Micing guitar cabs with 2 mics

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Goreliscious

New member
It occured to me today that I could mic my guitarist's amp with 2 mics to try and get the up close crunch as well as the further away beefier sound...it's raised a couple phase cancellation questions in my head:

If I follow the 3:1 rule, can I put one mic behind the other or have they got to be parallel?

Secondly, it's a 4 x 12 cab so there will be bleed from the other cones - should I mic the same cone with both mics or should I for example mic the top right cone at a distance of 1 inch and the bottom left cone at a distance of 3 inches?

Any help would be much appreciated.
 
Phase interactions are also affected by relative levels. The signals have to be at the same level for the phase interactions to be their worst. If one is 6dB or so lower than the other then cancellation is mostly eliminated.

If the room doesn't suck I like one mic out anywhere from 5-12' out from the cabinet in addition to the close mic. I use the far mic just to give a bit of space to the guitar by panning it to the opposite side, but not necessarily in a mirror image. That is, I'll use the close mic as the main "obvious" sound of the guitar, say 30% left (or whatever), then put the far mic 80% right and bring it up just enough to spread the guitar sound a little. It may be necessary to tweak levels, eq and panning to get just the right effect. I prefer a condenser or ribbon for the far mic.
 
Definitely something to try. Just check the phase and you'll be in for some interesting tones! Its exactly why AmpliTube 3 has 2 mics that you can mix and match.
 
bouldersoundguy said:
Phase interactions are also affected by relative levels. The signals have to be at the same level for the phase interactions to be their worst. If one is 6dB or so lower than the other then cancellation is mostly eliminated.
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So you're saying for example the close mic should be at 0dB and the distant mic to be -6dB? I.E I shouldn't up the gain on the distant mic to make both mics recieve 0dB?
 
Uh..... you don't want either mic peaking at 0dBFS. Nothing sounds worse than digital clipping.

More like -18dB. Record your track too hot and you don't have any headroom left for processing or mixdown.
 
c7sus said:
Uh..... you don't want either mic peaking at 0dBFS. Nothing sounds worse than digital clipping.

More like -18dB. Record your track too hot and you don't have any headroom left for processing or mixdown.
Click to expand...

I thought the idea with everything was to let in as much signal as possible before peaking, so -1dB is the target?
 
Goreliscious said:
I thought the idea with everything was to let in as much signal as possible before peaking, so -1dB is the target?
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I think that's analog levels. Digital is much lower ( e.g. -18dB ) due to the noise floor and other such technical things I don't spend a lot of time researching...
 
Cheers for that. I read it and didn't have a clue what he was going on about for the most part but the idiots summaries helped.

If I record at -18dB like they suggest, will my recordings sound 'woollier'? With my snare mic for example, does it mean that I'll get more of the room in and the snare will have less presence?

Can you do me a massive favour and check this link to my Korg recording gear and tell me if this -18dB level definitely applies to me? There were words in the article like 'buss' which I've never heard before. It's the most basic hard disk recorder I could find!

KORG D888
 
It's all really relative to your situation, whether you mic on the the same cone, close or far, and which mics you use should all be based on what you are trying to capture.

If you are after a beefy distorted sound I would suggest using an sm57 and a sennheiser MD421, have the sm57 pointing straight on around the edge/middle of the cone (not at the dust cap) have the 421 pointing just to the right of the dust cap on a 45 degree angle. These two together should give you a solid beefy sound. (The 421 sounds pretty rubbish on it's own but mixed with the 57 sounds great in this setup)

There are a few tricks to check your phase as well.

1. Record some mutes with a clean tone into your daw (not palm mutes, just full mutes with your hand over the strings) Zoom in and compare the waveforms phase relationship.

2. Unplug your guitar but leave your amp on and the lead still plugged into your amp (it needs to buzz), put some headphones on, input enable the 2 channels, put one channel out of phase, move the mics around until phase cancellation is at its greatest (when it is at it's quietest). Don't forget to invert the phase again.
 
Goreliscious said:
If I follow the 3:1 rule, can I put one mic behind the other or have they got to be parallel?

.
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The 3:1 rule is for multiple sources to minimize bleed.

Experiment with mic placement as a way to find the sound you are looking for. Listen from the control room while someone is moving the mics around for you. Listen move, listen move etc until you hear what you want to hear.
 
NYMorningstar said:
Listen from the control room
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Haha, good one. I'm a n00b with an 8 track mate. My "control room" would be next door in the bathroom...if the headphone lead is long enough.

