The interior sound field is measured again for those two new
cases, with results displayed in Fig. 6. It can be seen that further
sound reduction (compared to the non-partitioned case in Fig. 6
is achieved by partitioning the original backing cavity. A broadband
reduction of the sound pressure is visualized above 800 Hz.
As a matter of fact, the noise reduction after installing the partitions
can be readily perceived by ears during the measurement.
The effect of partitions on the longitudinal modes could be more
clearly observed at the first three resonant frequencies, which correspond
to the first three longitudinal modes of the system. With
no partition, these three resonances are nearly unaffected by the
MPP liner, in agreement with the observation reported in the
numerical analyses. When partitions are introduced, however,
these three resonances are reduced and slightly shifted, due to
the destruction of longitudinal modes in the annular cylindrical
cavity. It is relevant to note that, despite of the improvement on
the interior sound field, the two partitioning treatments are found
to provide roughly the same control performance. This is caused by
the symmetric acoustic source that the loudspeakers provided. In
fact, the loudspeakers installed coaxially to the pipe could only
effectively activate the longitudinal modes, but not the circumferential
ones, thus neutralizing the effect of the partitions along
the circumferential direction. This will be further demonstrated
in the next section. In brief, as a rule of thumb, if the acoustic
excitation is symmetric with respect to the axis of the cylinder,
partial partition would be sufficient for improving the sound
absorption of the MPP construction.