This study adopts a systems theory approach in its focus on the efficacy of a classroom sound field amplification system (SFA) for the development of language in children aged 4-7 attending 7 designated disadvantaged urban schools in Dublin. The multi-method case study design combines norm-referenced and dynamic language assessments to create multi-dimensional language profiles of 65 study participants. These profiles are augmented by observations of the elements of the child and classroom systems that maximise or hinder the efficacy of SFA. A systems perspective on the efficacy of this intervention is innovative in the context of SFA research to-date, which has been conceptualised predominantly in terms of a simple linear one-antecedent-one-consequence causal relation. Thus, it is argued that while the literature on the benefits of SFA is large internationally, it is still seriously underdeveloped, as it brings little new meaning to the understanding of this intervention. A series of case studies illustrate individual responses to the intervention. In addition to identifying situations in which SFA works, the analyses describe situations and environments where SFA does not work, thus introducing a heuristic element to the hypothesis, absent in previous SFA studies. SFA was found to bring particular benefits to children in junior infant classes, children with norm-referenced language delay at baseline and children with teacher-attributed attention difficulties, most notably in the area of language comprehension and classroom participation. The finding that children with weaker norm-referenced baseline profiles showed greater gains in the area of language comprehension than other study participants indicates that what is often termed the Matthew effect (Merton, 1968) was not observed in this study’s results. Overall, SFA is found to be beneficial under the assumption that it is brought into a system whose other elements are not operating in direct conflict with the goals of SFA. The findings of this study are in agreement with a systemic view of the education system and the somewhat obvious - yet ignored by previous SFA researchers — fact stemming from it, namely, that the same intervention may lead to different outcomes in different systemic conditions. Implications, based on key findings and interpreted within a systems theory perspective, refer to the professional development of teachers and to a need to interrogate theory more critically in other SFA intervention studies. This study constitutes the first large-scale study on SFA in mainstream education in an Irish context and the first study internationally that is entirely concerned with the effect of SFA on the language development of pupils in designated disadvantaged schools.