This article is on using a close mic on the guitar amp and one or more room mics. You may wish to use more than one close-mic on the guitar amp in addition to a room mic or two, so simply adapt as needed.
As with this multiple mic series, I suggest reviewing the archived article on Miking The Guitar Amp (Part 3) since so much important physical set up information will not be covered here.
Before we get into detail, there is one most important thing to remember: You can always add ambience to any sound source at any time as long as it was recorded close-miked (or direct) but you can't make a recorded ambient signal sound close. There are many ways to get a close-miked signal to sound roomy using reverbs with short ambient room programs, but there are no gadgets to remove the room reverb from a track recorded with too much ambience. Even so, every room has a sound of its own and is worth checking out.
(BTW, you can send any recorded track back through an amp and speaker(s) in a live room and then distant mic to get a room sound, and then record to an open track. This is an article on its own I will get to in the near future.)
Basically, this is a guitar overdub situation so all we will be recording is a guitar part. The exception is a studio that has a few rooms that are totally isolated and the guitar amp is in one room during a tracking session. This is pro stuff I'm talking about, you get the drift. OK, another exception is recording a band live in a big studio and using room mics. The latter is another article at some point in time.
Important! When using a room mic(s) for guitar amps, I usually set the amp a few feet off the floor on a riser, chair or anything stable that will not physically ring. I do this even if not using room mics to avoid low-end frequency build-ups from the amp sitting on the floor. There are no rules as usual so if you like the way the amp sounds with the speaker bottom on the floor, no problem, since the low end room rumble can be rolled out when EQing the room mic.
Typical Room Designs and their Acoustic Characteristics
Dead Room: Carpet floor, walls covered in cloth or a non-reflective surface and a ceiling that is also covered in cloth or non-reflective surface. In this case, the room mic will only act like a short delay line since the room is adding no reflections.
Slightly Open Sounding Room: Wood floors, walls covered with material, and a ceiling that is plaster with either some baffling or a ceiling that uses wood panels. The room sound will be bouncing off the floor and ceiling.
Typical Open Sounding Room: Wood floors, walls that have wood reflectors, and a ceiling that is plaster with either some baffling or a ceiling that uses wood panels. Now the room will give you more reflections off the walls.
Total Open Sounding Room: Cement and or plaster for all surfaces. This is an extremely bright sounding room with serious reflections. Such a room is similar to echo chambers in studios designed in the 50s and 60s BUT a room like this for recording is much bigger than a small echo chamber.
There are rooms that fall in-between the above designs and there are other materials that can be used. We will get back to the rooms as things unfold.
Room Mic Choices
Regarding mics used for room mics, this is a new ball game. Most rooms sound mid-rangy with the exception of the cement/plaster room, which is brighter. Condenser mics are the typical choice so as to get some high-end treble information be careful if the signal is very loud so as to not damage the mic diaphragm. If you have PZM mics, try taping these on reflective surfaces like glass or hard wood floors or walls. These mics like a hard surface. So dig, each room sounds different. Try every mic you can get your hands on to find out what works best for the room you are working in.
Note that most mics sound duller (more mid-rangy) the farther they get placed away from the sound source. There is one mic I know of that is designed to be a room mic and when placed far away from the sound source, the sound remains bright! This gem is the NEUMAN M 50 (nickel capsule is best). The drag is, even if you can find the mic for sale, the price will be at least 15 grand and probably more. A few major pro studios have these mics and I have used them for orchestral sessions. Theyre truly amazing! I have never tried using them for guitar but I am sure the sound would be amazing.
OK, back to reality. For mics that offer capsule patterns, you might think that this would be a good time to use the OMNI pattern, meaning the mic would hear 360 degrees. Yes, that should work, but in my case, my slightly open sounding room likes the cardioid pattern most of the time (the mic only hears from the front of the capsule). There are no rules so if the mic offers patterns, try using all mic patterns to find out what sounds best for the room you are working in. The typical selectable patterns are OMNI 360 degrees, FIGURE 8 (front and back are open but the sides are not active) and Cardiod (the front is all that is hearing the sound source.
Comb Filtering and a Math Concept
The deal here is the studio needs to be a room that is at least around 12 feet in at least one dimension (and bigger is better to a point). As we know, sound travels at approximately a millisecond per foot. If you're using a room that is 12 feet long, and if the guitar amp speaker was at one end and the room mic on the other end, that is about a 10 foot spread. So this equals around a 10-millisecond delay. You would think the comb-filtering effect is in play since it takes around 30 milliseconds to notice a distinct delay (as in, to get out of comb filtering land and into the range of distinct delays, which occurs at approximately 30 milliseconds). Technically, you should have comb filtering in this case, but the odds are good it will not be noticeable in a room that is at least slightly open sounding, since the tone totally changes compared to the close mic for a few reasons. The room mic waveform becomes defused due to air movement in the room as well as room reflections coming into play. The reflections bouncing around will arrive at many different delay times and the later the reflection, the darker the sound.
Note that if you are hearing some comb filtering cancellation in a small room between the close mic and room mic, you can always delay the room mic using a delay line. So lets use 30 milliseconds for the math base a guitar amp that is room-miked 10 feet away will have a natural delay of 10 milliseconds, and may require a delay of 20 milliseconds. Experiment as usual looking for a great sound.
