Yes, this is a fiddle question on a flute forum, but I know many here play both or know about the fiddle.
Question: does a fiddle tend to go sharp or flat after it’s warmed up? Flat perhaps because the strings loosen and create a longer wave.
Helpful info for in session or playing with fiddle if one knows over time if they do go flat while the flute goes sharp as we warm up.
Perhaps the fiddle stays pretty much the same.
Thanks for the info.
Jason
My subjective experience of being around stringed instruments - fiddles, guitars, bouzoukis, harps, etc… - is that they tend to go flat as they acclimatise to a warm room, but it may not be consistent across all their strings. Similarly, tune up indoors, go out in the cold to play and they go sharp. However, as they have other parts that can react to ambient environmental conditions, it isn’t necessarily so simple - the strings are more quickly affected, but after a while wooden or metal parts will also change. Overall they are probably more stable but less predictable in this regard than flutes and whistles - take us out in the cold to busk and we go way flat, bring us in to a nice warm fuggy sesh and we go sharp. We can warm up the instrument itself at the start of the evening and get it in tune, but as the crush in the room - and the temperatuure and humidity - increase through the evening, we’ll find we’ve gone sharp again and have to tune down. For us it is mostly about the temperature and humidity of the air-column within the instrument, of course, especially once the body of the instrument has acclimatised. Orchestral players have in the past told me that their expectation is for the strings to go out in the opposite direction to the winds if there are environmental fluctuations. Of course, stringed instruments themselves do not get warmed up just by playing in the same way that wind instruments do with our hot, damp breath going through them, but they are affected by, say, travelling in a cold vehicle and then being brought into a warm pub, or coming from a warm Green Room onto a draughty stage.
Thanks, Jem, for the detailed response. You’re a (shall I say it) gem…sorry, couldn’t help myself 
I’m a reformed orchestral player. Jemtheflute explained it well. Put an in-tune orchestra under warm stage lights and the strings drift flat and the brass and woodwinds go sharp. Sadly, it never quite counters the disastrous tendency of many violin/fiddle players to play sharp to get a “brighter” tone relative to the orchestra.
I second Elaine’s comment (especially the bright string part)! (and Jem’s)
But I think I read recently, here I believe, that strings do the opposite so it is nice to read confirmations of what I believed. Perhaps I should get my wife’s cello out and tune it then turn on the heater and see what happens.
Bill
Oooh, science experiment on the spouse’s instrument. Let us know how that goes for you!
!
an’ if ya make it back alive to let us know
we’ve lost so many that way
Years ago I saw John McCutcheon, the great hammered dulcimer player, in a venue where the warm-up room was about 20 degrees different from the performance room, I can’t remember whether hotter or colder. Anyway, he came out with his 58-stringed instrument and played one tune. Then re-tuned for about 10 minutes, played another, and so on. I think he spent at least half of the first hour re-tuning. He spent the whole intermission tuning, but with people going in and out, the room cooled off, then during the second set the room itself warmed up by probably 10 degrees from the doors being closed again and all the body heat warming it up. I’m sure it was a disaster for him.
A good friend of mine, the one who introduced me to Irish music, had a tin ear. And a hammered dulcimer. About once a week for awhile, he’d cook me dinner and I’d tune his dulcimer for him.
So if, anecdotally, string instrument go flat when warmed, the next question is … why?
already got hints/clues from Jem & Elaine…
Clues, schmoos … Hit me over the head. Jem and Elaine want to do that to me anyway. 
Seriously … metal/wood expansion, air density in the instrument body, speed of sound in the bridge … what’s really going on?
The strings warm up and get longer/expand, so they’re under less tension, resulting in a lower tuning. Metal(s) expands at another rate as wood, so the effects don’t cancel each other out. Actually every kind of metal expands/shrinks at different rates (bimetal cutout!), so the internal structure of wound strings might also change a little.
I spent the whole evening tonight playing fiddle next to a brick wall, on the other side of which was the pub’s boiler. I was sweating like a sexual professional in a house of worship, and my fiddle was pulling sharp. Cheers,
Rob
The whacky one is the winds. The speed of sound increases with increasing temperature. As the wind instrument warms, sound travels faster through the warmer air in the instrument. Speed = frequency * wavelength. The speed of sound depends on atmospheric conditions, and the wavelength is set by the shape of the instrument and distance between the toneholes, so the frequency (pitch) increases as the temperature and thus the speed of sound waves traveling through the instrument increases.
This effect overrides even thermal expansion in brass and metal whistles. It does not tend to override thermal expansion of the strings that Gabriel mentioned on stringed instruments.
Ouch, the picture in my head! Pleeez make it go away!
Thanks, guys (Gabriel, Elaine). Metal string contraction makes sense. For winds it’s clear; cold air in the tube increases the effective acoustic length.
Cold air in a string instrument’s body must affect the resonance and timbre, too. Do cold instruments sound boomier?
Rob: Do you use Perlon/synthetic cores, or metal/helicores? Maybe heat causes synthetic cores to contract?
I don’t know exactly what their properties in different heat and humidity would be or precisely how they react to changes in conditions, but obviously gut or nylon strings will behave differently to all-metal ones, and metal-wrapped ones with gut or synthetic cores will be different again, at least in speed of response to change. One might imagine that both gut and nylon or other synthetics would tend to soften with warming, thus becoming more elastic and therefore losing tension. Whatever, it is the behaviour of the tension-bearing core that will be most material to this discussion.
The influences of environmental fluctuations might in some circumstances act more swiftly upon the load-bearing parts of a stringed instrument than upon the strings, say softening it, thus making it more pliable and therefore slackening the strings; or expanding or stiffening, thus increasing tension… Of course, the whole instrument behaves as a complex system where an initial change in string tension may compress or release the structure which then affects other strings or gradually affects the whole set… a bit like post-glacial tectonic uplift. That’s probably why they spend so much time twiddling their knobs!
One could postulate a situation where intitially warming would affect the most-reactive strings, making them go flat, but over an longer period warm humidity might make the more slowly reactive wooden instrument body swell, or a steel neck-tensioning rod in a guitar expand, thus increasing tension on and re-sharpening strings that have already been tuned up to compensate for the original slackening…
Complicated!
I use Dominants, for the record.
Keep in mind, too, that the neck of a violin is made of two slabs of different woods, maple and ebony. It could be that these act like the bi-metal strip that switches on the thermostat in your HVAC system. Bi-wood strip?
Rob
What does your guitar do?
Good question. You know, I’ve never really paid attention, and I try not to let it get too hot or cold anyway. Since cold air in the guitar body increases the effective acoustic size of the resonant chamber, it seems logical that that would favor lower frequencies and make it sound boomier than at normal room temps.
Yes, that sounds very plausible, too. As Jem says, I think the situation with string instruments is much more complex than the fairly straightforward explanation for winds.