A material’s sound absorbing properties can be described as a sound absorption coefficient in a particular frequency range. Porous absorbers are the most commonly used sound absorbing materials, however they do not have good absorption properties at low and mid frequencies. In order to increase the absorption properties at low and mid frequencies, porous absorber should have larger thickness and size. By using perforated plates backing with porous absorber, it is possible to gain higher noise absorption at low and mid frequencies. This study was conducted to investigate the effect of perforation size ratio, mass, air gap and holes diameter of perforated plates as backing plate for porous absorber. Coconut fibres were used as backing medium for perforated plates. Perforated plates were fabricated using two different materials; aluminium and zinc plates with perforation size ratio of 0.10, 0.15 and 0.22 at thickness of 1 mm. The holes diameters of the perforated plates are 2 mm, 3 mm, and 4 mm, respectively. The tests samples were tested at the acoustic lab of the Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia according to ASTM E 1050-98 international standards for noise absorption coefficient. The data presented the perforated plates backing with coconut fibers have perforation ratio less to 0.22 showed the significant absorption at 800 Hz – 1800 Hz frequency range. The perforated plates backing with coconut fibers that their holes diameters are 4 mm also showed a good absorption at 600 Hz – 1800 Hz frequency range. However, perforated plates backing with coconut fibers showed a decreasing absorption coefficient at frequencies more than 1800 Hz.