which consists of a loudspeaker mounted on the base of a cone
cabinet. The acoustic energy feeding the small aperture at the
top of the cone is concentrated and radiates into the cylindrical
domain through a small opening on the wooden pad. The sound
pressure level inside the cylindrical enclosure for a point acoustic
excitation is measured again for the four cases, with the results
given in Fig. 8. It can be seen, partial partition treatment presents
better noise control performance than the non-partition treatment.
When the full partition treatment is installed, an overall best noise
control performance is obtained. Compared with the symmetric
acoustic excitation case, the non-symmetric acoustic excitation
effectively activates both the longitudinal and circumferential
modes in the annular cylindrical cavity. The full partition treatment
successfully destructs both types of modes, responsible for
the deterioration of the energy dissipation efficiency of the MPP.
In more general cases where the acoustic environment inside the
enclosure is either non-symmetrical or difficult to be predicted, a
MPP liner with fully partitioned backing cavity is preferred.