AbstractInsulation systems change the acoustic properties of solid walls. A thermal insulation can have positive or negative effects on the airborne sound insulation. The behavior of heavier walls (e.g., calcium silicate bricks or clay bricks) is very well documented. The acoustic behavior of autoclaved aerated concrete (AAC) walls with thermal insulation has not yet been similarly analyzed. Therefore, some tests on the acoustic behavior of AAC walls with thermal insulation were conducted. During the test, different insulation systems were attached to an AAC wall (expanded polystyrene (EPS), elasticized expanded polystyrene (eEPS), phenolic insulation (FP)). The mass of the outer plaster was also variated. Furthermore, an AAC cavity wall was tested. The tests show that the airborne sound reduction index of a monolithic AAC masonry wall can be considerably improved if the appropriate thermal insulation is selected. The use of an elasticized polystyrene insulation leads to an improvement of the sound reduction index between 3 and 5 dB. An additional plaster layer has further positive effects with an overall improvement of up to 9 dB. The tests also proved that a polystyrene insulation with a thickness of 16 cm does not lead to a decline of the airborne sound reduction index.