Realistic 3D avian vocal tract model demonstrates how shape affects sound filtering (Passer domesticus)

Despite the complex geometry of songbird’s vocal system, it was typically
modelled as a tube or with simple mathematical parameters to investigate
sound filtering. Here, we developed an adjustable computational acoustic
model of a sparrow’s upper vocal tract (Passer domesticus), derived from
micro-CT scans. We discovered that a 20% tracheal shortening or a 20° beak
gape increase caused the vocal tract harmonic resonance to shift towards
higher pitch (11.7% or 8.8%, respectively), predominantly in the mid-range
frequencies (3-6 kHz). The oropharyngeal-esophageal cavity (OEC), known
for its role in sound filtering, was modelled as an adjustable 3D
cylinder. For a constant OEC volume, an elongated cylinder induced a
higher frequency shift than a wide cylinder (70% versus 37%). We found
that the OEC volume adjustments can modify the OEC first harmonic
resonance at low frequencies (1.5–3 kHz) and the OEC third harmonic
resonance at higher frequencies (6-8 kHz). This work demonstrates the need
to consider the realistic geometry of the vocal system to accurately
quantify its effect on sound filtering and show that sparrows can tune the
entire range of produced sound frequencies to their vocal system
resonances, by controlling the vocal tract shape, especially through
complex OEC volume adjustments.