In a completely different genre, I was singing with the Vancouver Chamber choir, a professional choir in Vancouver, BC, in the early 80's. A new Abbey had just been completed in Mission, BC about 1hrs drive from Vancouver and we were scheduled to sing a concert there on a Sunday afternoon. Beautiful church acoustics, grand place. As it would be, the piece that we started with had the singers, 40 of us, break into three 'choirs'. The men started with a low A being sung across the choirs. While we were singing, the director, Jon Washburn, stopped us and asked the Alto's not to sing with us. He was hearing the upper octave A. We started and stopped again, and again he asked the alto's not to sing. No one was singing. The basses of the choir were so in tune, the acoustics so perfect, that we were creating the upper harmonic!! Even more incredible, the alto's had the same part a bit later in the piece, they produced the LOWER harmonic, such was their tuning and the wonderful acoustics!