The evolution of the syrinx: An acoustic theory
Fig 5
The interaction between sound source and filter is conceptualized as either Level 1 or Level 2 interaction.
Level 1 interactions are characterized by effects on the glottal flow, which will lead to acoustic effects, including changes in harmonic emphasis and/or the occurrence of nonlinear phenomena. Level 2 interactions are characterized by effects on vocal fold vibrations, which will lead to changes in fundamental frequency and/or also the occurrence of nonlinear phenomena.