Various forms of cover screens are used both to protect the porous absorbers from damage77 such as loss of fibers,14 to fine-tune their absorptive performance to meet practical demands,13 and to give a more esthetical appearance.10 Cover screens may be in the form of mineral wool felt sprayed on plastic, steel wool, mineral wool or glass fiber cloth; wire mesh cloth; or thin perforated metal. They are characterized by their specific flow resistance, r, and their mass per unit area.17 The addition of a cover screen increases the sound absorption at low frequencies substantially, if it is not in contact with the porous material and able to vibrate freely. However, this may be at the cost of attenuation at high frequencies if flow resistance, r, of the screen is higher than 1 pc.35 If the cover screen has very low porosity77 or is in contact with the porous absorber, the result will be decreased high frequency absorption with unaltered low frequency absorption14. The thinness and the lightness of the film in­crease the absorption.75 Rebillard et al.78 reported that a heavy film behaves like it is in contact with the porous absorber whether or not it is so. Jayaraman et al.47 found the presence of PVC film on the side facing the sound source had a positive impact on NAC values in the frequency range below 4500 Hz. In this case, the curve has a completely different shape, similar to a bell shape rather than a typical “S” curve in 500-6400 Hz frequency range due to the decrease in higher frequencies. The maxi­mum absorption was recorded to take place at 2200 Hz. When the PVC film was placed at the backside, it caused a slight increase in NAC values.

Similarly, Seddeq et al.56 reported a shift to the lower frequency value of maxi­mum sound absorption from 6300 Hz to 2250 Hz ofjute mats with the incorporation of a perforated sawdust board as a cover plate. In terms of the absorption coefficient for frequency values higher than 3500 Hz a drastic decrease was observed, as “S” curve became a “bell” curve similar to that reported by Jayaraman et al.47 When a cover plate and a back air space were incorporated to the porous absorber at the same time, multipeaked graphs of sound absorption was observed as seen in Fig. 5.11.