I just learned of this New Zealand teenager named Adrian Mann, who built what is described as “the world’s longest piano” (I’m not convinced of that claim) from scratch. The thing is 18 feet 9 inches long. It has only 85 keys, for some reason, instead of the canonical 88, apparently choosing to cut off a few from the top end. I would understand stopping at some point at the bottom end due to the continued logarithmic lengthening of the strings, which could become extreme.
In any case, I presume the object of making such a long instrument was to avoid the foreshortening of the bass end strings which are traditionally made heavier by winding metal wire layers over the core string. (A wound string is more flexible than a solid string of similar mass.) The shorter the string, the more/thicker overwinding has to be done to compensate, and this results in more complex and chaotic overtones and basically a stiffer string that will have less sustain. This is why the bass end of a grand piano sounds clearer and stronger than the same notes on an upright. So this maker has avoided the issue by letting the strings be as long as they need to be. I expect the tone on the bottom is very clear, but I’m not convinced that’s all there is to it. You’re going to get a lot more standing wave artifacts on a very long string. I would bet a hard strike would have a noticeable distortion on the attack and possible residual subharmomic standing waves. In any case, kudos to the kid for his chutzpah. He’s clearly got a lot of talent and drive.