Directional Mic - using only aural tubes

[Canyonland]

On my factory floor we use a purely acoustic device, much like a
stethoscope, to speak to the office - everything has to be dielectric.
The tube goes through the glass window and into the noisy factory.
It is only one way communication, as instructions are only passed from
the factory to the office. Our proplem is that as our machinery is
getting louder, the office staff are finding it too noisy when
listening on the tubes.

My question is- How can I modify the tubes so as to eliminate
background noise as much as possible and still allow the voice of the
tube holder to come through to the office ?

I have been researching how directional mics work and other things
like; frequency filters, comb filters and phase cancellation.

Would something like a shotgun mic work? Or, I have heard a more
common approach these days is to have a single tube, with sound entry
ports all the way along it, simulating the effect of many tubes.

We can't use anything electric
or with metal parts, not even small batteries. I was hoping to
overcome this problem, by design with some sort of mechanical noise
reduction... I wonder if this is posible ?

Regards....John

 

[Kasper Pedersen]

On my factory floor we use a purely acoustic device, much like a
stethoscope, to speak to the office - everything has to be dielectric.
The tube goes through the glass window and into the noisy factory.
It is only one way communication, as instructions are only passed from
the factory to the office. Our proplem is that as our machinery is
getting louder, the office staff are finding it too noisy when
listening on the tubes.

How much is pickup through the tube?
An idea could be to do fiber optic pickup from a plastic membrane - it
can be made to look and feel like an electrical microphone. You can even
put a PTT 'switch' on it for the office people's comfort.
If you build the microphone open, it will be pressure-gradient sensitive
instead of absolute-pressure sensitive, cancelling out ambient noise
quite well.

I think 'Phone-or' would be a good thing to put into google. They make
FOMs

/Kasper

 

[Canyonland]

Thanks. I'll look into some of your suggestions. It's something I
would never have thought of... Cheers.
-John-

 

[Canyonland]

How much is pickup through the tube?

Oh, Pickup is strong. We just use a funnel shape. Is there a certain
shape mouth piece that, when pointed at the face of the speaker, could
help to block some surrounding noise ?
-John-

 

[Rich Grise]

Yes, it looks something like a jet pilot's oxygen mask. They were the
mic part of headsets we used near running airplanes in the USAF. They'll
work if people don't mind sticking stuff right against their face. Maybe
you could get little face-sized ass gaskets.

Good Luck!
Rich