Two types of modelling have been used by MACH in establishing the performance of open windows; Finite Element Analysis used to determine the exact frequency performance of different opening arrangements, and Finite Time difference modelling is used to illustrate and present the passage of sound through windows.
MACH Acoustics appreciate the importance of modelling and we have therefore used a world recognised FEA provider, Comsol, to ensure the accuracy of our modelling. Due to it’s cost, Comsol is mainly used by product designers, however we see that it has the ability to model exceptionally complex problems and offers exceptional accuracy.
These images show the frequency response of a top hung window opening inwards (left) and opening outwards (right). As noted, FEA places a mesh over the test subject and then derives a series of differential equations, such to determine the acoustic frequency response across the mesh.
The challenge with both FEA and acoustic testing is that you cannot see how the sound passes through a window, or how it bends around an acoustic baffle.
MACH Acoustics in conjunction with Southampton university have developed our own in house software tools based around Finite Timne Difference Modelling methods.
As seen from the following videos, the core advantage of this technique is the ability to visualise the sound. This in turn can be used to inform the design team of the elements that impact upon the performance of vented facades.
Finite-Difference Time-Domain (FDTD) is a numerical analysis technique used for modelling the transition of a wave between one point and another. By arranging a grid, it is possible to see how sound moves through the grid placed over a model scenario. The propagation of this wave is dependent upon the ease (impedance) of how each point in the model moves.
The principle is seen in the slinky and dot model shown, where the top of the slinky represents a node. The wave moves by vibrating each of the nodes in turn, along the length of the slinky
As an example of FDTD, this image illustrates a grid placed over a casement window. In this case, the grid is square in shape. The nodes at each of the points where the grid meets the model, are fixed solid. Alternatively, nodes can be made semi flexible such to represent acoustic absorption.
The grid is then excited at a fixed point, resulting in a sound wave propagating across the grid, which in turn allows MACH to visualize the visualization of sound.
Computational Fluid Dynamics
It is clear that there is little point in enhancing the acoustic performance of an open window, if the acoustic enhancement restricts the airflow through these windows.
MACH Acoustics utilises the test facilities at the Hive such to measure the on site pressure drop/discharge coefficient of windows. This is also complimented with our in house modelling software, allowing for detailed Computational Fluid Dynamics assessments to be undertaken to window types being proposed by MACH Acoustics.
The Hive – Sound Testing
The HIVE is a £1m ground-breaking building project designed and supported by the University of Bath to allow for research into construction materials, and is the first building project being developed at the Building Research Park.
MACH Acoustics has access to this facility as a result of our lecturing and research at the University of Bath. As shown, MACH Acoustics have been adapting a ceil at the hive into an acoustic labratory standard test facility.
As a result, MACH Acoustics now has the facility to undertake both Acoustics testing and Air Permeability Testing to open windows and other air vents used in low carbon buildings.