"Singers who had learned their songs isolated from radio broadcasts, recorded music and tempered instruments played 'classically' had overtones of fourths and sevenths above the fundamental notes they sang, while singers who had learned their songs immersed in the milieu of radio , recordings and performances on instruments played 'classically' had overtones of thirds and fifths above the fundamental notes they sang."

Rowan, can I ask what you mean by overtones?