Internal project of the Applied Mechanics Group
Dr.ir. Ysbrand Wijnant (Y.H.)
My research is oriented towards the development of new methods for measuring sound absorption. Current methods are all based on an overall model of the sound field that is present during the measurement. However, such models are represent an idealized environment, and accordingly, incorrect results may be obtained if the actual sound field deviates from the model. To be able to obtain accurate results in non-ideal sound fields, we have developed new methods for the measurement of the normal- and oblique incidence sound absorption coefficient.
The first newly developed method is the Local Plane Wave (LPW-) method, and is suitable for normal or near-normal incidence. By using a simple sound field approximation based on a local plane wave assumption, the active- and incident acoustic intensity can be measured with a single measurement. By spatial integration of both intensities, the area-averaged sound absorption coefficient is determined.
An example of a measurement is the measurement of the sound absorption coefficient of a road noise barrier in Enschede, see Fig. 1.
Figure 1 Road noise barrier with marked scan area. Inset: the fibrous material leads to a porous surface
Three measurements were taken, resulting in the three curves in Fig. 2:
Figure 2 Normal incidence sound absorption coefficient vs. frequency (3 curves)
Another example is the measurement of the spatial variation of the absorption coefficient for a car seat, as shown in Fig. 5. The three color maps on the right side show the distribution of the absorption coefficient for thee octave bands. It is noted that sound absorption varies strongly with position, and that single structural details of the seat become increasingly well visible with an increase of frequency.
Figure 3 Distribution of the normal incidence sound absorption coefficient for a car seat.
The second method is conceived for both normal and oblique incidence and is called the Local Specular Plane Wave (LSPW-) method. Again, the sound field is approximated by an incident- and a reflected wave, however, both waves now represent a local specular reflection. In combination with spatial averaging, measurements can be performed in non-ideal sound fields. As an example, the sound absorption coefficient was determined for incidence angles varying between 0 and 60° for a perforated panel, see Fig. 3.
Figure 4 Sound absorption coefficient vs. frequency for varying angles of incidence for a perforated panel, backed by a 50 mm cavity filled with a mineral wool. The black squares indicate a measurement of the same panel in a reverberation room acc. ISO 354.
Furthermore, a new 3D sound intensity probe was developed within this project. Figure 5 shows the probe in front of a metal plate. This probe consists of 8 digital MEMS-microphones. The quality of such microphones is highly consistent, and therefore, such microphones exhibit very good phase matching properties. The electronic prints of this probe were supplied by CAE-Systems (www.cae-systems.de).
Figure 5 3D sound intensity probe (8p-probe)
OutlookIn combination with a non-linear solving method, the LSPW-method has potential for application in situ. By means of a non-linear solver, at each frequency, the angle of incidence and the corresponding sound absorption coefficient are then determined. Application for non-locally reacting surfaces, or even scattering surfaces, may become realistic if the local field assumption, on which the LSPW-method is based, is extended with more waves. However, a sophisticated numerical solver will be required to be able to determine the amplitudes of the incident and reflected waves.
See link on my personal page.