The acoustical transmission loss of an isotropic material can be estimated by Eq. (13), which characterizes the sound blocking capabilities of sound barrier materi­als. This equation states that for every doubling of the frequency the transmission loss increases by 6 dB. For a uniform density of the material, the transmission loss also increases by 6 dB. This is true below the first coincident frequency of the panel made out of such materials. However for composite materials Eq. (13) is not ap­plicable since the mass density is not uniform. It is recommended to measure the transmission loss in such case. The transmission loss (TL) of materials can be de­termined experimentally and standards exist for the same37,38. A view of the setup used to measure the TL of the biocomposite panels is shown in Fig. 6.10. The two microphone sound intensity probe is used to measure the sound intensity with and without the sample whose transmission loss is to be determined. A random noise source is placed at the bottom of the box39.

FIGURE 6.10 Transmission loss being measured.

TL = 20log10(m/) -42 [dB]

where, m is surface density in kg/m2 and f is frequency in Hz.

The coincident frequency, / for a panel can be estimated by Eq. (14). At the coincident frequency there is drop in the value of the transmission loss of the material.

where E is the modulus of elasticity of the material, h is the thickness of the panel, c is the speed of the longitudinal sound wave in the material, rs is the density of the material and v is the poison’s ratio.

The natural rubber based jute composites were prepared as per the following procedure39. Jute felt of 400 gsm specimens were dried in an oven for lhour to re­move the water content in the specimen. The jute felt were treated with 1% NaOH (alkali) solution for 1 h. This alkali treatment was used to remove the impurities in the specimens. These alkali treated jute felts were again washed by water till they became alkali free. The washed jute felts were dried in an oven at 80°C for an hour. The dried felt was then dipped in 1% (by volume) natural rubber solution for 1 h. Excess rubber latex was drained off and the rubber treated jute felts were dried in a dry room for 1 h. Jute-based natural rubber latex composite was prepared by press­ing ten pieces of natural rubber treated jute felts in a hydraulic press at 140°C with a load of 8 ton for 15 min. Similarly 2.5% natural rubber, 5% natural rubber and 10% natural rubber jute composites were prepared keeping all other parameters same. In all the sample preparations, natural rubber was used as bonding agent between the interfaces of the fibers.

The measured TL of all the samples are given in Table 6.1136. Usually a single number is preferred to represent the transmission characteristic of a barrier material known as the sound transmission class (STC)37. The STC ratings of the measured natural rubber latex treated jute composite of 5 mm thickness is shown in the last line of Table 6.9. These jute base composite panels of 5 mm thickness have compa­rable STC ratings to that of 3 mm Aluminum plate. The mass density of the panels have a strong influence on the TL. Higher the surface density higher will be the TL.