Improvements to the Johnson–Allard model for rigid-framed fibrous materials

Measurements of the surface impedance and the physical parameters of seven glass wool samples and six polyester fibre samples with flow resistivities between 4100 and 69,900 Pa s m−2 have been made. Comparisons of measured absorption coefficients and those predicted from the Johnson–Allard formulae using the measured and deduced physical parameters show discrepancies that exceed 20% for some samples and frequencies. By modifying the Johnson–Allard formula for effective density and by introducing a correction factor that is a function of flow resistivity based on data fitting, it has been found possible to improve the predictions. However, it has also been found that a similar modification of the formula for bulk modulus is necessary to reduce the discrepancies with data to below 5% in the frequency range between 800 Hz and 5 kHz.