Electret condenser microphones directionality...

Hi. Is there a way to tell which of these are omnidirectional, and which directional ?

They all appear omni type for lower frequencies. For high frequencies, like most mics they are more sensitive when the source is in front of the mic.

A directional mic has a sensitivity pattern which is cardioid (heart-shaped). I believe it has to do with a barrel around the mic, having holes drilled at certain locations, to create a cancelation effect of sounds whose source is in back of the mic. There is also a variety known as the 'rifle mic' which may combine more than one mic and more than one barrel.

Thanks.
So the venting holes in the back in some of the above are not indication of directionality; are those for noise canceling feature ? Their front perforations being all the same, a single centered port ?

There is no sure way to tell without looking at the data sheet, which will identify if it is unidirectional or omnidirectional.

You better ask the datasheet, but a few obvious properties can be determined though. A microphone with only front opening is a pure pressure transducer and can't be directional. Microphones with multiple evenly spaced holes on the backside are candidates for directional types.

The vents in the rear or sides of a mic do not cancel noise, they cancel all sounds from the rear making the mic directional. But I think a vented directional mic must be close to your mouth or it also cancels your voice.

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.

I was wrong. A directional mic does cancel background noise. But it also cancels your voice if you are not near its front.