Do parabolic mics work through thin walls?

trecker4

Member
2010-09-17 12:53 am
Hi,

hard to find a right internet forum for my problem, I try it here.

My neighbour is able to to hear very quiet sounds (like breathing) through a drywall between us. I know because he does not keep it secret and makes a "counter noise" each time, some kind of stalking.
So I'm very interested in what he uses. I've tried an el. stethoscope once on a drywall and realized that's not possible with it, you would hear clear spoken conversations but not more, instead loud noise from the whole house eg water pipes.

So the drywall is thin, even without auxiliary means I can hear single loud spoken words on the phone when I'm 3 metres away from the wall and my neighbour uses the phone near the wall, maybe 2 metres away. I can also roughly locate him left and right when he makes louder noises eg. when he tidies up.
The sound might also travel partly through or underneath the two doors which are not on that wall.


So my question is: Apart from bugs in the same room, can a parabolic mic achieve that (ie hearing you breathing or very quietly moving on a sofa)? Usually you are told that you need a sight line for parabolic mics to work but how is that with the mentioned thin wall?
If one can listen up to 100 metres outside with a parabolic mic, I imagine it's possible for a few metres inside, with a wall, but also without any wind noise, I have no idea.

Any help of anyone who has used a parabolic mic would be greatly appreciated.
(if you don't have a drywall, can you hear through doors?)
 

Ron E

Member
2002-06-27 10:41 pm
USA, MN
laser listening devices can be quite effective on membranes, like panes of glass, or perhaps a wall. I would think that any listening device would have to deal with a lot of low frequency noise. Hold an spl meter up to your chest once and see how loud your heartbeat is with a mic in contact....
 

Rodeodave

Member
Paid Member
2004-12-26 9:38 pm
in the alps
Yep, that's a creepy thread, especially the title on it's own.
I don't think a parabolic mic works that way. From further away someone's mouth talking appears as a point source, the parabolic thing around the mic itself collects a larger volume of pressure-modulated air and focuses it on the membrane. Sound through a wall is not coming from a point source anymore, it's a plane. Pointing a parabolic mic at a wall would just collect the sound emitted from a small section of said wall.

The question is if this is enough, but I'd suspect that a stethoscope would work just a well in this situation. A laser mic would certainly do the job, but afaik it's not too easy to setup, not too cheap and in order to work needs readjusting and maintenance.
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
Actually a parabolic mic could work, just not as well as in open air. Since it is very directional, it will tend to exclude the noises in his room while picking up sounds coming thru the wall it's pointed at.
Add some amplification and he may be able to hear thru your wall just fine.

Or he is simply pressing his ear against the wall. Try banging on the wall while he's listening. :D
 
Sounds like a job for a pile of bass shakers (find the studs), some 3" drywall screws, and infrasonic signal generator. Play around with it until you can get stuff to rattle on his side of the building. Whenever he gives you the "signal" you can return the favor. If the floor joists run across both rooms you can put some on there too. Or put a surface mic on the wall and send the output through a cheap delay/reverb unit to the power amp and turn up the gain just under the point of feedback. Just for fun, let it actually feed back once in a while. You could even run a line in from the bathroom. See how he appreciates that.
 
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trecker4

Member
2010-09-17 12:53 am
Ok, thanks for the answers.

Maybe it depends on the actual implementation, shape of the deflector and type of (directional) microphone and amp.

the most probable alternative imo, is, he found a hole in the wall or made one and put a simple electret microphone with a wire just before the wallpaper. A cheap amp would suffice then. But a parabolic mic would be easiest because it leaves no traces plus one might get directional information (?).

White noise generator is a good idea but it will go on my nerves. I would prefer sound proofing but then again it would be useful to know what type of microphone is used because it's expensive...Until then: Hip-Hop ;)


ps authorities won't believe you if you have no proof. They will think you're insane then which can be even further exploited by harassing neighbors. The only solutions in such a case are, if you think about it, defense (ie sound proof or moving away) or to take equal measures, that's the creepy thing.
Of course eavesdropping is a huge waste of time for everyone involved. I also suppose the person who does it gets an irrational feeling of superiority over time and can't stop it, it could be like a drug.
 
My neighbour is able to to hear very quiet sounds (like breathing) through a drywall between us.

(if you don't have a drywall, can you hear through doors?)

i lived in this building for a while (the larger one) -

[IMGDEAD]http://img338.imageshack.us/img338/2158/lasalle.jpg[/IMGDEAD]

a stout building,

all concrete, and steel.

it's truly amazing what can be heard,

especially at night.

sometimes i'd lay in the bathtub, water covering my ears.

and just listen.

refrigerators turning on, sound much like the 8-track,

in those soundesign receivers.


[IMGDEAD]http://img375.imageshack.us/img375/6662/lasallehotel.jpg[/IMGDEAD]


at night -

[IMGDEAD]http://img191.imageshack.us/img191/7453/lasallebodine.jpg[/IMGDEAD]
 
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Yes, a parabolic mike will indeed work through a thin wall.

A parabolic reflector is designed to take a planar wavefront and reflect it into a focal point with time coherence. If a wall is vibrating as a result of sound energy, wherever it is vibrating in a planar fashion, the reflector will work correctly.

What will be lost is the directivity information if the wall acts like a piston source. In general, it will not be possible to discern the origion point of the source on the other side of the wall. But the parabola will need to be pointed directly at the wall, as any angle will destroy the pickup ability, with hf being lost first as a result of wavelength issues.

Cheers, John
 

Rembrant

Member
2007-03-17 5:44 pm
As others have previously noted, there are simple and effective ways to end your problem. My suggestion involves the early hours of the morning and a couple of packs of black cat fire crackers. If you really want to be creative, you can use a metal trash bin as the blast protection device and a sound "enhancer"..:D
 
Would a sufficiently thin wall act more like a planar array of sources, excited by the incoming wavefront, rather than a piston? There would be the problem of reflection, and some phase shift at the edges becuase they would be less free to move, but I would expect a parabolic system to work reasonably well. You might even be able to achieve partial cancellation of reflections by using another mike and a time delay.

You could check for a mike in the wall by using a metal detector, such as those used by electricians to stop them drilling through wires.

Just one question: you are the recipient of this unwanted attention, and not the perpetrator seeking better technology?