Acoustic models of the vocal tract for articulatory speech synthesis often neglect a range of acoustic effects that are known to exist in the human vocal tract. Here we extended a basic acoustic vocal tract model by three features: the piriform fossae, transvelar acoustic coupling of the oral and nasal cavities, and sound radiation from the skin of the neck. The main goal was to find out how these features affect the naturalness of the synthesized speech. To this end, ten German words were synthesized with different combinations of the additional features, and listeners compared the naturalness of these stimuli. Surprisingly, all three features reduced the perceived naturalness, although they should make the synthesis more realistic. A closer analysis revealed that all new features emphasized the low frequencies compared to the high frequencies of the synthetic speech, leading to slightly more muffled speech with the used glottal excitation. An additional perception experiment with synthetic stimuli with a slightly more tense voice revealed no perceptual preference for the synthesis with or without the piriform fossae. These results indicate that the examined features play a minor role for the naturalness of articulatory synthesis compared to the voice source characteristics.