This is probably one of those dumb questions but...
 
Can reflections in a small room cause a single mic to record a signal that is out of phase with itself?  Here's why I ask the question.  When I play through my amp live I hear a nice strong midrange, a nice high end and low end.  Everything is there and it sounds great.  However, when I record the amp I get a strong high and low end but the mid just seems to disappear.  The mids aren't nearly as strong in the recorded track as they are live. 

Yes they can. However, I would look to the mic pattern first, its tonal quality second, before assuming destructive interference.

If you're close to the amp, the phase issues (which will be mainly comb filtering) should be pretty negligible due to ratio of source to reflections. Things that make it worse:

 - if you're close to a reflective surface such as a wall or hard concrete floor (or any floor)
 - the further away your mic is from the amp
 - the polar pattern of your mic - an omni is going to pick up more room and maybe more phase detrimental reflections
 - etc

If this is the case, a bit of experimentation should see the issue rectified.

I think it's 1ft is 1 ms delay. Comb filtering can occur over a wide range of delays. It's frequency dependent too. Around 1-2ms will really get the top end, and as you increase the delay, you'll hit midrange and finally the low range. Couldn't tell you off the top of my head what delays will do it. So any wall could theoretically affect you noticeably from a pretty decent distance. Now it all comes down to signal to reflection ratio. Fix that and you'll fix your issue - if that's the cause.

More likely though, I'd say it's due to your mic positioning and/or choice of mic.

Any microphone in any room is going to pick up some out-of-phase reflections. Always. 

However, if the direct sound is significantly louder than the reflected sound, it dominates and the reflections become insignificant. That's why you put the mic right up to the grill, or an inch from the tom tom, or just in front of (or inside) the kick drum. It's why you set a small amp on a chair to get it off the floor. And it's why the 3:1 rule exists for stereo pairs. And it's the reason you don't stand in front of a window while singing.

bitflipper


...And it's the reason you don't stand in front of a window while singing.

I thought that was if you were naked.

marcos69

bitflipper


...And it's the reason you don't stand in front of a window while singing.

I thought that was if you were naked.

 
'Made me smile.

bitflipper


Any microphone in any room is going to pick up some out-of-phase reflections. Always. 

However, if the direct sound is significantly louder than the reflected sound, it dominates and the reflections become insignificant. That's why you put the mic right up to the grill, or an inch from the tom tom, or just in front of (or inside) the kick drum. It's why you set a small amp on a chair to get it off the floor. And it's why the 3:1 rule exists for stereo pairs. And it's the reason you don't stand in front of a window while singing.

OK...well, I have the mic (sm57 or Senn 421) about 1" off the grill.  Even at 1" there's a heap of low end proximity effect.  The room is a converted attic space.  It's about 8x10 with slated ceiling (8' tapering to 5') and one of the walls is a cut out for the dormer (window.)  So, it's very small and irregular shaped.
When sweeping the EQ I've discovered that there's a huge 150hz buildup.  Slicing that out with a parametric helps but I'm thinking about just using a hi-pass set at 150hz.  I'm also experimenting with using acoustic panels to isolate the speaker a little bit from the rest of the room.  ‘Any thoughts?

That's not a phasing issue, it's one of resonance. You can't stop it in a room that small, but you should be able to at least partially mitigate it. You could build an isolation booth for the amp with gobs of absorption, but a more cost-effective first step would be moving the amp and mic around to different spots in the room and making test recordings of white noise played through the amp at each trial location.

bitflipper


That's not a phasing issue, it's one of resonance. You can't stop it in a room that small, but you should be able to at least partially mitigate it. You could build an isolation booth for the amp with gobs of absorption, but a more cost-effective first step would be moving the amp and mic around to different spots in the room and making test recordings of white noise played through the amp at each trial location.

How, exactly, would I conduct the white noise test?

Me, I don't use a room mic for low end and I will roll it all off on that track.  I use it simply to define the space by panning it opposite of the source.  In fact, I go so far as to delay it even further by putting a delay in full wet mode in the FX bin on the room mic.  Every 10 ms is like an extra ten feet of distance from the source to the boundary.

Sometimes I turn my room mic off axis of the source and try to catch a reflection off a wall to get more delay.  It's one of those things you have to experiment with and find out what works and what doesn't.

Remember that just because you recorded something, you not forced to use it.