How loud exactly is pianissimo when you’re singing?

My choir is singing Carmina Burana’s Floret Silva Nobilis and the soprano parts have very high notes that you have to hold at pianissimo.

The second sopranos hold a G5 and the first sopranos hold a B5, for several measures, at pianissimo repeatedly throughout the whole song.

I don’t know how it’s possible to hit a high B at pianissimo, that’s the equivalent of a whisper, right?

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