The ISO 23351-1 DS,A figure from a named, independently accredited laboratory. This metric directly quantifies speech level reduction — the acoustic performance dimension most relevant to the Irrelevant Speech Effect and cognitive performance. Ask every supplier: “What is your ISO 23351-1 DS,A figure, and which laboratory tested it?” If they cannot provide this, they cannot support their acoustic performance claims. HIGHKA’s DS,A = 29.4 dB is independently verified by SGS.