In singing, the fundamental frequency determines the pitch, which is specified by the composer or performer. Singers can significantly increase their loudness by adjusting the resonance frequencies of their vocal tract to closely match the fundamental frequency or harmonics of the pitch. Sopranos can sing with fundamental frequencies that considerably exceed the values of R1 for normal speech, but if they left R1 unaltered, they would receive little benefit from this resonance. Consequently they tune R1 above its value in normal speech to match fo and thus maintain volume and homogeneity of tone. This is often achieved by opening their mouth wide as if smiling or yawning for high notes, which helps the tract act somewhat like a megaphone. (This tuning of R1 away from its values in normal speech, as well as the large spacing between harmonics, has implications for intelligibility, and is one reason why singers can be hard to understand at very high pitch.) Singers at lower pitch sometimes tune R1 to match harmonics (for example, 2fo) rather than the fundamental, but do not usually practice resonance tuning as consistently as sopranos.So there we have it.
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