Influence of PCB acoustic vias on microphone trans…

With the IM68A130V01, the manual states that the acoustic opening diameter in the PCB should be larger than the acoustic opening diameter of the MEMS microphone to ensure optimal performance. The recommended acoustic opening diameter of the PCB is 0.6 mm. We tried 0.4, 0.6 and 1.2 mm in our tests and found that different sizes of acoustic apertures have a frequency selective effect on picking up sound signals, i.e., the size of the aperture affects the output of each frequency band, and it will amplify the output of a certain frequency, with different sizes of apertures amplifying different frequency bands. Have there been cases of this before, please? Is this the Helmholtz resonance effect?