Tech Time: Signal Phase vs. Signal Polarity
Someone elsewhere asked about double miking snare drums, it raised (but) one of my pet issues -- proper terminology when talking about issues of signal phase and signal polarity...
You reverse polarity in order to get the two signals from mics on opposite sides of a drum head to reinforce each other instead of (potentially) partially canceling each other out.
By contrast, when someone suggests moving the mic to change the relative distances from the drum head of the mics, he is talking about changing the relative phase relationships of the two signals, vis a vis the sound emanating from the head.
But phase is a term strictly relative to the frequency of a wave. Since the cycle period of every frequency is different, those phase relationships are strictly related to the frequency of any given wave component of the aggregate sound. And that is something a lot of folks who spend a lot of time talking about recording either don't get -- or simply are too lazy to address correctly.
With a single skin drum, by moving one mic far enough away from the struck skin so that the signal reaching the mic is now delayed by precisely 1/2 the wavelength of the fundamental, we achieve a change in that phase relationship equivalent -- in a sense -- to reversing the polarity -- but only at that fundamental. Other frequencies will have varying amounts of cancellation or reinforcement when the two signals are summed, often leading to the familiar 'comb filter' effect.
So, under that latter scenario (moving one mic), if we consider the fundamental of the drum to be our primary concern with regard to phase (assumptions are often dangerous in audio) and the fundamental to be 800 Hz (to pick a number I've heard a few times, though, of course, the fundamental tone of a drum, and the concentration of energy at a specific frequency depends in large part on how -- and how well -- it's tuned. (Info on snare drum physics: The Snare Drum)
Here's a (simple) wavelength calculator (it assumes 'standard' values for temperature and altitude/air pressure): Wavelength
From that we get a ballpark figure of ~17 inches for the WL of our 800 Hz fundamental. So, to change the phase of that signal in such a way as to invert an 800 Hz tone 180 degrees, you would move that mic ~ 8.5 inches farther (or nearer) the drumhead, vis a vis the other mic.
But it's important to remember that that snare sound is not only comprised of its fundamental pitch -- drums -- and particularly snares -- have a tendency to produce extremely complex waveforms with a lot of different frequency components. There are the myriad of issues revolving around the complex character of the snare drum, particularly the fact that (while many drummers remove the bottom skins from most of the drums in their recording kits) the drum will generally have two skins. At lower frequencies, the skins will tend to move in the same direction. But at the higher mode formed by the enclosed space, the skins will actually be moving apart, making the sound quite complex. And then there is the snare 'spring' itself.
So, in all likelihood, moving one of the snare mics may produce pleasing results in the sum of the signals but it will not be that much like what would be accomplished by having both mics equidistant from the drum head and reversing signal polarity of one of them.
The 3:1 relative distance rule of thumb will help save your sanity. Keep in mind it's a relative guideline ballparking the relative levels of a given signal reaching each mic. (You're basically trying to get the level of a given drum loud enough in its own mic that it will dominate when that mic is summed with other mics, and relative effects of cacellation are minimized.)
When considering phase relationships in complex drum miking scenaria, another sanity-saver is to focus on one drum and its relationship with its mic vis a vis the other mics around the kit at a time.
Since sound radiating in a free space is basically inversely proportional to the square of the distance, we know that if mic X is one unit of measure away from a single point sound source and mic Y is three units away, the level of the signal reaching mic Y will be nine times (3 squared) less than that reaching mic X. (However, there is a lot more chaos there, though, since a drum head is certainly not a single point source.)
You reverse polarity in order to get the two signals from mics on opposite sides of a drum head to reinforce each other instead of (potentially) partially canceling each other out.
By contrast, when someone suggests moving the mic to change the relative distances from the drum head of the mics, he is talking about changing the relative phase relationships of the two signals, vis a vis the sound emanating from the head.
But phase is a term strictly relative to the frequency of a wave. Since the cycle period of every frequency is different, those phase relationships are strictly related to the frequency of any given wave component of the aggregate sound. And that is something a lot of folks who spend a lot of time talking about recording either don't get -- or simply are too lazy to address correctly.
With a single skin drum, by moving one mic far enough away from the struck skin so that the signal reaching the mic is now delayed by precisely 1/2 the wavelength of the fundamental, we achieve a change in that phase relationship equivalent -- in a sense -- to reversing the polarity -- but only at that fundamental. Other frequencies will have varying amounts of cancellation or reinforcement when the two signals are summed, often leading to the familiar 'comb filter' effect.
So, under that latter scenario (moving one mic), if we consider the fundamental of the drum to be our primary concern with regard to phase (assumptions are often dangerous in audio) and the fundamental to be 800 Hz (to pick a number I've heard a few times, though, of course, the fundamental tone of a drum, and the concentration of energy at a specific frequency depends in large part on how -- and how well -- it's tuned. (Info on snare drum physics: The Snare Drum)
Here's a (simple) wavelength calculator (it assumes 'standard' values for temperature and altitude/air pressure): Wavelength
From that we get a ballpark figure of ~17 inches for the WL of our 800 Hz fundamental. So, to change the phase of that signal in such a way as to invert an 800 Hz tone 180 degrees, you would move that mic ~ 8.5 inches farther (or nearer) the drumhead, vis a vis the other mic.
But it's important to remember that that snare sound is not only comprised of its fundamental pitch -- drums -- and particularly snares -- have a tendency to produce extremely complex waveforms with a lot of different frequency components. There are the myriad of issues revolving around the complex character of the snare drum, particularly the fact that (while many drummers remove the bottom skins from most of the drums in their recording kits) the drum will generally have two skins. At lower frequencies, the skins will tend to move in the same direction. But at the higher mode formed by the enclosed space, the skins will actually be moving apart, making the sound quite complex. And then there is the snare 'spring' itself.
So, in all likelihood, moving one of the snare mics may produce pleasing results in the sum of the signals but it will not be that much like what would be accomplished by having both mics equidistant from the drum head and reversing signal polarity of one of them.
The 3:1 relative distance rule of thumb will help save your sanity. Keep in mind it's a relative guideline ballparking the relative levels of a given signal reaching each mic. (You're basically trying to get the level of a given drum loud enough in its own mic that it will dominate when that mic is summed with other mics, and relative effects of cacellation are minimized.)
When considering phase relationships in complex drum miking scenaria, another sanity-saver is to focus on one drum and its relationship with its mic vis a vis the other mics around the kit at a time.
Since sound radiating in a free space is basically inversely proportional to the square of the distance, we know that if mic X is one unit of measure away from a single point sound source and mic Y is three units away, the level of the signal reaching mic Y will be nine times (3 squared) less than that reaching mic X. (However, there is a lot more chaos there, though, since a drum head is certainly not a single point source.)
posted by TK Major at
9:25 AM
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