I think it's this. Sounds are very different based on the temperature and humidity of air. I have a piano at home and the difference is huge and very apparent over time.
Piano soundboards are almost always wood. There are a handful of companies that offer carbon fiber soundboards but they are very rare and expensive. As the soundboard gains moisture, it swells and buckles as it has nowhere to go, which makes it louder and also raises the pitch a bit as the strings are pushed a bit. When it gets drier, it goes flat (structurally and musically). If it has been too moist, the wood cells will be compacted and crushed. If it is then dried, it will form cracks as the wooden structure rips itself apart when shrinking again.
A piano soundboard that has been built and kept in a desert for instance will not crack. But if you take a piano from a ~55% humid country and put it in a desert (or overly air-conditioned room! or right in front of a heat radiator!) it will die.
The harp doesn't actually lie flush on the soundboard; there should be some space in between. The harp is just there to bear the tension of all the strings. The strings are connected to the soundboard via a wooden so called bridge, which is a raised wooden element glued onto the soundboard. It has pins in it which the strings are held against in tension. Bridges have a coating of graphite usually, and combined with the fact that most notes have 3 strings, it creates kind of a blocky pattern.
Here you can see a soundboard, bridge, and hitch pins on the harp. The hitch pins on the harp can be seen up top, then in the middle the bridge and bridge pins, and then below that the soundboard. The hitch pins are located on the opposite side of the keys in a grand, or on the bottom of the piano in an upright. You can see on the right of the image that the harp has a structural element which floats over the bridge, with plenty clearance.
It’s important, and can change the tuning in spring and fall cycles
most of the US is too dry in the home, and humidification is helpful for all wood in your home - to a point of course. Approx 55% is optimal
Of course in the south it’s the exact opposite - but A/C usually solves the problem.
Every homeowner should monitor their indoor humidity, it’s important for, wood floors, door trims, wood furniture, and our own health. Too much causes mold, too dry is hard on your respiratory system
Absolutely not. I first noticed this when visiting Iceland. The hot streams bubbling out of the ground sounded the same as boiling hot water being poured from a kettle, not the kind of cold-water stream sound you'd normally expect in the north in winter.
You can try it yourself. Take two identical kettles, boil the water in each of them and pour it somewhere. Obvious difference.
My piano has a Dammp-Chaser automatic humidifier and dehumidifier system that works well. My piano tech uses a moisture meter to check the soundboard at tuning time and the wood’s moisture has been fairly stable for more than two decades.
