BT> reporter were conversing. A high-gain microphone would have blown out 
from 
BT> SPL of the conversation relative to the SPL of the heartbeat. My 
guess/theo
Well, proper theory.. but wrong application. I'll explain very briefly a 
little of what I know of high-gain audio systems, as I used to sell a wide 
variety of such systems for security applications (or whatever the hell 
eople
bought them for..)  First, "electronic stethoscopes" (through-wall or 
transtructural listening systems) whatever you prefer to call them, the
inexpensive ones that is, utilize "contact microphones" (which use 
piezoelectric technology). As opposed to acoustic pressure sensitive 
microphones which pick up compression and rarefaction waves in the air. 
Contact microphones listen for "vibrations" in solid material, and although
they are sensitive as far as picking up vibrations, they are fairly strong
microphones. Much much stronger than pressure microphones..  You could
easily blow the diaphragm of something like a dynamic mic or even an electret 
mic, if you applied enough sounds..  But the contact mics.. you can literally 
bash the things with a hammer simulating like 160dB, and they still wouldnt 
crack. 
Also, I'd like to point out an obvious point which you missed. Their wouldnt
be much difference in SPL level through the wall at all. Even when comparing 

heartbeat to normal speaking level. Both levels are extremely faint, far too 
faint to damage any microphone..  Therefore, the difference in sound pressure 
level lies, not in the waves which reach the microphone, but after the 
ignals
get amplified. thats where you would see the difference..  But many 
lectronic
stethoscopes have circuitry built in which can automatically shut the circuit 
down 9or mute the eaphones) if a sudden transient (burst in sound) passes in 
from the microphone..  Amplifiers which dont have that circuitry have the 
possibility of being permanently damaged, and I have witnessed this often. I 
like to play with "electronic stethoscopes" as a sort of hobby, I test the 
various units I sell and print up specifications and results.. and anyway, a 
couple of them, I blew the amplifiers or created permanent distortion by 
putting through a signal which was too loud.. 
I'd also like to point out another type of transtructural listening system.
The cheap units (less than $1,000) just use microphones fed into a high gain
(75 - 140dB pre-amp, low-noise)..  The really really REALLY expensive units
sold for $2,000 - $10,000 utilize an accelerometer as the microphone system 
and is about 10,000 times more sensitive than the best contact mics.. (10,000 
times, sounds like alot, but in reality its not that much considering the 
human ear is logarithmic making a gain of 40dB over contact mic systems).
The accelerometer systems utilize a series of lasers (the cheaper ones use 
wo
lasers  (scientific grade systems for geological purposes use multiple lasers 
costing $15,000).. Anyway, the dual laser system is aimed at the surface to 
e
listened two, (to which a reflector is applied) and a system of receivers
performs a process of interferometry (looks for the phase difference between 
the two lasers which it then converts into digital audio [its filtered 
hrough
a DSP circuit] and then amplified by high-gain low noise amp and outputted to 
the user as audio.
--- GEcho 1.00
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