To measure the airborne sound insulation of glazing in accordance with Annex D of ISO 10140-1, laboratories must verify their procedure by measuring reference insulating glass units (IGUs) annually. However, it has proved difficult to reproduce the reference IGUs used in the original round robin. A more stable reference sample that can be easily obtained and stored could solve this problem. Within the WG Acoustics of ENBRI (European Network of Building Research Institutes), the use of a steel plate as a reference sample was proposed. This paper presents the result of a round robin test organized by ENBRI with 7 participating laboratories. Each laboratory performed 7 tests on a 2 mm thick steel plate, fixed to a wooden frame according to clear specifications. Both repeatability and reproducibility standard deviations were overall lower than ISO 12999-1 values. To check the repeatability of the installation in the small test opening, the sample and frame was remounted twice in each laboratory. The mounting affects the repeatability especially at higher frequencies, indicating that careful mounting is necessary to avoid leakage around the perimeter.