There is little data available of low frequency impact sound response of light weight wooden buildings. The ISO 140-7, 717-2 standards normally used of impact sound is limited down to 50 Hz. The response in low frequency area is of interest for human comfort. In the present work low-frequency impact sound measurements were carried out in two modern office buildings with lightweight timber frame. The purpose was to assess the levels of impact sound transmission below 50 Hz in these construction types. The low frequency impact sound levels are compared to the higher. Both the tapping machine and the impact ball are used for excitation. It is seen that the present constructions have their highest levels below or close to 50 Hz when excited by the impact ball. From the office rooms sharing joist floor with corridor there is seen increased levels of low frequency impact sound. Up to 10-15 dB higher impact sound was detected compared to room with joist floor separated from corridor.

Using wood as the main construction material is a potential solution to achieve sustainable buildings. Previous research has shown that frequencies below 50 Hz are of significant importance for the perception of impact sound by residents living in multi-story buildings having light weight wooden frameworks. The standards used for impact sound measurements today are developed for diffuse fields above 50 Hz. For instance due to requirements concerning wall reflections, these methods are not applicable for low frequencies within small rooms. To improve measurement methods, it is important to know the nature of the full sound distribution in small rooms having wooden joist floors. Here, impact sound measurements with microphone arrays are made in two small office rooms having the same dimensions. The rooms represent two extremes in design of joist floors; one with closely spaced wood joists and the other with widely spaced joists. An impact ball is used for excitation the room being measured from the room above. The results show that there are significant variations in the sound pressure, especially in the vertical direction. Here, measurement techniques of impact sound in the low frequency range in small rooms in wooden buildings are evaluated and potential improvements are proposed.

In recent years research has shown that low frequency impact sound is of significant importance for inhabitants´ perception of impact sound in buildings with light weight wooden joist floors. The tapping machine is well defined as an excitation device and is a standard tool for building acoustics. However, the excitation force spectrum generated for each individual floor is unknown when using a tapping machine. In order to increase the possibilities to compare simulations to impact sound measurements, there is a need for improvement of impact sound measurement methods. By measuring the input force spectrum by a modified tapping machine and the sound in the receiver room, transfer functions can be achieved.In the light weight wooden building used for the evaluation test of the proposed method, structural nonlinearities are evident in the frequency response functions stemming from different excitation levels. This implies that for accurate FRF-measurements in low frequencies, excitation magnitudes that are similar to these stemming from human excitations should preferably be used.

The purpose of this paper is twofold. The first aim is to develop a numericalanalysis procedure, by combining FRFs from FE-models with analyticalformulas for sound emission and transmission from the ceiling anddownwards within a room with four walls. The aim is to, by applying thisapproach; accomplish a tool which calculates the relative impact soundbetween different joist floors, in the low frequency range. The second aim is tobenchmark a thin hybrid wooden based joist floor with similar thickness,surface weight and global bending stiffness as a concrete hollow core floorstructure. What will be the difference in sound transmission? The question isrelevant since it may be necessary to make thinner wood based joist floors inhigh rise buildings, if wood should stay competitive against concrete. Theresults show that the direct transmissions of impact sound are very similararound the first bending mode. As the frequency increases, the modes in thestructures differ significantly. Below 100 Hz, the concrete floor has 4 modes,while the hybrid joist floor has 9 modes. As the frequency increases the soundradiation characteristics differs. The results show that it is possible to havesimilar sound transmission properties around the first bending modes for ahybrid based joist floor and a hollow core concrete floor structure with similar thicknesses. At the first modes of the structure, the information about thesurface weight and global bending stiffness are useful for prediction of soundtransmission properties but for higher modes, they are not sufficient.