For my project – “beam configurable boundary microphone” I using 2 MEMS ADMP521ACEZ mics with ADAU1761 DSP. The project is almost complete, but I was curious about low frequency respond difference between data from “ Beamforming calculator” and DSP equalization settings. For example, for edndfire array configuration and 21mm spacing I need to boost 100 Hz frequency up to 20db to achieve 0db out as 1KHz, based on calculator data. But actual DSP setting for 100HZ is +10db. I think may be PCB surface acts like a low frequency booster. When I closed acoustical ports with 2mm thick tape, I got approximately 7db attenuation. See attached graphs for open and closed audio ports. My question is: is it PCB or something else boosting low frequencies. Also is there is any formula or calculator for acoustical port opening, based on board thickness and how it’s correlated to frequency respond.