Sound Absorption Measurements for Micro Perforated Plates: The Effect of Edge Profile
Regular paper
Eindhoven University of Technology
Wednesday 3 june, 2015, 15:40 - 16:00
0.7 Lisbon (47)
Abstract:
In this study, the effect of perforation edge profile in micro perforated
plates (MPPs) is investigated and compared with the theoretical sharp edge
case. MPPs are metal plates with small holes (perforations) that absorb
incident sound waves using the Stokes boundary layers developed inside and in
the vicinity of the perforations. The perforation diameter should be optimized
so that the Stokes layers cover almost the entire perforation thus maximizing
the absorbed sound energy due to viscosity. Hence, designing MPPs is a tuning
problem.
The existing analytical model for evaluating the transfer impedance of the MPPs
is based on the assumption that the edge profiles of the perforations are
sharp. In other words, the angle between the plate surface and the inner
surface of the perforation is 90<sup>o</sup>. This case is not very practical
to achieve in mass production of MPPs, since the perforations are formed by
drilling and / or punching in manufacturing.
To investigate the end profile on the performance of the MPP, we design
samples
with different edge profiles. These profiles contain various combinations of
sharp, chamfered and inverse-chamfered cases for the sake of geometrical
accuracy. We carry out open-end reflection coefficient measurements in a
semi-
anechoic room with 6 microphones. The relative error of this setup is measured
to be less than 0.3% for closed-end reflection coefficient between 100 Hz and
700 Hz in the linear regime. The measurements are also compared with a linear
numerical model.
In our measurements, we observe that the edge profile can have an effect up to
30% in resistance and 18% in reactance.