Vocal tract wall softness and open-end impedance mismatch lower resonance Q

Evidence

The Q of these resonances depends on how easily the sound reflects at the open end—that is, the impedance mismatch there—and the friction and sound absorption of the soft walls of the tube that lowers the Q and broadens the resonances. — Eric J. Heller, Why You Hear What You Hear- An…, 1667