I built one of those gadgets back in my high school days
for science class. I glued several types of microphones
into holes drilled in suction cups of all types. Then built
a high gain amp and used headphones. I recall we used
plumbers putty around the edge of the cup for use on
locker room cinder block walls, all that was needed was an
air tight seal. The thing worked real good and was a lot of fun.
Not much high tech needed for this application.
Speaking of walls.. I wonder what happened to the guy who wanted to use
a dog repeller on some neighbor's mutts that were several hundred feet
I was talking to a lady the other day and she said that they had bought
a dog repeller to use on their neighbor's dog. They're right next door,
and they put it on the side of their house. But she said that it had a
very limited range, so even tho the device worked on the side of the
house, the dog would just go into the back yard and keep on barking.
THis unit has a bark sensing circuit, and an ultrasonic element to
produce high levels of sound near the unit. But I guess ultrasound
doesn't travel as far as regular sound as it gets absorbed and
dissipates faster. So her unit isn't effective enough to cover the
whole area. She saisd that it cost something like $70 from some company
If you have physical access, you can put a little transmitter in the
room ( a 'bug' .) There are various circuits on the internet you can
use to build an FM transmitter. They are suprisingly simple to build.
Tune it in on your stereo, and you have it.
If you don't have physical access, then you are limited to
amplification of some signal. The problem with this is that the
amplification will also amplify ambient noise. You can limit this by
attempting to muffle stray noise. Get a bucket, stuff it with rags or
something like that, and put a microphone in the center on top. Plug
the microphone into your stereo, put on headphones, and turn it on.
Put the bucket against the wall, and turn it up.
This is clearly an invasion of privacy, and possibly illegal, so I
wouldn't actually do it.
Well... my knowledge/experience is a bit dated.(used to work in
surveillance year back.)
But will share what I know.
The do make variuos "Big Ear" microphone rigs that consist of a
variable electret mike and amplifier with closed air headphones.
I've seen some stick those up to a wall with some success.
There are some mikes designed for this use as well.
Not legal in some states I've been told. You can do a web search for
spyshop or spystore, they'll have them for sale.
If you can dig in the wall as suggested above you can use the method
we used to use in the military years gone by.
use an iron or wood spike driven into the wall with a transducer
pickup on it fed into a amplifier rig that'll both amplify the
vibration and shift them down to the audible spectrum.
They also make spike mikes that do work on regular(drywall etc..)
walls. you might predrill and give one of those a try.(you;ll have to
judge the thickness of the cinderblock or concrete and under drill
just a 1/16 or so to avoid detection.)