Optimized sound absorption of a rigid polyurethane foam

In this study, cast panel received as sound insulator was
implemented as sound absorber. The panel is composed of prepainted metal skins
with a rigid self extinguishing polyurethane foam. For the achievement of such
material as sound absorber, holes are uniformly drilled in the sample to allow
relatively easy access of sound to the interior structure. Using the two
microphone impedance measurement tube, the sound absorption coefficient was
studied for samples of different, coverings, opening pores (2 mm, 3 mm), open
areas (0.04, 0.16), and at air space depths: 0, 5, 10, 20 and 40 mm from the
reflected wall. The normalized acoustic impedance was also studied. The results
revealed that when the sample was uniformly drilled with pores of opening 2~mm
each to have 0.16 mm open area, and positioned at air space depth (5 mm), it
showed optimum sound absorption greater than 0.80 in the frequency range from
700-1600 Hz. In dealing with noise reduction, this absorption gives, 6.9 dB drop
in noise level.