Background: Teachers' behaviours drive motivational climates that shape children's engagement and well-being in the classroom, but few studies examine how specific teachers' behaviours such as wording, body language, or voice contribute to these outcomes in isolation of one another. Aims: This pre-registered experiment sought to examine the often-forgotten role that teachers' tone of voice plays in children's education. Informed by the theoretical framework of self-determination theory (SDT; Ryan & Deci, "Self-determination theory: Basic psychological needs in motivation, development, and wellness," 2017), conditions manipulated controlling (pressuring, demanding), autonomy-supportive (inviting of choice), or motivationally neutral, tones of voice to explore their effects on children's self-reported psychological needs satisfaction, well-being, intention to self-disclose to and intention to cooperate with their teacher. Sample and Method: Children aged 10-16 years (n = 250) heard pre-recorded teachers' voices holding sentence content and speakers constant across conditions, but varying tones of voice. Results: We hypothesized a-priori and found that when children heard controlling sounding voices, they anticipated lower basic psychological need satisfaction, well-being, and intention to disclose to teachers, as compared to neutral-sounding voices. We also anticipated beneficial effects for autonomy-supportive versus neutral voices, but pre-registered analyses did not support these expectations. Intention to cooperate with teachers did not differ across conditions. Supporting relational motivation theory (RMT; Deci & Ryan, "Human Motivation and Interpersonal Relationships," 2014), exploratory analyses showed that hearing autonomy-supportive sounding voices increased autonomy and relatedness need satisfactions (but not competence need satisfaction), and through doing so indirectly related to beneficial outcomes (well-being, intention to cooperate and self-disclose). Conclusion: In summary, tones of voice seem to play an important role in shaping teachers' impact on their students.