In this paper, mechanical impedance plate and a Helmholtz resonator
were introduced into the traditional MPP structure as a possible
solution to improve low-frequency absorption. The sound
absorption effect of the micro-perforated panel is mainly focused
on the middle and high frequency when the cavity is restricted.
In order to have a sound absorption effect, the micro-perforated
plate is needed to increase the cavity distance. Mechanical impedance
plate is provided by sticking a layer of thin viscoelastic
material along the rim of the micro-perforated panel. The resonance
frequency of the MIP is designed near the low frequency.
Based on the mechanism of mechanical resonance absorption,
the sound energy at low frequency, which penetrates the prepositive
MPP, can be effectively absorbed. Therefore, there is one
absorption peak between 200 Hz and 400 Hz. Meanwhile, a Helmholtz
resonator was fixed on the MIP to make full use of the cavity
behind MIP. The resonance frequency of Helmholtz resonator is
controlled by the length of the tube to the needed frequency.
Through the experiment, the Helmholtz resonator, using its own
cavity resonance mechanism, has a good sound absorption near
500 Hz. Additionally, that the number of Helmholtz resonators
affects the absorbing performance was discussed. Following this
introduction, absorption coefficient of this composite structure is
calculated in Section 2. Experimental verification is showed in Section
3. The effects of the Helmholtz resonator’s parameters are
exhibited in Section 4. Finally the conclusion is displayed in
Section 5.