let's say that both of them put out a positive voltage (on pin 3 with respect to pin 2) when they experience a positive pressure on their diaphragms.
Now, let's hang our pair of SM57s on a snare drum: one 5" above the top head, and one 5" below it. And we'll hit the snare once, nice and hard. What happened?
Let's model the behavior of the snare as putting out a single pressure pulse when the stick hits the head, and let's ignore the contributions of the bottom head and the snares (and reverb from the floor, and son on...!) for the moment. That's horribly oversimplified, but for the purposes of illustration we'll go with it. The two mics are symmetrically 5" away from the top head, one above, one below.
The head on top experiences a negative pressure pulse: the stick moves the head away from the mic, creating a lower pressure above the head than below. So the upper mic sees a negative pressure pulse, and the lower mic sees a positive pressure pulse: they see essentially the same signal, but with opposite polarity.
Let's say the top mic puts out -1V when we whack the snare- then the bottom mic will put out +1V. Identical mics, but the pressure environment is different, because they are on opposite sides of the head. Sum their outputs, and you get zero, because they are equal, opposite polarity, and not delayed with respect to each other. Good so far?
This is the really simple case: a single head, mics identical distances away from it on opposite sides. In the real world it is _never_ that simple... Just for grins, let's take that bottom mic and move it 3" further down: top mic is 5" off the head, bottom is 8" off the head. We'll still get about +1V from the bottom mic, but we'll get it 3/1100*12 seconds or .22 milliseconds later. It's now no longer purely the opposite polarity: it's now delayed, and that delay may cause some problems if we sum the outputs of the two mics, some of the components will cancel, and some will reinforce. 3" is the wavelength of a 4400Hz signal- so we'll exactly cancel out everything at 4400Hz (because the two mics see opposite polarity signals, remember), creating a huge upper midrange suckout. Maybe we need that effect, maybe we don't: but the distance between the two mics controls where the first cancellation occurs. It's basically an EQ knob!
Here's a fun trick: if you have a polarity switch for the lower mic, switch it now, without touching anything else. And you'll find that the suckout moves from 4400Hz to 2200Hz (one half wavelength of the signal gives us the opposite polarity, instead of one full wavelength with the _mic_ giving us the opposite polarity).
Clear as mud, probably: I wrote it, but I doubt that that will help clear up much...
Anyway, if you are having phasing issues with a multiple mic setup, by all means play with the polarity switches at the preamp. Changing the polarity of one or more mics may change an awful comb-filtered sound into a *pleasant* comb-filtered sound: creative use of interference is the order of the day. Changing the polarities can move those notches and peaks around, and sometimes that's exactly what you need (for example, to tame a "pingy" underdamped snare sound...).
Multiple mics on a single source _will_ create cancellations and reinforcements, and it is a game of inches. That's why everybody always says "experiment with placement": a move of 6-8 inches can move the first comb filtering notch right down to 1kHz, and that's in the most sensitive part of your hearing range. Little moves make _big_ differences. And hopefully, all this verbiage will help explain why...
Light, back to you at the studio!
Let's see if I can come up with some more words to help with this. We all know that sound waves are really just a series of disturbances in the pressure of the air: the air is compressed (positive pressure), and then rarefied (negative pressure), as the waves pass any given point in space. Let's take an identical pair of SM57s. And just for argument, let's say that both of them put out a positive voltage (on pin 3 with respect to pin 2) when they experience a positive pressure on their diaphragms.
Now, let's hang our pair of SM57s on a snare drum: one 5" above the top head, and one 5" below it. And we'll hit the snare once, nice and hard. What happened?
Let's model the behavior of the snare as putting out a single pressure pulse when the stick hits the head, and let's ignore the contributions of the bottom head and the snares (and reverb from the floor, and son on...!) for the moment. That's horribly oversimplified, but for the purposes of illustration we'll go with it. The two mics are symmetrically 5" away from the top head, one above, one below.
The head on top experiences a negative pressure pulse: the stick moves the head away from the mic, creating a lower pressure above the head than below. So the upper mic sees a negative pressure pulse, and the lower mic sees a positive pressure pulse: they see essentially the same signal, but with opposite polarity.
Let's say the top mic puts out -1V when we whack the snare- then the bottom mic will put out +1V. Identical mics, but the pressure environment is different, because they are on opposite sides of the head. Sum their outputs, and you get zero, because they are equal, opposite polarity, and not delayed with respect to each other. Good so far?
This is the really simple case: a single head, mics identical distances away from it on opposite sides. In the real world it is _never_ that simple... Just for grins, let's take that bottom mic and move it 3" further down: top mic is 5" off the head, bottom is 8" off the head. We'll still get about +1V from the bottom mic, but we'll get it 3/1100*12 seconds or .22 milliseconds later. It's now no longer purely the opposite polarity: it's now delayed, and that delay may cause some problems if we sum the outputs of the two mics, some of the components will cancel, and some will reinforce. 3" is the wavelength of a 4400Hz signal- so we'll exactly cancel out everything at 4400Hz (because the two mics see opposite polarity signals, remember), creating a huge upper midrange suckout. Maybe we need that effect, maybe we don't: but the distance between the two mics controls where the first cancellation occurs. It's basically an EQ knob!
