ON EXPERIMENTAL SOUND POWER DETERMINATION IN A CIRCULAR PIPE WITH UNIFORM MEAN FLOW

The in-duct method to determine the sound power spectrum in pipes with circular cross-section and with mean flow by means of the azimuthally averaged sound pressure spectrum has been re-examined. It is found that the sound power determined in this way is correct only for a very special source mode composition and for zero mean flow Mach number. A determination of the sound power by means of the area-averaged sound pressure spectrum is less sensitive to the source mode composition. A correct determination of the sound power spectrum and modal sound power is feasible by means of the area-averaged sound pressure cross-spectral density up to a dimensionless frequency kR = 8, if the measuring accuracy of the sound pressure cross-spectral density is of at least three significant digits. However, this method is very time-consuming, since it requires a large number of measurements. © 1990.