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.)

What mastering is for...

Talk about a bad penny topic...

Someone was asking recently about a problem he was having with his mixes sounding good in his studio -- but falling apart in the car or on other playback systems. He asked if that was a problem or if that could be fixed in mastering -- and someone helpfully answered, Yes, that's what mastering is for.

In today's paradigm, I'd make that a highly qualified maybe.

Even the best ME can't turn a sow's ear into the proverbial silk purse. If a track is mixed badly or has fundamental flaws, you can try to put sonic band-aids on it, but it's always going to be fundamentally compromised -- and it will be likely to cost you more money as the ME struggles to overcome problems with tracking or mix.

In the old days of then-high tech, computer controlled cutting lathes, creating a master for pressing disks required very expensive gear and a lot of skill and knowledge to use -- making the process quite expensive -- fixes at the cutting lab were for last minute and emergency fixes.

Also, in those days, mastering labs were seldom outfitted as high quality mixing rooms. Not only was it wildly expensive to do a fix there, but it was entirely likely that if you tried to make decisions there, they would be compromised by far less than ideal monitoring.

That's why mastering jobs were, by accepted practice, submitted with an edit list of any EQ or other fixes one wanted imposed. The ME might change the sound (say rolling off bass) in order optimize the signal for the rigid requirements of vinyl (narrow dynamic range, limits on bass levels) but there were other things like phase content that he typically had little control over (unless it was simply a polarity error).

In those days, the ME was expected to not make aesthetic decisions but only apply the requested changes or those absolutely necessary for the format.

But mastering changed with digital -- but the cost of entry was even higher at first. But as CD-R masters became acceptable at rep houses, it became entirely possible for the average home recordist to prepare his own replication masters.

And at that point, mastering houses -- and those who had simply noticed that mastering houses had traditionally commanded hourly rates sometimes 5-10 times higher than studios -- realized there was a challenge but also an opportunity -- to expand on the traditional last-minute-fix aspect into something oriented to corrections few would have made -- or wanted made -- in the old days and to plant the idea that was 'normal practice' and that a mix was not 'finished' unless it had been 'fixed again' by an ME.

So is recording history rewritten to help shore up demand for what is often an unnecessary -- and sometimes aesthetically disastrous -- step.

Now there is another aspect to mastering for those putting together album packages. It's also one where the traditional role of the ME has been expanded. That is in last minute fixes to help try to give some consistency in timbre as well as level to album tracks that may have been recorded in different times and places by different production staff, as so often is the case these days.

When one is putting together a package for replication and commercial release, it may well make perfect sense to use an ME in order to assist in producing a package that fits together well.

And in an era when many of those recording do not have long experience, necessarily the greatest gear (and likely not the knowledge of how to get the most of it), that court of last resort at the ME's may well make some kind of sense. Do make sure that you have chosen an ME that is not just experienced but thinks like you do. There are a lot of different approaches. Validity of approach is contingent on the nature of the project/genre.

But when there is no budget, when you are releasing the music for free or as one-off sales through online stores, I recommend at least trying to do it oneself.

Really, the right place to get things right is in tracking and mixdown. Learn to get that right and, even if you always have everything externally mastered, you'll still be helping to make sure that things end up sounding  as close as possible to what you want and that the ME has to make as few of his own aesthetic/corrective decisions as possible.