Here's a fun trick: if you have a polarity switch for the lower mic, switch it now, without touching anything else. And you'll find that the suckout moves from 4400Hz to 2200Hz (one half wavelength of the signal gives us the opposite polarity, instead of one full wavelength with the _mic_ giving us the opposite polarity).
Clear as mud, probably: I wrote it, but I doubt that that will help clear up much...
Anyway, if you are having phasing issues with a multiple mic setup, by all means play with the polarity switches at the preamp. Changing the polarity of one or more mics may change an awful comb-filtered sound into a *pleasant* comb-filtered sound: creative use of interference is the order of the day. Changing the polarities can move those notches and peaks around, and sometimes that's exactly what you need (for example, to tame a "pingy" underdamped snare sound...).
Multiple mics on a single source _will_ create cancellations and reinforcements, and it is a game of inches. That's why everybody always says "experiment with placement": a move of 6-8 inches can move the first comb filtering notch right down to 1kHz, and that's in the most sensitive part of your hearing range. Little moves make _big_ differences. And hopefully, all this verbiage will help explain why...
Light, back to you at the studio!
cordura21
New member
maybe this chart can help.
This is the way I understand it (See the chart below).
The y axis is the amplitude, and the x axis is distance. You see 3 waves, which is the same wave sampled at different times. You also see 2 squares: a blue one for where the mic A goes, and the green one, where the mic B goes.
First sample (thick red line):
Mic A gets a +1 reading, while mic B -0.5 (more or less) reading. If you add them you get +0.5
Second sample (less thick red line):
Mic A gets a little more than zero, while mic B reads almost -1. So that sum will fall around -0.8
Third sample (the 3rd. line):
Mic A reads -0.5 and mic B is near -1, resulting in -1.5.
If mic A was at position 6 and mic B at position 9, both readings will be completely out of phase. If they were at position 6 and 12, they will result in a wave that's 2 times the first one in amplitude.
I hope it makes sense. Cheers, Andrés
This is the way I understand it (See the chart below).
The y axis is the amplitude, and the x axis is distance. You see 3 waves, which is the same wave sampled at different times. You also see 2 squares: a blue one for where the mic A goes, and the green one, where the mic B goes.
First sample (thick red line):
Mic A gets a +1 reading, while mic B -0.5 (more or less) reading. If you add them you get +0.5
Second sample (less thick red line):
Mic A gets a little more than zero, while mic B reads almost -1. So that sum will fall around -0.8
Third sample (the 3rd. line):
Mic A reads -0.5 and mic B is near -1, resulting in -1.5.
If mic A was at position 6 and mic B at position 9, both readings will be completely out of phase. If they were at position 6 and 12, they will result in a wave that's 2 times the first one in amplitude.
I hope it makes sense. Cheers, Andrés
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Digidude824
New member
Light said:
Are you saying this switch is useless, and has no meaning? Does everything have to be a conspiracy, or was it just named that to help prevent mass confusion for the people that don't know the full details of phase and polarity? I'm just a bit confused by this comment. Nice info though ty.
skippy
New member
He's saying that the "phase switch" is really misnamed: it really should be a "polarity switch". They are very useful tools, of course: they are just poorly named, and that misuse of the word "phase" leads to confusion. Like what we have here.
Light
New member
Digidude824 said:
I find I keep saying this. What Skippy said.
Light
"Cowards can never be moral."
M.K. Gandhi
VesuviusJay
Poser Roaster
Listen for yourselves
Hey light, correct me if I'm wrong but I could hear the phase and polarity phenomenon by using my wireless headphones and messing with mic position in the soundroom. By separating mics and micing the back of the amp respectively. Each produced a profound effect in the mixed output.
Now what it sounds like when you have it perfect, Im still working on that one. :s
Hey light, correct me if I'm wrong but I could hear the phase and polarity phenomenon by using my wireless headphones and messing with mic position in the soundroom. By separating mics and micing the back of the amp respectively. Each produced a profound effect in the mixed output.
Now what it sounds like when you have it perfect, Im still working on that one. :s
Light
New member
Re: Listen for yourselves
I find headphones have insufficient isolation for me to do any critical listening in a live room environment, but if it works for you, great.
Light
"Cowards can never be moral."
M.K. Gandhi
VesuviusJay said:
I find headphones have insufficient isolation for me to do any critical listening in a live room environment, but if it works for you, great.
Light
"Cowards can never be moral."
M.K. Gandhi
VesuviusJay
Poser Roaster
OK how does this pertain to miking the back of the amp?
OK boys and girls,
I bought a neutrik XLR cable from guitar center today. I am going to cannibalize one of my regular mic cables and reverse the polarity( pins 2 and 3). Then I am going to mic the back of my 2X12 with the mic and the reversed cable. This is according to Mr. Harvey Gerst's mic thread. With the polarity regular, I stick the mic in the back of the amp and it sounds like crappola. Hopefully this will produce better results.
