This study examined the efficacy of classroom sound field amplification system for the language development of children, aged 4-7, in early primary years in schools designated as disadvantaged in Dublin, Ireland. The language development of 65 study participants from 14 classes in 7 schools was assessed in a multiple baseline AB case study design supplemented by norm-referenced language assessments pre- and post-installation of the amplification systems. One hundred and seventy three 30-min classroom observations were conducted over a 9-month period. The intervention was found to bring statistically significant gains in the area of language comprehension and classroom participation, particularly in classes in which the support was introduced at the start of schooling. The study constitutes the first large-scale study on sound field amplification systems internationally that is entirely concerned with the effect of this intervention on the language development of children experiencing socio-economic exclusion.