Wooden multi-family residential houses continue to gain in popularity. Compared to concrete, wooden constructions have low mass and are subject to relatively poor sound insulation at low frequencies. In a series of Swedish research projects (AkuLite, Aku20 and AkuTimber), the correlation between the measured sound insulation in lightweight, cross laminated timber (CLT) and concrete buildings, was compared to the self-rated annoyance from the residents. Impact sound insulation, LnT,w and LnT,w+CI,50−2500, showed weak correlation with the self-evaluated annoyance. Particularly residents living in wooden houses were irritated by footstep noise generated by a sound source with significant energy content below 50 Hz. When determining impact sound insulation, reverberation time (RT) is measured and normalized to 0.5 s. Below 50 Hz, the uncertainty of the measured RT has been reported to be an issue. In this thesis, empirical studies of the spatial variation of the RT are performed in two empty bedrooms of different construction, the measurement methods ”interrupted noise” and ”integrated impulse response” are compared. The spatial variation of the RT is studied. The uncertainty, in terms of standard deviation, is not found to be larger for frequencies below 50 Hz compared to higher frequencies. The bedrooms are then furnished and the RT is measured with the integrated impulse response method, found to be favorable below 50 Hz. To what extent furniture contributed to the absorption, and thereby affected the RT, is studied. At frequencies below 50 Hz, no statistically significant difference in the absorption, due to furniture, is found. Consequently, below 50 Hz, there should be no need for the normalization of 0.5 s when impact sound insulation is evaluated. From the scientific literature, there is an evident need to develop and upgrade sound insulation prediction methods of CLT panels. A finite element (FE) model is provided, capable of predicting the airborne sound insulation of typical CLT wall configurations. Single and double CLT panels, with air and mineral wool respectively in the cavity, are studied. A 2D-3D Hybrid FE model is introduced that rotates the panels to capture a diffuse-like sound field with reduced computational time compared to a full 3D model. The purposes are to predict the weighted sound reduction indexes, Rw and Rw + C50–3150, as well as to study the response as the materials and dimensions alter. For single walls, the differences regarding Rw and Rw +C50–3150 are within ±2 dB compared to analytical calculations and laboratory measurements. For double walls, the Hybrid model can provide an indication of the airborne sound insulation and serve as a tool for relative comparisons, but due to the lack of comparative data, the absolute accuracy cannot be stated. Metamaterial, an engineered material with properties not found in naturally occurring materials, may at a later stage be added to the FE model with the aim to improve the sound insulation below 50 Hz of CLT constructions. Passive vibroacoustic metamaterial, designed according to the localized resonance principle where the eigenfrequency of the unit cell (building up the metamaterial) coincides with the wanted wave length to decrease or cancel, is of particular interest. One such example is the Mie resonator which are studied in a pilot study. Two critically coupled Mie resonators, tuned to have monopolar and dipolar resonances in the same frequency region, will produce the highest sound absorption and thereby the highest sound insulation of the CLT construction. The Mie resonator was designed to absorb sound within the 31.5 Hz octave band, the result from the pilot study seems promising.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:ltu-91134 |
| Date | January 2022 |
| Creators | Pettersson, Maria |
| Publisher | Luleå tekniska universitet, Drift, underhåll och akustik |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Licentiate thesis, comprehensive summary, info:eu-repo/semantics/masterThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | Licentiate thesis / Luleå University of Technology, 1402-1757 |
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