Adagio per Voce Umana

The composer states:
Per organo antico Italiano
Principale e Voce-Umana

Voce Umana:
An important Italian stop of the 16th century onwards; it had treble compass Principal-scaled pipes mistuned with the Principale 8' and thus producing an undulating effect (Schwebung), more singing and less reedy than 19th-century céleste stops.

It was sometimes called Voce Umana.
Also called Fiffaro or Piffaro.