ESDU 74003 gives a method of estimating these acoustic properties of fibremetals and polyimide laminates used to line aircraft propulsion ducts. The materials are 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 160 dB in the absence of flow. The absorption coefficient is a function of the physical properties of the liner@ and the impedance a function of both the liner physical properties and the noise environment. The method has been proved against test data for frequencies of 1 to 6.5 kHz@ for air gaps of 0.25 to 2 in (6 to 50 mm)@ for liner thicknesses up to 0.06 in (1.5 mm) and porosities (that is@ one minus the ratio of material density to fibre density) of 0.8 and for normal temperature and pressure.