So in short I won't be creating phasing issues regardless of whether I mic 1 cone with both mics or do a cone per mic as long as I follow the 3:1 rule? And because I'm using a digital recorder I shouldn't go above -12dB with any instrument?
 
Goreliscious said:
So you're saying for example the close mic should be at 0dB and the distant mic to be -6dB? I.E I shouldn't up the gain on the distant mic to make both mics recieve 0dB?
Click to expand...

Record them both at a normal level, like -12dBFS peak. It's in the mixing phase (heh) that you want to have one lower than the other. That works out pretty well because you'll probably want the far mic lower anyway for a cool ambient effect.
 
Hello again - someone on another forum told me that this whole -18dB thing only applies to 24 bit digital recorders and 16 bit digital recorders should be used more like analogue recorders and record hot...anyone agree / disagree? I'm none the wiser. My basic Korg D888 is a 16 bit unit.
 
Goreliscious said:
Hello again - someone on another forum told me that this whole -18dB thing only applies to 24 bit digital recorders and 16 bit digital recorders should be used more like analogue recorders and record hot...anyone agree / disagree? I'm none the wiser. My basic Korg D888 is a 16 bit unit.
Click to expand...

16 bit audio has a lower S/N ratio, so there's some reason for running a bit hotter, but you still need headroom. Going over is still just as bad. I recall that the meters on many 16 bit units read 3dB low, so even hitting 0 on the meter meant you had another 3dB of headroom.
 
Goreliscious said:
..So in short I won't be creating phasing issues regardless of whether I mic 1 cone with both mics or do a cone per mic as long as I follow the 3:1 rule? And because I'm using a digital recorder I shouldn't go above -12dB with any instrument?
Click to expand...
Mics picking up a sound from different distances means one is delayed. It's the delay that causes the tone change when they are combined due to the phase being misaligned at many frequencies.
Mics at equal distances = in phase for the most part. Yes there is 'errors, side-to-side bleed included, but so what? You put them somewhere they sound good to you. The reasons why are good to know but mainly so you can make smarter guesses. :p

The whole point of 3:1 is to use distance to attenuate unwanted bleed between two mics, picking up two different things. This doesn't apply at all since you (may) want to mix these together at any level you please later!

You should experiment with the out of phase tones of different distances to get a feel for this. The effect is very much the same as 'flangers and 'phasers. As the time difference increases from 'zero, starting with the highs, the effected frequencies get lower and lower. The 'effect is deepest when the two are at equal volume.

As you move the 2nd mic far enough away from the direct sound and the room reflections become strongest, the 'phase effect diminishes and it starts sounding like a room mic.
Why
Because now the mic is picking up all sorts of different reflections coming in at different times. You get 'smooth and mooshy instead of a few deep holes and peaks. :)
 
Recording with 24 bits and peaking 18 dB below full scale ~ recording with 21 bits.
Recording with 16 bits and peaking 18 dB below full scale ~ recording with 13 bits.

That's potentially a significant difference.

Two recordings with different peak levels (so long as both are below 0 dBFS) will differ
- in the digital realm: not at all, except that quantization noise will be 1 dB higher for every dB you're under full scale;
- in the analog realm (i.e. everything between the output jack of whatever's producing your sound and the string of data that your converter makes out of that signal): totally depends on your equipment, including your converter, and how well it's designed and built.

Lowering the signal going into your converter will raise the noise that's present in the analog signal path, relative to the signal. It might possibly lower distortion, though that depends entirely on your equipment. Whether anything will be more or less "wooly" depends on what "wooly" means, and possibly the presence or absence of sweaters or actual sheep.

You'll never have a problem with being short of additional bits for processing and mixing: you can always reduce resolution; you just can't increase it. Besides, the processors are probably working with 32 bits.
 
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bouldersoundguy said:
I recall that the meters on many 16 bit units read 3dB low, so even hitting 0 on the meter meant you had another 3dB of headroom.
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Yeah I've noticed that even when the meter reads as full, the red light on that track doesn't light up.

Thanks for the reply sjjohnston, I appreciate your effort but my knowledge is really limited and I didn't understand a lot of it. Don't worry about trying to explain it again, though I'd really appreciate it if you guys could suggest a dB level to record at considering I have 16 bit 8 track - regardless of whether I understand the theory behind it, I'll understand a number!
 
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