Now, recently I've been trying to ignore technical issues in favor of KISS (keep it simple, stupid), because creating a great recording is only 10% gear, maybe less, BUT when you're recording and mixing in tiny, poorly designed rooms, its nice to have lots of things in your mental toolbox.
The biggest boogy monsters of most home studios are phase problems. Phase problems can result from all kinds of different scenarios... cancellation from multiple sound sources (as when you're listening on your studio monitors), cancellation from multiple capture sources (most often from attemping to use multiple mics to capture the same sound source, or capturing different sounds in close enough proximity that you're getting alot of bleed), and they can even be introduced as a result of the distance at which you mic a source.
Which seems to make this a good time to explain what phase cancellation is. We even have a neat little visual thanks to wikipedia. Basically, in theory, if a particular soundwave is 180 degrees out of phase with another wavelength of the same frequency, those two completely cancel each other out, resulting in no sound.
Now of course, this isn't exactly the way it happens in real life (but this is the principle that all these new fangled "sound-cancelling" headphones are based on) because very few sound waves come completely isolated, but think about the implications: The closer two wavelengths get to being 180 degrees out of phase with each other, the more they're going to start disappearing. This means, not only do you have to worry about the scenarios I described in the last paragraph, you also have to start thinking about different sound sources that have similar frequency information (for instance, the low end of an electric guitar might share alot of frequency range with the mids of a bass guitar)... this is generally thought of as "masking".
If you're thinking about this alot right now, it might seem like a big scary mess, but dealing with it in the studio isn't actually that difficult. There's some simple techniques that will bring you "phase enlightenment". First, in your monitor setup, you *need* a mono button. The easiest way to know if there are stereo phase issues in your mix is to switch it to mono and listen for what disappears. Even if you can't nail it down to something specific, but it sounds weird in mono... you have phase problems... go back to anything thats panned and work with it till the mono mix is as solid as the stereo mix. If you're thinking to yourself, "who cares though, if it sounds kick ass in stereo, no one's going to actually be listening to it in mono"... remember that there are thousands of different stereo speaker setups in millions of different rooms out there, in which you have no control over how those sound waves are going to be interacting... thats part of the job of someone mixing or mastering a record, to make it as solid as possible for all of those combinations (I'll address some other ideas on that specific topic in a later post).
One more note on stereo phase cancellation. Anytime you're trying to get a stereo mix from a pair of microphones, remember the 3-1 rule. Any microphones that are micing the same source should be at least 3 times as far from each other as they are individually from the source. Which means for drum overheads, if they're 4 feet above the drums, they need to be 12 feet from each other, which will not be practical for most situations. In some cases, you might be better off using an xy pair, which is a very specific method of stereo micing. Take two mics of the same type and line the capsules up, facing each other at exactly a 90 degree angle (or more). This way the overlap of the two pickup patterns is minimized, resulting in a lower chance of phase cancellation.
As we slowly move to less complicated sound fields, the solutions become slightly more complicated. Anytime you start to feel like two instruments are "fighting" each other in the mix, they're trying to share the same frequency ranges and there's just not enough space for both of them to be dominant across the whole spectrum. This may or may not be phase-cancellation related, but if you use this technique, it will help eliminate phase-cancellation issues between different instruments in your mixes. Think of your mixes like a sandwich. What needs to be at the base, and what needs to be at the top? What needs to be the dominant flavor, and what should merely be an accent? If you apply that theory to mixing, keeping in mind what you can compromise on, your mixes will clean up right fast. So your electric guitar sounds beautiful and full with this huge bottom you love, but the bass has no definition; what's the important part... can you sacrifice some of that low end on the guitar for the bass? Each instrument needs its place in the frequency spectrum. Not to say that there won't be overlap, there will be alot, but if you map out the frequency spectrum based on where your different instruments really shine, they stop fighting with each other, and where they do, you know where you can start cutting frequencies out to make room.
Worst case scenario, when two instruments are fighting each other, you can always solo them out to try and make them play nice, isolate where their overlap is, and mold them together. However, it's always a good idea to not spend too much time with anything solo'd. You start to have the tendency to create a great sound for the solo'd instrument and forget about the mix, then when you switch back, you've changed the whole landscape and you have to start over again.
Alright, so what if the problem is just one instrument fitting into the mix? The first thing to do is to invert the phase of that instrument in the mix and see what happens. It might just be a sonic revelation! I hate using exclamation points, but when you hear a phase problem pop out at you, it really can be eye-opening (or ear-opening I suppose). Most audio sequencers will have an invert-phase plugin you can put on a channel, otherwise, if you're still in the recording phase, you can try to invert phase by keeping a few mic cables around that are wired backwards. Make it a weekend project, so you have them around when you need them in a pinch. Just swap the hot and cold wires on one end of the cable... easy to do with some cable ends and a soldering iron (what?!? you don't have a soldering iron? how long have you been running a home studio?).
One last scenario in this long winded post... and really this is what motivated me to write this in the first place this morning. When you're recording an instrument and the sound you get on the recording just doesn't line up with the way you hear the instrument. In this case, it's time to start thinking about mic placement (which you've already thought of right? that should be priority #1, but that's another article too). So, it is possible to have phase cancellation using one microphone recording one sound source. This is particularly true with instruments that produce purer notes and waveforms (say, violin) than more complex ones (distorted electric guitar), but its another thing to add to your toolkit, nonetheless. If you try to imagine a sound as the physical waveform that it really is you can see where overlapping waves from the same sound source can produce phase cancellation at the right distances. There's a funny thread over at the sweetwater forums that talks about a specific scenario with a violin like this, combining reflections and direct sound at particular wavelengths. The nice thing about that thread is the note at the end from a proffessional engineer reminding us to not get too caught up in technical details, which is full circle for this post. The bottom line is: move the microphone. Don't try to fix it in the mix. Keep moving the microphone until you have the sound you want. Microphone selection, but really microphone placement is the holy grail first important step of a good recording. No amount of great eq'ing or fancy preamps and effects will do you better than moving the microphone around and getting used to really listening to things and how sounds change.
So, basically if you're letting all this information turn into soup in your head like I did, you might be curious about the physical wavelengths of different frequencies... I stumbled across this wavelength calculator. I want to reiterate not to think about this stuff too much when you're actually working, trust your ears, but its nice to have some idea of the theory, and kind of cool to think about the lengths of different sounds. Just remember, there's no secret sauce.
What is the boogeyman's worst enemy? Mic placement. Know how to position your mics and experiment with moving them around in relation to each other and to your sound source and you can solve almost any problem. Some of the other tricks I've mentioned will still be super useful, but the fundamental is always where you put the mic and how the thing sounds in the room. Concentrate on what you hear and where you put things in the room... don't pull out the big complicated words if you can fix things by moving a mic.
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