Likely, the most subjective microphone placement in all of recording is the room mic for a drum set (or perhaps a room mic for any instrument setup for that matter).  As with any recording, the sound concept you have in mind should lead the decision making process.  In other words, what is the intent?  Are you seeking a huge cavernous drum sound for your material or something tight and punchy?  Are you looking to use the room mic to add realism to your recording or are you looking to create an ‘effect’?  What kind of material are you referencing to inform your sound?  Above all, start with these considerations and once you have determined those answers, it is time to consider what is available to you in order to achieve the sound of your dreams.

Cavernous Drums

This one is tricky because a larger room with a high ceiling is needed.  A good commercial studio facility will usually have the appropriate space (and some certainly sound a lot better than others of course), however there are few home studios that will fit the bill (unless you live in some sort of mansion or repurposed you’re a-Frame great room (much to the dismay of your co-inhabitants).  The trouble with the typical room in a residential home studio (often a garage) are the low, flat ceilings and the square room design’s parallel walls.  A flat ceiling can be mitigated with height.  Parallel walls can also be mitigated to a degree with size (ie distance between the walls), but unless an acoustician expertly treats the room, it will still be problematic.  The goal is to achieve a roomy sound without actually hearing the room itself.  Pop in a Led Zeppelin album and you will hear a fantastic example of an expertly recorded cavernous drum sound (it helps to have Bonham behind a Vistalite kit with a 26-inch bass drum of course). 

Let’s assume you’ve got the golden room (with your favorite room mic) and it is simply about picking a placement.  It helps to think geometrically as sound has measurable speed and reflection behaviors, but it helps more to consider where things sound best to you.  Most of us are not acousticians and at the end of the day, it is art.  Every room sounds different, as does every drum set and every drummer.  Keeping in mind the microphone is classically designed to mimic the inner workings of the human ear, you can rely on a good condenser to replicate the exact sound you are hearing when standing in any given area of your room.  Find the spot that sounds good to you, and put the microphone there.  It is actually that simple.

Tight and Punchy

For these applications, you are a lot more reliant on your close mic setup and your overheads.  The tightness and punchiness of your drum sound is primarily achieved with skillful dynamic processing and is also largely dependent on the arrangement of your program material.  Do you still need a good room?  Absolutely.  However, it is less crucial than what is necessary for a more ‘roomy’ drum sound.  Even so, a room mic can still pay dividends, even when playing a smaller part in your overall mix.  You may not use the room mic at all.  That said, if you have the means (ie the inputs and the awesome microphone) record it. 

The same placement philosophy applies.  Where does the room sound best when your drummer is playing?  Stash the microphone in that spot and don’t be afraid to make a position change if you are not satisfied.  Laziness doesn’t pay when it comes to recording.  Fixing it in the mix is far more challenging than getting it right in the early stages.

Room Mic Effect     

In some cases, a recorded room mic can provide an artistic opportunity to add some unconventional effects to the overall drum sound as well as influencing the finished mix as a whole.  The important thing here is to record the room mic, regardless of room size, so you have this option at your disposal.     For example, applying extreme compression to the room mic channel, while removing the mid-range frequencies (ie everything from 300hz to 5K) and boosting the lows and highs can yield some pretty cool results.  Mix this channel into your overall drum mix and notice the distortion-like effects.  This is just one of an infinite number of room mic effects you can create.  In addition, depending on the distance of your room mic placement, you will hear a natural delay against the close microphones surrounding the drum set.  Sound has a defined speed of travel.  The further microphone from the drum set, the longer it will take the sound to get there.  The delay becomes audible (and visible if looking at the transients in the waveforms) pretty quickly.  The technique of simply mixing in the room mic sound will create realism in your drum mix, due to the natural delay created by the distance miking (similar to reverb).  Engineers will remove this delay by aligning the transients against a close microphone (such as a kick drum mic) to achieve a tighter sound.  Experiment with this as well.    

One Room Mic or Two?

Mono or stereo room mic considerations are purely subjective.  Keep your room size in mind when making your decision.  Also consider you are likely using two overheads.  If you’ve been recording drums, you are probably familiar with the phase relationship challenges in a multiple microphone environment.  Using two room mics will add to those challenges.  If wider stereo imaging is your goal, and you have the room size and contour to support, experiment with two room mics.  Be sure to review your room mic sound (as well as overheads and overall drum sound) in mono to check for phase cancellation issues.  If frequencies are disappearing from the drum mix when you listen in mono - that is your signal to adjust the placements of your microphones.  If punchiness is a priority, stick with a mono room mic.  These of course are suggestions, not rules.  In many cases, you will achieve a lot more with one room mic and two overheads, rather than two room mics and two overheads.  It is really within the ears of the beholder.  Trust yourself and enjoy what you’ve created.