I considered beginning with the question of "what should I record?" but then I figured that the answer to that would be way broader than the scope of this series of articles or even of this blog, so instead I'll assume that you've decided what you're going to record and are now on to the next step of the recording thought process:
"How many channels should I use?"
The answer here, as in all of the questions is "it depends," so let's look into exactly what it depends upon, and what factors will move us in one direction or another.
In any recording situation you're limited to the number of channels and mics that you have available, so if your only recorder is a zoom H4n then you're pretty much locked into a max of 4 channels and two of the mics are built into the thing. If you have a larger project and any budget at all, you can start to get into gear rentals - which can broaden your recordings considerably. The trick is to use your resources efficiently, and not to rent/use channels that won't serve a specific purpose in your recordings.
All of this is to say that you may have in your mind a reasonable number that is the max number of channels you can record this sound with. If the answer to the question of how many channels isn't immediately "all of the channels that I have or could rent" then you have to approach this question with a little more thought.
- Practical considerations:
Adding channels increases the amount of time that it takes to setup, troubleshoot, monitor, record, organize, sort, edit, master and catalog anything. As such, the two most pressing time issues are subject availability and deadlines.
This means that the closer your deadline the more likely you should be to go with fewer channels, since time spent sorting and editing recordings that are only moderately useful or moderately different can be a detriment to the work you do on the other primary tracks. Also, this means that the narrower your window of subject availability, the more likely you should be to go with fewer channels.
An example where you'd be able to go nuts with all the mics and channels you have access to would be if you're recording your own vehicle and have an entire weekend to do it, and no pressing project deadline to hit. An example where you'd probably only bring one or two channels of audio based purely on time considerations would be if you have a 30 minute window of availability with an exotic animal for use in a project due tomorrow.
- subject dynamic range
The vast majority of subjects to be recorded have dynamics that fall within the recording capabilities of a wide variety of mics. Things like human voices, dogs, acoustic guitars, business machines, gas powered portable generators and anything in between can be recorded beautifully with 3 or fewer channels.
There are certain subjects, though, whose dynamics may require as many channels as you can muster in order to fully express what they sound like in a recording. Highly transient subjects like weapons, explosions, baseball bat hits and metal crashes can require a lot of channels to properly record all of the various elements of the noises that they make. This is both because playback systems only have so much dynamic range with which to use and also because highly dynamic sounds tend to be defined by how they sound in the spaces in which they exist. This means that that you'll often want mics up just to catch the reverberation in space in addition to the various parts of the sound that's causing them.
The rule of thumb is that the more dynamic (and loud) a subject is, the more channels you'll need to get the type of coverage that works.
- motion of the subject
All of sound is motion, but some subjects move while creating their noises more than others. If a subject is sitting static in space you're more likely to get a good recording of it with fewer mics than if it's hurtling through space in front of you.
If something is zooming in front of me I tend to want both onboard and exterior stationary channels to cover the various perspectives of all of the noises being made. Objects in motion can tend to double track counts.
If you don't want to be seen recording, you're probably stuck in stereo or mono based on the fact that you'll need to hide both the mics and the recorder on your person.
- setup and mobility logistics
Bringing more channels means bringing more of everything - mic stands, mic mounts, wind protection, cables, cases, etc. All of this extra physical, mental and logistical baggage need pretty specific justification once you get past about 4 channels. Big multichannel shoots also tend to require setup and staging areas. If you lack a setup area having 12 channels of recording may be out of the question.
When I recorded a printing press recently, I ended up bringing more channels than I used due to setup constraints in the facility. I just didn't have a safe staging area and had to leave a fair amount of gear unpacked both for that reason and for time considerations. I also was faced with a vast warehouse of crazy noisemaking machines and I opted for breadth rather than depth of recordings. This meant that I had to be mobile, and while repeatedly moving and setting up 3 mics is doable, any more than that and I would have missed the chance to record entire machines.
The more restrictive your recording environment is to big shoot logistics, the more likely you should be to roll with fewer channels.
- width of the subject's sound creating parts
Certain subjects are pretty big and can have different noises coming out of all different parts. Things like industrial machinery and vehicles can require many channels to get all of their sounds, even when they're stationary.
The more spread out the physical soundmaking elements of the subject are, the more likely you should be to roll more channels.
Rolling more channels reduces the risk of failure if one of the channels doesn't get what you need. This is not to say that you should roll multiple channels of the same perspective for the purpose of redundancy, but rather to say that if you roll a lot of different perspectives, having one or two channels give you nothing is not a tragedy. Getting zero useful stuff from a channel happens all the time in complex vehicle and weapon shoots.
- Artistic considerations:
If you're out recording a vehicle for use in a film, then a lot of the context will be defined by the cut of the picture you're working with. Video games are a little more open ended, but they're still pretty well defined.
This means that if you're only ever going to see that aston martin in a few wide shots cruising down the street then you probably don't need to bring a hundred mics to the shoot. Focus on what you need to get for the project, and then get any extra goodies afterward.
- reality vs hyper reality
The more you're looking to get a realistic image of a subject, the more likely you're going to be to use a small number of mics to do the job. Again referring to context, this is because the specific reality you're looking to capture is already likely to be very well defined, and you're not looking for a lot of contingency perspectives.
The more hyper-real your aesthetic takes you, the more like you are to need many more channels of audio, because the process of creating hyper real sonic images involves isolating and accentuating the individual elements of a given sound, meaning that you'll need as much distinct isolated coverage as you can get.
The process of deciding just how many channels to bring to a shoot can be a complex and interesting one, as is every decision one makes when deciding to record something. Thoughtfully planning out this step of the process is an important part of getting great recordings.