Hello! Sorry, I don't have much of a formal education, but am an amateur singer (female, age 66, tenor) who experienced a BIG surprise during one of our choral concerts a few weeks ago. While a fellow tenor was singing a solo (Gounod "Sanctus"), I heard distinct undertones (fourths). I was so shocked I lost my place in the music (but quickly recovered). At my age, I have heard overtones many times, but this was a first! The concert venue was a stone church in the VA. countryside; the undersides of the roof stones were exposed into the church's interior. Once, during a rehearsal there, the acoustics were so exquisite I burst into tears. Whew! Was I really hearing undertones, or was it just a miraculous moment of transcendence??? Whatever it was, I will NEVER forget it! Thanks for your advice.