Why can't I keep my instrument tuned? - Apr 2001
If your instrument is always out of tune, the instrument's tuning pegs/machines may be worn or its set-up may be imperfect. In either case, your instrument will need attention from someone who understands the mechanics of tuning & set-up.
Three simple tests can determine if your instrument needs repair. In each test, use an electronic tuner to tune the strings, then...
if the strings won't tune to correct pitches, the instrument needs attention
if the strings tune correctly, check the tones produced when you finger the strings exactly half-way between the nut & the saddle (this location is at the 12th fret on fretted instruments); if the note on each string is not exactly an octave higher than the open string, the instrument needs attention
if the instrument passes both of these tests, let the tuned instrument stand where temperature & humidity are constant; check the tuning after 10 minutes, 1 hour & 1 day; if the tuning is correct each time, the instrument is probably fine. If the tuning shifts (and temperature & humidity has not changed), the instrument probably needs attention.
If the instrument passes these tests and is out of tune only after long periods of not playing or when the temperature or humidity changes, the instrument is probably fine (all you need is to be aware of how changes in environment affect tuning).
To be responsive to a player's touch, instruments also respond to a host of other stimuli like temperature & humidity even though we'd prefer that they didn't.
String materials (steel, brass, bronze, synthetic, gut...) each respond differently to temperature & humidity. In addition, wood responds differently than string materials (in fact, different woods used a single instrument respond uniquely to heat & moisture). So, when you tune an instrument in one environment & then play it in a different one, it will go out of tune. And, while the exact interaction between tuning & environment are complex, the general trends are fairly simple:
Metal strings generally go flat with increasing temperature (metal expands faster than wood) & go sharp as humidity climbs (wood swells).
Conversely, gut strings generally go flat with increasing humidity (gut swells more).
If humidity changes quickly, synthetic strings generally go sharp (wood absorbs water more quickly), but are relatively unaffected if humidity changes slowly (the synthetic & the wood swell almost equally).
Synthetic strings generally go flat with increasing temperature (synthetics expand faster). None of this knowledge relieves the frustration of going out of tune, but it at least helps us know if the instrument is responding as it should, or if it needs help.