Why the shape of the throat and vocal folds sets the voice
The voice starts with anatomy. In the larynx, two folds of tissue vibrate as air passes between them, and the size and shape of those folds determine how the airflow behaves and how much the tissue deforms. Computational models of nine three-dimensional vocal-fold geometries, built with different lengths, thicknesses, and depths, found that thinner and longer folds produced noticeably more tissue displacement, while fold depth strongly affected flow rate [2]. The pressure drop across the glottis, by contrast, was nearly independent of fold length [2]. In plain terms, some features of the folds change how much the tissue moves and how much air gets through, while others barely matter for pressure.
The vocal tract above the folds acts as a resonator, and its length shifts the frequencies that come out. A shape model built from magnetic resonance imaging of 41 speakers, covering 1,947 spoken sounds, found that lengthening the vocal tract by one centimeter in either the vertical or horizontal direction lowered formant values by about 7–8% [4]. Formants are the resonant frequency bands that give vowels their identity, so a longer tract means a lower-sounding voice. Most of the variation between speakers came from independent changes in horizontal oral length and vertical pharyngeal length, with the vertical dimension capturing male–female differences [4].
Anatomy helps decide who can learn new sounds
Vocal learning — the ability to pick up new sounds by imitation rather than being born with them — is not evenly distributed across animals. A review of vocal learning in animals and humans ties the topic to vocal physiology, indicating that the physical vocal apparatus is part of what makes vocal learning possible [3]. This matters for interaction because two individuals can only share a vocal repertoire if both can physically produce the relevant sounds. A species whose anatomy limits its range of movements is constrained in which sounds it can copy, and therefore in how finely it can match a partner's calls.
The human case shows the same constraint from the other direction. Because vocal-tract length and fold geometry vary from person to person, speakers build their own articulatory strategies on top of different anatomical starting points to reach the same acoustic goals [4]. Two people saying the same vowel may be moving their vocal organs differently to arrive at a similar sound. That flexibility is what allows humans to converge on a shared sound system despite different bodies, but it also means the underlying anatomy is never irrelevant — it is the substrate the strategy works on [4].
What happens when the voice belongs to a machine
Robots do not have vocal folds or a vocal tract, so their voices are synthesized rather than produced by tissue. The question then becomes whether the form of the robot and the presence of a voice change how people respond to it. One study on robot morphology and trust reports that giving a robot a name and a voice helps establish greater trust in what it communicates [5]. The same work frames the issue around what robots resemble — human, animal, or something else — and measures responses using human–robot interaction ratings [5].
That finding is about trust in information, not about vocal physiology, and the available summary does not report how different robot forms compared. So the honest reading is narrower than it may first appear: a name and a voice are associated with greater trust in at least one study [5], but whether a robot's body shape constrains its vocal interactivity the way anatomy constrains a human or an animal is not established by this evidence. The parallel with biological voices is suggestive rather than demonstrated.
Where the human–animal–robot comparison actually holds
Across humans and animals, the constraint is concrete: fold geometry changes airflow and tissue motion [2], tract length shifts formant frequencies [4], and vocal physiology is bound up with the capacity for vocal learning [3]. These are physical limits that shape what sounds can be produced and, by extension, what can be shared between two talkers. The comparison to robots is weaker. A robot's voice is generated by a synthesizer, so the relevant constraints are design choices rather than tissue properties, and the evidence here speaks to trust rather than to acoustic or articulatory limits [5].
A workshop series devoted to vocal interactivity in and between humans, animals, and robots treats the three as a single subject of study [1], which shows the comparison is taken seriously as a research area. But that framing is a research agenda, not a finding about how physiology constrains interaction. The specific mechanisms documented here come from human modeling and from the study of vocal learning in animals and humans [2][3][4]; the robot side contributes evidence about trust in a voiced, named machine [5], not about morphological limits on vocal production.
Sources used in this answer
Proceedings of the 4th International Workshop on Vocal Interactivity in-and-between Humans, Animals and Robots
The paper is a workshop proceedings on vocal interactivity among humans, animals, and robots, but it does not explicitly discuss vocal physiology or morphology.
Gender in human phonation: Fluid–structure interaction and vocal fold morphology
Computational models of nine vocal-fold geometries indicate that gender-related differences in laryngeal dimensions notably influence glottal airflow and tissue deformation, with thinner and longer folds increasing maximum tissue displacement and fold depth affecting flow rate.
Vocal learning in animals and humans
The paper examines vocal learning in animals and humans with attention to vocal physiology, though it does not explicitly discuss interactivity in communication.
Morphological and acoustic modeling of the vocal tract
A shape model of the full vocal tract from 41 speakers found that a 1 cm lengthening in the vertical or horizontal direction lowered formant values by 7–8%, with most inter-speaker variance tied to oral and pharyngeal length variations.
Can you trust what a robot says? The influence of robot morphology on trust
The study reports that giving a robot a name and a voice helps establish greater trust in what it communicates, based on human–robot interaction ratings of robots resembling humans, animals, or other forms.