When tuning a guitar, what is the best way to strike a string to get it as close to pitch as possible? I'd like to get it as close to the right pitch as I possibly can. I've noticed with my tuner (it's an iPhone app) that the pitch isn't perfectly stable, but tends to fall down 5-10 cents after the initial attack of the note. (I'm not sure whether this is instrument error or a physical effect.)
All stringed instruments, to my knowledge, do this; the note starts out sharp, and stabilizes after a half-second or so. The lower the string is in pitch, the longer the effect lasts.
Pluck the string gently, not near the bridge (over the 12th fret is good, just stay away from the bridge), in order to get the most even tone possible, wait a hair, then tune. I generally tune without a pick for that reason. Some tuners don't pick up the lower-pitched strings well; I'll tune to a harmonic if that's the case.
(Edit: As has been pointed out in the comments, an exception to the above is when you're going to be playing just short, choppy notes in a song. In that case, you might consider tuning to the beginning of the note, not the tail of it.)
As to why this happens, perhaps that's a question for physics.se.
More information about tuning than you ever wanted to know: Guitar Tuning Nightmares Explained
This is a major reason why I prefer to tune by ear whenever possible (obviously, on stage in the middle of a gig I'll use a tuner and turn my volume off; and even then I'll recheck by ear as soon as possible... quietly of course). Generally I check it a variety of different ways; open strings against one another (4ths except for the 2nd & 3rd together... that's always a problem spot), 5th & 7th fret harmonics against each other (again... except for the 2nd & 3rd... and you can use 5th fret low 6th for the 1st string's pitch and the 7th fret same low 6th string for the B natural 2nd string). Also an Emaj7 at the 7th fret uses all six strings together...
But my favorite is stacked A's and E's... starting on the high E string (1st) I make sure open E and 5th fret A are both in tune (seperately of course, I'm talking about open 1st and 5th fret 1st... same string) with a sustained A=440Hz (i.e. a metronome, tuning fork... ummm... whoever I'm playing with or whatever I'm playing to or recording over) after that I add a string at a time making sure each new string is in tune with every other string I've tuned so far (from A 5th fret on the top high E string down) and my A440 all together. Basically, if you fret 5th fret on the 1st & 2nd (E & B) with your pinky, and the 3rd and 4th strings at the 2nd fret, and then leave the bottom 2 lowest strings (5th & 6th) open you will have stacked 5ths a 4th apart (A's & E's... AEAEAE from high to low) on all 6 strings and all should sound properly and naturally in tune with A440 (after you tune of course)... whether you play harder or softer (remember, everyone you play with is going to have that same variance of effort, and it will change during the course of a song).
Why do I say this? Because the vast majority of the time that you hear something being out of tune, it's not because it's out of tune with the bass player or pianist for instance... It's because one of your guitar strings is out of tune with all the other strings on your guitar (the guitar is out of tune with itself). Remember what was stated above... that "all stringed instruments vary pitch in this way" (paraphrased). Of course because of the imperfect intonation capabilities of guitars (especially non compensated bridges on acoustics) but any guitar in actuality (even those with that... I think it's the "Buzz Fieten Tuning system"... Yep, found it); doesn't intonate every single fret spot on. You will also find times where maybe one of the open strings needs to be slightly out to compensate for a problem fret that may only show up in a certain key (or be guitar dependent).
I've recorded too many guitarists who only tune with a tuner... and at least 90% of the time I've ended up stopping them in the first minute of recording because they're still out of tune. Basically, if it sounds out of tune, it is (I don't care what some cheap tuner says, and a real quality strobe tuner's dial never really stops 100% perfectly still for longer than milliseconds)... IME.
Now don't get me wrong... everyone should own a tuner, but... in my experience they aren't the final arbitrator, your ear is. (This actually makes me want to bring up proper intonation and cheap and/or old tuning machines... watch out for "vintage" instruments with old tuning machines... keep the old ones for resale, but vintage guitars don't sound so good when they don't stay in tune for the whole song... of course intonation is beyond the scope of this question).
I did notice on that buzzfieten.com that he recommends tuning the attack and not the flattening decay of the strings pitch to avoid going noticeably sharp when playing harder. I generally pick at a few different intensities when I’m being really anal about my tuning, but most of the time I would qualify it as moderately firm (but never enough to buzz).
I now notice I left out the usual way to tune by ear (the 5th fret compared to the open string above… except for the 2nd/3rd of course), but I assume that other link posted covers that one (I didn’t check).
Kind of a hard thing to describe with just words... (I hope that all made sense).
Oh... I guess an exception would be while using an E-Bow (you are completely altering your ADSR profile then... "Attack Decay Sustain Release").
Upgrading my earlier comment to an answer.
It depends on your playing style. Remember that the aim is for your notes to be in tune when you're playing.
So, if you play mostly long sustained notes, aim for the correct pitch during the sustain. If you mute or re-pick most of your notes early, aim for the correct pitch during the attack.
If you play softly, pluck gently when tuning. If you play hard, pluck vigorously when tuning.
It's a physical effect. Strings, particularly the thicker ones, vibrate at different pitches depending on the size of the vibration. Strobe tuners allow you to see this, even with some background noise, and there are several strobe tuner apps available for the iPhone.
How to pluck and what part of each string's bending pitch to tune is an artistic musical decision. You get to decide which part of each strings pitch profile is most important to your music. If it's the pluck, or attack, and you rarely let notes linger, you want to tune the pluck or earliest pitch. If you are vocally harmonizing to a long sustain, then you might want to accurately tune that part of the strings "singing", and let the pluck be sharp. Or something in-between.
I've found that I get the best tuning when plucking the string near where I usually pluck it. I'm guessing this is because I play mostly chords, and there fore short notes, and that tuning to "compensate" a bit for the effect you noticed helps. I wouldn't be surprised (but I don't know) if classical guitarist rather like to pluck the string near the 12th fret.
I've also noticed that this mostly is a problem on cheap guitars, probably because the tuning incorrectness induced by this problem is then made worse by bad intonation.