The in-duct attenuation performance, or insertion loss, of a range of absorbers was measured, and guidelines for the design and application of absorbers in air ducts were developed from the measured data. A test facility that met the requirements of ISO 7235 was designed and constructed. Insertion loss measurements were made using the test facility to investigate the effects of absorbent material and absorbent thickness, facing type, airflow conditions, and fixing method It was found that absorbent flow resistance was the most influential factor in determining the insertion loss of an absorber. The insertion loss of wall absorbers was significantly modified by changes in absorber thickness, which was attributed to the consequential change in flow resistance. Similarly, the insertion loss performance of bar absorbers with significantly varying thickness, and therefore varying flow resistance, was greater than that of bar absorbers with approximately constant thickness. The insertion loss of wall absorbers was very sensitive to impedance conditions at the boundary between the rear surface of the absorber and the duct wall. Insertion loss performance was improved by applying thin facings to the exposed surface of wall absorbers. Airflow velocity did not affect insertion loss over airflow velocities typical of building services ducts.