Modal identification of aeroacoustic systems using passive and active approaches

The noise generated by a ducted diaphragm configuration, taken in this paper as a prototype aeroacoustic system, is investigated experimentally using a two-port source identification method [Lavrentjev, Åbom, and Bodén (1995) J. Sound Vib. 183, 517-531]. The method requires successively the determination of the so-called passive scattering properties of the ducted diaphragm system using loudspeakers, and the extraction of the active noise that would be emitted by the aeroacoustic source itself in an anechoic duct. These steps involve assembling a non-singular acoustic load matrix based on experimental data, and its inversion. This inversion can present numerical robustness issues, especially when higher-order modes are included as in the present study. Another issue is related to the alteration of the duct-termination reflective properties that external loudspeakers modules can cause, yielding a change in duct scattering characteristics for the active and the passive measurements. The present paper revisits established practices in the field and presents an experimental procedure in which the acoustic load matrix is assembled using the aeroacoustic source instead of the loudspeaker excitations. The proposed approach enables improving the conditioning of the load matrix and accurate determination of the duct termination reflectivity. The use of external loudspeakers however, remains necessary for the measurement of the scattering properties of the source itself.