Acoustic parameter for timber buildings (part 2)
Acoustic parameter for timber buildings (part 1)
21 October 2020
Acoustic parameter for timber buildings (part 3)
22 October 2020Impact sound insulation: how it is predicted and improved.
Mathias Barbagallo, PhD and senior consultant, Brekke & Strand Akustik AB,
Look at the timber joist floor in Figure 4. It is a complex structure to study in respect to impact sound, most of us acousticians will have a hard time studying it. Which are then the available tools to investigate impact sound insulation for timber structures and therefore to understand how to improve it?
Shortly answered, deterministic methods (like FEM, see Figure 5) may work well at low frequency, energetical methods (like SEA and ISO 12354, see Figure 7) may work well at mid and high frequencies. All methods must however cope with unknown or changing material properties, difficult or impossible to model structures as the one above, unknown boundary and coupling conditions, a wide variety of products and building techniques on the market and not least uncertainties due to statistical variation of build-up structures at mid- and high-frequencies. Therefore, analysis of previous acoustic measurements, simplified calculations and educated mind-experiments and are often solid approaches in consultancy.
Figure 5: Finite element model of a CLT volume element where the top CLT plate is excited by a tapping machine where the arrow points. Velocity response at 20 Hz is shown. Warmer colors indicate higher vibration velocity. Such models allow for more advanced studies, but take time and are limited to analysis at low frequencies.
A simple yet useful mind-experiment for impact sound insulation between two vertically placed timber rooms may be done in terms of energy, see Figure 6. The tapping machine excites the timber floor by injecting power in it: due to its low mass per unit area and low bending stiffness, the timber floor will vibrate a lot, especially at low frequencies. Part of the energy is dissipated within the floor thanks to its internal losses, part flows elsewhere in the building, and part flows to the other elements of the receiving room – e.g. walls, floor and ceiling: this is the so-called flanking transmission. Finally, acoustic energy is radiated from the structural elements in the receiving room into air: the direct sound radiated by the floor construction sums to the flanking sound radiation from the other building elements.
By identifying the various paths where vibroacoustic energy flows, we also identify strategies to improve impact sound insulation. Reduce the input power from the tapping machine into the floor, increase the losses in the floor, decrease the energy flowing from the floor to other elements, reduce the radiated power from the structural elements in the receiving room into the air.
The input power from the tapping into the floor can be reduced by decreasing its input mobility at low frequencies so that it will vibrate less. This is achieved making the floor construction stiffer and heavier: thick CLT elements, concrete or plasterboard coverings, gypsum boards, shorter spans in timber joists, supporting pillars are possibilities. A floating floor will both increase losses in the floor and its mass, as well as decouple it from the structure; rubber interlayers between volume elements will decrease flanking transmission.
Figure 7: Calculation of vertical impact sound between two rooms separated by 175 mm CLT floor with and without floating floor (FF) (SS 12354 supporting calculation down to 50 Hz). The floating floor consists of 13 mm parquet, 50 mm gypsum concrete, 25 mm rubber mat. The heavy floating floor improves insulation below 80 Hz and above 160 Hz.
This qualitative energy model may be refined – imagine a timber joist floor to which a suspended ceiling is attached. The energy from the tapping machine flows into the floor, then it is partly radiated into the air cavity between the floor and the suspended ceiling and partly flows in the joist themselves, which are then connected to the suspended ceiling. Two additional strategies to break the flow of energy are identified: increase the damping losses in the cavity by adding mineral wool and diminish the energy flowing from the joists to the suspended ceiling by using resilient mountings.
Once a qualitative energy analysis has been performed, it is possible to predict impact sound insulation using, e.g., ISO 12354 series and commercial software allowing for quick calculations. However, the reliability of calculation models to date is still questionable due to the complexity of the constructions and lack of extensive measurement data input to calculation models covering all construction solutions. The final design shall always be based upon previous experiences and available measurements, and by checking meaningful parameters, such as the mass per unit area of the floor construction, which plays an important role in determining its impact sound insulation – more mass, less low-frequency vibrations.
In the next and final article, we will look at building acoustic regulations and how they relate to acoustic comfort in timber buildings.
Brekke & Strand Akustik is an independent specialist company with acoustics, noise and vibrations as its field of expertise. Our continued focus on technical expertise, independence, cost-effectiveness and good customer service ensures that we can contribute to the largest and most prestigious acoustic tasks in Sweden and Norway where we operate. We are more than 100 specialists within acoustics, with a master’s degree in engineering, a doctoral degree or equivalent. Together we have accumulated more than 1000 years of experience in the field. If you would like to get in touch, just contact Mathias Barbagallo for acoustics of timber constructions () or Johanna Carpelan for other inquiries ().
