It should not be A surprise that a singer would choose natural intervals rather than well tempered ones, because they are already aware of the key in which they are singing. A piano keyboard is usually adjusted so that the intervals sound correct or approximately correct in every key. It has often been noticed that a piece changes its feeling subtly when placing a different key on the piano because equal tempering does not in fact make every key sounds exactly the same. There is more going on in a piano because the strings are struck usually at 1/12 of their length to make the best compromise between the string frequencies and the bar frequencies of the high tension strings. This compromise is what gives the piano it’s unique sound which can then be altered by different makers by tiny adjustment of these basic parameters. This is also why a piano tuned below concert pitch, in order to make it keep in tune for longer, has a different tone which can be recognised by musical experts. As you reduce the tension in the steel string the ratio between the string frequencies and the bar frequencies is changed and therefore the compromise set by the manufacturer for concert pitch is changed. This complicates the question of what a fifth, for example, sounds like in every key that you can play on the piano.
sicut vis videre esto
When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
Originally Posted by Ken G