Acoustic properties of wooden floor slabs

The acoustic behaviour of altogether 15 different wooden floor slabs and some modifications to them were measured by the acoustics group of VTT Building Technology. The measurements included both the airborne sound insulation and the impact sound insulation. With regard to the airborne sound insulation, only the Rw rating according to ISO 717 was used. In rating the impact sound insulation, the methods proposed by Bodlund and Fasold, the energetic sum Ln,sum according to ISO/DIS 717-2.2, and the traditional Ln,w according to ISO 717-2 were also used. The load-bearing structure of the floors was either wooden thin web beams, ordinary timber joists or a concrete-wood composite construction. The floors were so-called floating floors, i.e. the top boards of the floor were laid on a layer of a resilient material. On the underside of the floors there was, as a rule, a resilient board ceiling (acoustic profiles). Some floors were also measured with alternative ceiling structures on the underside. When the measurements were carried out, the floors were either non-loaded or loaded according to ISO 140-8. Measurements were also carried out with the floors carpeted and without the floating structure. The purpose of these tests was to study the effect of different floor sub-systems (e.g. different floating floor structures, different heights and spacings of the joists, different ceiling structures) on the insulation properties. Attention was mainly focused on the impact sound insulation, since that was known to be the most problematic property. In addition to normal insulation assessments, the sound pressure levels caused by a person walking across the floor (living walker) was measured in the room below. Vibration levels in the different layers of the floor structure were evaluated using a tapping machine as the source. The results show that when the floating floor is loaded the impact sound insulation deteriorates, the impairment in Ln,w being from 3 to 5 dB. The corresponding impairment in the airborne sound insulation Rw is of order of 1 dB. The experiments with a living-walker, although very limited, clearly show that the sound spectrum produced by a living walker differs greatly from that induced by the tapping machine. The sound levels of low frequencies, say from 32 to 200 Hz, are in some cases most critical when the sound pressure levels induced by walking are considered. The scores obtained for the wooden floors from subjective judgements may be difficult to evaluate exactly with any rating method proposed in the literature (even if they take the frequencies under 100 Hz into account). It may be that some kind of noises like "thumps" can not be avoided totally when using light-weight floors. Anyway, some suitable rating methods can be developed for the floors if the overall spectra of impact sounds is taken into account. However, it is important to ensure that the rating methods used do not overemphasise the influence of the lower frequencies, which are also associated with severe practical measuring difficulties. In the future it would be most desirable to carry out an extensive experimental programme to determine the relationship between the subjective scoring and test rating of different floor structures. In this context the current impact-induced sound spectra should be adequately clarified. The programme should include a sufficiently wide range of floor types (concrete, composite, wooden, steel, etc.) in order to obtain an exact basic for assessing the subjective performance of different types of floor structure.