Room Mic Placement
With any of the room designs, the basic concept is to start by placing the room mic about 10 to 15 feet away from the amp speakers. The idea is to set the mic height so as to look directly towards the center of the speaker. If youre using a speaker cabinet with more than one speaker, face the mic towards the center of all the speakers. The reason to face the mic looking at the speakers is to get more upper frequencies. If the mic is not looking at the speaker(s), typically, the sound will be more mid-rangy. As mentioned above, room mics will sound mid-rangy unless the reflective surfaces are cement or plaster.
Back to the rooms for more definition:
Dead Room: Carpet floor, walls covered in cloth or a non-reflective surface and a ceiling that is also covered in cloth or non-reflective surface. In this case, the room mic will only act like a short delay line since there are no reflections. Let's say the room is small. Position the amp at one end of the room (leave about a foot near the wall if youre using an open-backed speaker cabinet so the amp can breathe) and position the mic at the other end of the room looking directly at the amp speaker cone. The sound would be kind of soft. Note if the room is around 25 feet or longer, you may want to shorten up the distance if the sound is too soft.
Slightly Open Sounding Room: Wood floors, walls covered with material, and a ceiling that is plaster with either some baffling or a ceiling that uses wood panels. The room sound will be bouncing off the floor and ceiling. So what is the room size? If its small, put the amp at one end of the room (leave about a foot near the wall if a open back speaker cabinet so the amp can breathe) and set the room mic on the other end. In this case, you might try positioning the mic up about 6 feet but still looking towards the amp speaker. This will add longer floor reflection delay. You might also put the amp/speaker(s) on a chair and angle up towards the ceiling for more of the ceiling effect. The sound will surely have mid-range characteristics. Note that if the ceiling is plaster, there may be a ringing sound that is not desirable. You need to hang baffle boxes that are filled with fiberglass or install plywood reflectors.
Typical Open Sounding Room: Wood floors, walls that have wood reflectors, and a ceiling that is plaster with either some baffling or a ceiling that uses wood panels. Now the room will give you more reflections off the walls. This opens up more possibilities for mic possibilities so start with the procedure in the above paragraph and experiment with mic positions. This room sound will be mid-rangy as usual but should be slightly brighter than the above.
Total Open Sounding Room: Cement and or plaster for all surfaces. This is an extremely bright sounding room with serious reflections. Such a room is very bright sounding and is similar to echo chambers in studios designed in the 50s and 60s. This is an echo chamber so the deal here is if you want that sound, mic close to a corner of the room. OK, if you want a more up front sound, mic as in the above paragraph.
There are rooms that fall in-between the above designs but you get the drift. The above miking positions are typical and for now are the best way to mic the room. You need a 2nd engineer (helper) to slowly move the mic around the room as the guitarist plays the part for the song (or all over the neck). The 2nd engineer will be wearing headphones so you can communicate (as well as to not blast out the 2nd if the guitar is high in level). As the mic is moved in all directions slowly, when you hear a good sonic spot, tell the 2nd to stop and mark his feet using artist tape as well as noting the height and angle. The hunt continues and in time you will find the best spot for the mic placement.
Other than a dead room, there are so many other ways to look at this situation since the room has tons of places to pick off reflections using other mics or a totally different concept. When my recording technique book writing partner Craig Anderton had a discussion site on AOL, I was posting about room mics and a site visitor posted some interesting stuff: The guy mentioned running white noise through the guitar amp. The 2nd engineer would walk abound the room moving the room mic around while the engineer looked for the highest level point on the recorder meter. The idea is that white noise has all frequencies and when seeing the most level, that was the point of the most room build up.
The bottom line is there are no rules here for sure. You may like a sound that is very dark, such as miking a wood wall looking at a different direction in regard to the amp.
Using More than One Room Mic
If you're using two room mics, the positioning possibilities are endless so mic position experimentation is most important. Here are three basics concepts:
1. As with one mic, our basic starting concept is we positioned in line of sight towards the guitar amp speaker so as to hear more upper frequencies. For the second room mic, position behind the first mic, back at least 5 feet or so. This way, both mics are in line of sight towards the speaker.
Even though the mics are not split to the sides of the room, meaning both spread apart left and right from the amp speakers, there will definitely be a spread when panned on account of separate delay times and reflection times.
2. You could position the mics in a triangular setting from the amp speaker(s). Try about 10 to 15 feet back as a starting place (one mic 5 feet left of the amp speaker(s) and the other mic 5 feet to the right of the amp speakers and both 10 or 15 feet back). Face both mics towards the amp speakers.
This triangular shape positioning will sound more mid-rangy the farther away the mics are positioned from the speaker cones, but this sound may be what you are looking for.
3. With the mics next to each other around 10 to 15 feet back from the amp speaker(s) looking towards the speaker in the same fashion as the mono mic set up, turn each mic about 22 degrees away from each other (so they are no longer looking straight at the speaker cone but are now pointed off to either side of the speakers). This will allow for some of the directional sound from the amp speaker(s) as well as acting like how the human ears hear the sound.