This study explored how the frequencies and bandwidths of the acoustic resonances of physical tube models of the vocal tract differ when they have hard versus soft walls. For each of 10 tube shapes representing different vowels, two physical models were made: one with rigid plastic walls, and one with soft silicone walls. For all models, the acoustic transfer functions were measured and the bandwidths and frequencies of the first three resonances were determined.
For the models with soft walls, the resonance frequencies and bandwidths below 2 kHz were consistently higher than for the models with hard walls. These results confirm the general predictions of mathematical lumped-element models for the mechanical impedance of the vocal tract walls. The resonance bandwidths of the silicone models were well within the range of 50-90 Hz measured for humans, while they were too small for the rigid tubes below 2 kHz.