Abstract:

ESDU 00024 introduces a computerized method for estimating the sound power attenuation of absorbent linings in constant section, straight, rectangular ducts with axial flow. The method is based on a theoretical model that takes the velocity profile across the duct, assumed constant axially, into consideration. The method can also be applied to annular ducts with inner radius to outer radius ratios as low as 0.5, below which the risk of error increases. Up to ten duct sections with different linings in each can be considered. The programs, ESDUpac A0024 and VIEWpac M0024, permit the user to output results at third-octave band centre frequencies, at specified frequencies, or over a frequency range with constant step increment between the frequencies. Equal energy in each mode or equal amplitude in each mode can be selected. Individual modes can be weighted. The output can be total attenuation at each frequency, or the total attenuation and the attenuation in each mode at each frequency. The format of the input file is explained and a second ESDUpac, B0024, facilitates its construction. The input requires values for the normalized acoustic impedance for each lined section and, for low Mach numbers, values may be taken from ESDU 74003 and ESDU 74004, while for higher Mach numbers, the manufacturer's values will need to be used. Worked examples illustrate the various features of the program.