The researchers found that the early Italian instruments produced human-like “formants”, the harmonic tones that correspond to resonances in the vocal tract. Specifically, the Amati violins produced formants similar to those from bass and baritone singers, while the Stradivari instruments had higher-frequency formants, closer to those of tenors and contraltos. Hwan-Ching Tai, an author on the study at the National Taiwan University, said Stradivari violins are often described as having “brightness” and “brilliance”, both qualities that could be rooted in the higher-frequency tones that make the instruments sound closer to female voices. “Some Stradivari violins clearly possess female singing qualities, which may contribute to their perceived sweetness and brilliance,” he said. Female singers tend to have higher-frequency formants than males because their vocal tracts are shorter.