On the paradox of brick influence on vertical sound insulation

In Spanish buildings, sound transmission between two vertically adjacent rooms is often dominated by flanking paths involving the inner walls. These walls are mainly built with ceramic bricks and there is a rigid junction between floor and walls, though elastic layers at the bottom are more and more used since a new build regulation asks for higher insulation levels. Different brick thickness are used, from 4 to 9 cm, with increasing masses and sound insulation. These walls are usually lined with 1cm of plaster. Common floors are beam and pot style, covered with some elastic element to improve impact sound insulation. This paper shows some surprising results, as overall sound insulation decrease using a 7cm thick brick instead of a 4cm one, and raises again using the 9cm brick. This result is however explained because of the effect of the brick mass on both sound insulation and vibration attenuation through the joint.