I don't know why these sorts of topics always seem to generate argument, but it is a necessary topic to discuss anyway.  We'll just have to give it a try and hope any debate doesn't devolve into.. oh darn, what is that expression?  You know, when a whole flock of chickens are all running around raising a fuss?  Ah yes.. a fluster-cluck.  I'm pretty sure that's it.

I ran across something that just might help some folks.  Friend of mine was over and we were working a bit on a song he wants to learn.  He hasn't been playing a long time, but he has a pretty good ear.  He was having a "bad intonation day" though and was being a bit frustrated with it.  It was inconsistent.  He'd play the notes, going slow, and be right in pitch sometimes and other times almost everything would be a bit off.

If I was backing him up, he was fine, but playing solo, it kept wandering a bit.  I had a crazy notion and took him to another room and he did better.  

Theory:  I think what was throwing him off was the sound from the refrigerator motor running.  Like most appliances and noises in the world, it isn't on a musical pitch.  At a guess, I'd say it's between a Bb and a B with a sort of throb that feels like maybe 10 Hz or so to it.  I think there may have been a confusion resulting from sort of trying to play in tune with that and the memory of where the right place for his fingers should feel like on the fingerboard.  He kept missing notes by a little and then overcorrecting and etc.

So I tried a little experiment.  I took my mp4/mp3 player where I have scales and drones recorded for practice, loaded up an A 220 drone (the piece we were working on was in Am), then set the volume on one earbud to just loud enough to be definitely louder than the refrigerator.  Had him put it in one ear and try playing.  Bang.  He did fine, and when he did miss by a little, he'd hear it and do the note over or just "lean it in" if it was very close. 

I think that some of the problem that some folks may have when practising/playing by themselves may be environmental noise/sound, throwing off their pitch. 

I do not have perfect pitch, and I can't say how pitch recognition works for other people.. But what I have is a fairly good relative pitch.  How that works for me is I can remember sounds/notes.  I can compare a note being played to my memory of a note I've heard played on an in-tune instrument or other pitch source.  That's how I personally can tell if a note on an instrument is in-tune or not.  With my violins, for example, when I pick them up, I'll very lightly strum all 4 strings.  Since I know how it should sound when it is in tune, I know if I need to tune up, and about how far out it is.  It isn't "perfect pitch" though, since I can be thrown off pitch if I have to play with someone who is out of tune or if I'm around a sound (like a computer fan) for a long time that is "off" from a musical pitch.  People with actual perfect pitch, I'm not sure how it works, but they don't get thrown off by such things.  I've known a couple such people over the years, and like probably a lot of folks, I definitely tried to see if they could be thrown off.  And no, they couldn't.    

But anyway, maybe not everyone can remember pitches steady enough to be able to tune against them.  But a drone note playing quietly, just loud enough to cover computer fans, air conditioners, refrigerator motors, etc.. It might be enough to help. 

I use drones in practise to play scales and melodies against, particularly melodies where I haven't worked up a backing track for them yet.  It might be helpful for learning to recognize by ear the intervals needed for a particular melody.  I'm not sure on that, since I learned what intervals sound like long enough ago that the memory of it is a bit hazy.  I also learned originally on guitars and keyboards, where intonation is an adjustment made to get the instruments right, not a skill to learn.

But on the chance that this idea might be helpful to anyone or someone might have some good insights on it, I'm posting it.  It is a tantalizing notion that the problem that at least some folks have with learning to be able to hear if they are playing in tune might be as simple as environmental sound throwing their ear off kilter.  If that does happen, and such noise producing devices probably aren't on all the time, or one might be playing closer to one on one occasion, and closer to another on another occasion, it could be causing an inconsistency that would make it harder to learn to play in tune in the first place.

Just a thought.