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Direction microphone questions Answered

A few days ago a friend gave me a directional mic he used with his camera.
Of course I don't get these things as a working present but to repair them :(

Taking it apart to expose the inside was the first nightmare as for some reason my friend decided to use a splash proof mic on a totally unprotected cam.
If you ever tried to take one of these aquarium tube heaters apart you know what I mean...
To my surprise this formerly expensive toy failed due to a bad solder connection, easy to fix.
And lucky as well because the actual microphone capsule was glued in place pretty good.
A quick test on the computer with the supplied microphone amp showed it worked fine again so I wanted to find out how it actually works.

They toy ones use a half sphere or mini dish to reflect the soundwaves onto the mic.
My friends however was an aluminium tube of about 20cm lenght.
15mm in diameter with the mic being just 5mm and sitting in what seemed to be the end of a little funnel inside the tube - right at the end.
So in theory I would now just say the thing gets what comes in from the front, but well that would be far too easy...
When I pulled that fluffy sock off the mic I realised two weird things:
6 small holes going straight through the mic funnel and outer tube.
A glued in relatively soft plastic sleeve with a wave pattern.
It seemed this pattern changed on the way down but I could not really see inside properly.
Closing the front hole and the mic had no problems getting all sorts of noise from all directions, quite lower in volume compared to a direct hit from the front though.
Closing the small side holes with some tape and the mic is still very directional but also really noisy.

It seems some acoustic magic is happening in the tube.
No real clue though but I guess the sleeve acts like the sound dampening in a test room.
Without it all sound entering the tube would reach the mic, while with it really only a very tiny, front facing angle is able to reach the mic.
The aluminium certainly transmits some sound onto the mic and the glue appeared to be rock hard as well.
So I have to assume that the sound entering through the tiny holes somehow cancels out the noise transmitted by the tube itself.
With just 15mm in diameter however I can calculate what I want but am unable to get any true 180° phase shift happening.
Looking up how other types of directional microphones work was a bit helpful here but not so much in understanding the holes and liner.
My friend said it is well over 30 years old and that he got dirt cheap at a garage sale.
No markings anywhere and the small amp only has the connectors for mic, power and output.
Did not check the circuit board inside as the housing was glued together.

Why holes in the side right at the height of the mic?
Why does it "hear" all surrounding sound when the front is closed but only directional with the front open?
And I mean directional even with some music playing close the mic :(
By the way: Battery pack for the mic was the classic four pack at 6 Volt.


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Best Answer 1 year ago

From https://www.audicus.com/directional-microphone/

How Do Directional Microphones Work?

There are a few ways that directional microphones can function. A microphone can be made directional by adding ports on both sides of the sound-receiving sensor.
Sounds that come from both sides, which is most often background buzz,
cancel each other out and allow the listener to concentrate on sounds
coming from one direction.

The most advanced directionality
comes from using multiple microphones. With two or three microphones,
the directionality is improved because the shorter distance between
microphones allows them to be hyper-selective about which sounds to
cancel and which sounds to concentrate on.
With this it is clear:
- Holes to let the surrounding sound in and cancel itself out and making the mic directional
- If front is closed, the directional properties is lost and it simply becomes a omnidirectional mic again
- If the holes are open, it is a directional mic again and does its job: Only if sound is generated in front (or back) it wont cancel out. From all other directions, it will cancel itself out doe to the stregically placed holes letting in the sound from the front and the sides to both sides of the diaphragm of the mic. If you examine the sensor itself, you will propably find entry-ports in the front and the back/side of the mic-tube itself. the side-ports propably lead to the back of the diaphragm.