Normal specific sound absorption coefficient of the materials has been determined by using an impedance tube, two microphones, dual channel frequency analyzer and the Indian Institute of Technology Kharagpur developed MATPRO software36. Test has been done as per ASTM E-1050 standard19. The impedance tube fabricated at the Indian Institute of Technology Khaargpur is used to measure the normal specific sound absorption coefficient of the biocomposite materials is shown in Fig. 6.6. Noise reduction coefficient (NRC), a simple quantification of absorption of sound by an acoustical material was calculated by averaging the four values of acoustical normal specific absorption coefficient at specified octave band levels. The NRC val­ue lies anywhere between 0 and 1. Higher the NRC value better is its sound absorb­ing property. Figure 1.7 gives a comparison of the normal specific sound absorption coefficient of some of the sound absorbing materials used for noise control. Each of the material in Fig. 6.7 is of 25 mm thickness with a rigid backing. The normal specific sound absorption coefficient of all the materials increases with frequency. Fiberglass and wool have high sound absorption coefficients. Gypsum board has a low sound absorption coefficients since it is denser with less porosity. The natural materials like jute, cotton and coir also have relatively high sound absorption coef­ficient. The polyurethane open cell has a sound absorption coefficient higher than that of gypsum though less than the porous natural materials and fiberglass.

Table 6.9 gives the values of the normal specific impedance with both the real and imaginary part of a 50 mm jute felt with a density of 117.2 kg/m3 as a function of frequency from 100 Hz to 1000 Hz. The jute felt was provided with a rigid backing. The data in the Table 6.9 can be used in numerical simulations20.