4. Conclusions

This chapter has studied the behavior of nanofibrous membrane in a synergy with a grid over the sound waves incidence. The sound absorption coefficient of a frame-based element in the form of linear structures overlapping the nanofibrous membrane over its entire back surface has been measured. A nanofibrous membrane having a basis weight of 6, 3, 2, and 1 g m<sup>2</sup> covering a rectangle grid with the different side dimensions, 4.1 4.3 mm, 9.4 4.1 mm, 9.0 9.4 mm, and 9.0 14.2 mm, has been placed at a constant distance of 30 mm from the reflective wall. Two-microphone impedance measurement tube typ. 4206 was used to measure the absorption coefficient in the frequency ranges 50 Hz to 6.4 kHz according to standard ISO 10534-2.

By measuring the sound absorption of nanofibrous layers with the same area weights covering the grids of different mesh sizes, it was found that the nanofiber layer applied on smaller mesh achieves better sound absorption where two absorption peaks occur at approximately 1500 and 3500 Hz, while nanofibrous membrane applied to the larger mesh grids does not absorb the sound of these frequencies enough. From the analysis of sound absorption measurement results, it can be assumed that the mass of the carrier grid alone does not significantly influence the course of the sound absorption curves. The influence of basis weight on the sound absorption coefficient is not clear for all measured configurations. For the smaller meshes, the sound absorption increases with decreasing basis weight of nanofibrous membrane. Then the antiresonance effect of heavy membrane where the acoustic element loses sound absorption ability (approx. 2500 Hz) occurs due to undamped vibrating membrane. The maximum values of the sound absorption coefficient occur at resonant frequencies of the sound absorption system. The resonant frequencies of 1 g m<sup>2</sup> membrane have been compared with the calculated values, whereas the calculated and measured values are in a good agreement.
