Abstract: In its first aspect the invention provides an electromechanical acoustic transducer (5) which is resistively terminated at the rear with a closely-coupled sound absorber (7) made from an aerogel with good acoustic absorption at low frequencies. The invention also provides an aerogel suitable for this use and which is a conglomerate of small particles packed so that there are spaces left therebetween to allow the passage of sound. Moreover, the invention provides an aerogel suitable for this use and comprised of multiple layers (17a etc.) of materials of graded properties, or comprised of material of continuously varying properties throughout the thickness of the absorbing structure, in the principal direction of sound from the source.