Colm Tóibín at the London Review of Books:
Besides the complexes that blokes in the 21st century may have about castration and the shivering joy many take in explaining all this to a psychoanalyst, there is another reason the castrato may continue to fascinate us. It is the old idea that while heard melodies are sweet, those unheard are haunting. Feldman writes that castrato voices had ‘strong resonance … understood as relative loudness and intensity, with timbral richness’. If we want to imagine what a castrato sounded like perhaps it would help to listen to a recording by a deep and powerful contralto – Hilde Rössel-Majdan, for example, or Maureen Forrester – and then follow this by listening to a countertenor, David Daniels, for example, or Andreas Scholl, or Iestyn Davies (or go on YouTube and listen to a recording of the last castrato, Alessandro Moreschi, who died in 1922, singing the Bach-Gounod ‘Ave Maria’, with what Feldman called a vibrato that is ‘often lush and plentiful’, and the ‘Crucifixus’ from Rossini’s Petite messe solennelle). But all of these offer merely clues.
Some of the clues are fascinating, however, perhaps because the language used to describe a castrato singing has its own luscious, plaintive sound. The French soprano Emma Calvé wrote in her autobiography about hearing the castrato Domenico Mustafà in 1891: ‘He had an exquisite high tenor voice, truly angelic, neither masculine nor yet feminine in type – deep, subtle, poignant in its vibrant intensity … He had certain curious notes which he called his fourth voice – strange, sexless tones, superhuman, uncanny!’ Another writer wrote of a castrato voice that it was ‘so soft, and ravishingly mellow, that nothing can better represent it than the Flute-stops of some Organs’, which themselves were ‘not unlike the gentle Fallings of Water’.
more here.