OK boys and girls,
I bought a neutrik XLR cable from guitar center today. I am going to cannibalize one of my regular mic cables and reverse the polarity( pins 2 and 3). Then I am going to mic the back of my 2X12 with the mic and the reversed cable. This is according to Mr. Harvey Gerst's mic thread. With the polarity regular, I stick the mic in the back of the amp and it sounds like crappola. Hopefully this will produce better results.
D
Digidude824
New member
Just for shits and giggles, I made a session in Pt's and recorded a pure sine wave using the generator. I then copied that same wave to another track. Now I added a 1 band EQ to both tracks so I could flip the polarity back and forth. So in doing so to one track I had total cancelation, as theory predicts. Now to address the phase (if i understanded it right), I put both tracks back in polarity and then moved one of the tracks ahead of the other to model a 90 degree, and then a 180 degree phase difference. As far as sound is concerned the effects were the same as changing the polarity.
This is what's supposed to happen right?
This is what's supposed to happen right?
D
Digidude824
New member
I guess what I'm really getting at is, how would you know if you're having a phase problem or a polarity problem, giving an extreme example of coarse.
skippy
New member
Well, I assume that you put the straight-up signal on one channel, and the delayed signal on the other channel, and listened to them- right? In that case, sitting in one spot, you might not notice much difference: simple sine waves are not really the best vehicle for demonstrationg this phenomenon.
In particular: the 180-deg phase shift you created is precisely the *same* as a polarity reversal in this oversimplified case: that's the one special case I talked about above when the two are one and the same. Phase differences really become obvious when you have complex signals that have many different frequency components, because the various frequency components are affected differently.
This is especially true if you listened with headphones, instead of monitors. The brain is not that sensitive to absolute phase. It is _very_ sensitive to relative phase, but primarily if that is changing over time (e.g., as you turn your head or move around the room), since that is one of the mechanisms your brain uses to localize the source of a sound. With headphones, the phase difference doesn't change as you move your head- so you might not hear it at all. Headphones can drive you nuts in this respect, because your brain expects the phase relationships of the signals you're hearing to change as you move... You need to be hearing the two signals with *both* ears, so that they can interact with your complete hearing mechanism. Then, as you move around the room, you'll hear suckouts and peaks as the signals interact and cancellations and reinforcement.
If you are talking about two mics: you will almost always have phase (time-delay-related) problems, not polarity problems. The exception comes when you put your two mics in acoustic environments that specifically emphasize polarity differences: above/below a drum head, or in the front/in the back of a speaker cabinet, for example. In that case, you know going in that the soundfields will be intrinsically opposite in polarity. In that case you'd probably start out with the polarity inverted on one of them: you're expecting it.
Regardless, the polarity switch is your friend: if you are having spacing/time-delay-created comb filtering, changing the polarity of one of the signals will change the position of the peaks and notches created by the resulting comb filter. Always try the polarity switch: the nature of the comb filtering will change when you do, and you'll like it better in one setting than the other. And there's no hard and fast rule about _which_, because it depends upon too many factors to list. Just try it...
In particular: the 180-deg phase shift you created is precisely the *same* as a polarity reversal in this oversimplified case: that's the one special case I talked about above when the two are one and the same. Phase differences really become obvious when you have complex signals that have many different frequency components, because the various frequency components are affected differently.
This is especially true if you listened with headphones, instead of monitors. The brain is not that sensitive to absolute phase. It is _very_ sensitive to relative phase, but primarily if that is changing over time (e.g., as you turn your head or move around the room), since that is one of the mechanisms your brain uses to localize the source of a sound. With headphones, the phase difference doesn't change as you move your head- so you might not hear it at all. Headphones can drive you nuts in this respect, because your brain expects the phase relationships of the signals you're hearing to change as you move... You need to be hearing the two signals with *both* ears, so that they can interact with your complete hearing mechanism. Then, as you move around the room, you'll hear suckouts and peaks as the signals interact and cancellations and reinforcement.
If you are talking about two mics: you will almost always have phase (time-delay-related) problems, not polarity problems. The exception comes when you put your two mics in acoustic environments that specifically emphasize polarity differences: above/below a drum head, or in the front/in the back of a speaker cabinet, for example. In that case, you know going in that the soundfields will be intrinsically opposite in polarity. In that case you'd probably start out with the polarity inverted on one of them: you're expecting it.
Regardless, the polarity switch is your friend: if you are having spacing/time-delay-created comb filtering, changing the polarity of one of the signals will change the position of the peaks and notches created by the resulting comb filter. Always try the polarity switch: the nature of the comb filtering will change when you do, and you'll like it better in one setting than the other. And there's no hard and fast rule about _which_, because it depends upon too many factors to list. Just try it...
Light
New member
What Skippy said. Though you should have heard a difference between 90 degrees out of phase and180 degrees out of phase.
Light
"Cowards can never be moral."
M.K. Gandhi
Light
"Cowards can never be moral."
M.K. Gandhi
D
Digidude824
New member
i did. 90 sounds weeker, like half as loud.Light said: