Are you making sure that the other strings are not vibrating in sympathy.
You could be having a sympathy between the G & a harmonic of the D which, when you add in the A, could be causing the two apparent tones to appear, but on different strings.
You're probably hearing right, and it's all on one string, but with strings that length and with that much energy to throw around, and with any experiments in harmonics, it's simply prudent to make sure the other strings aren't contributing. So it's worth check.
My first guess was that I was setting of a sympathetic vibration in the A string, so I tried it again muting the other three strings before I sat down to write about it, and the effect was still there. It's all the one string.
(no subject)
You could be having a sympathy between the G & a harmonic of the D which, when you add in the A, could be causing the two apparent tones to appear, but on different strings.
You're probably hearing right, and it's all on one string, but with strings that length and with that much energy to throw around, and with any experiments in harmonics, it's simply prudent to make sure the other strings aren't contributing. So it's worth check.
(no subject)