ESDU 74004 gives a method of estimating these acoustic properties of thin perforated liners used in aircraft propulsion systems. The material is spaced from a solid backing sheet by a honeycomb layer@ and the spacing is critical in determining liner response. The absorption coefficients and impedances are estimated for noise environments up to 150 dB in the absence of flow. Both are functions of the physical properties of the liner. The method has been proved against test data for frequencies from 1.5 to 6 kHz@ for air gaps up to 2 in (50 mm)@ for perforate thicknesses up to 0.1 in (2.5 mm)@ for porosities (that is@ the ratio of holes to the total liner area) up to 0.3 and for normal temperature and pressure.