Thanks gentlemen, learned something today.
Went digging deeper and reading patents, transcribing portions of interest about the venting holes here :
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Patent US 4281222 A ...unidirectional electret
...A sound hole is made for intercommunication of the rear compartment with the outside to achieve unidirectional directivity.
...The rear compartment communicates with the outside through sound holes, so that a sound reaching the back of the electret diaphragm from the outside through the rear compartment and a sound reaching the front of the electret diaphragm from in front are properly selected in magnitude to achieve unidirectional directivity.
Patent US4117275 A Non directional electret...
...a novel casing structure for the non-directional capacitive microphone. The casing of the present invention is completely sealed air-tightly except a front wall thereof to prevent any mechanical noise generated behind the casing from entering into the casing and allow the both sides of the diaphragm in the casing to be communicated with each other so that the air pressure behind the diaphragm in the casing is responsible to the atmospheric pressure,...
Patent US3895194 A Directional condenser electret
...Port 74 alone will establish essentially bidirectional operation. If the port is covered by acoustical resistance material 80, spaced from rear member 16 by spacer 76 having opening 78 therein, a unidirectional characteristic preferential to the front is established...
Patent US8509459 B1 noise canceling microphone
...Noise cancellation port 114 is used to cancel out undesired ambient or background noise which arrives from a different angle and originates much farther from the microphone than the voice of the user. Sound waves that arrive at opposite sides of the diaphragm in equal phase and amplitude do not induce diaphragm vibration. This condition is referred to as acoustic cancellation.
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Seems like ... No venting holes=no directionality.