It is truly amazing how much data can be collected by a skilled physician using only two basic
human senses - touch and hearing. The skill of diagnosing through touch is called "palpation"
while the art of listening to patient sounds is called "auscultation". Stethoscope is a device
that doctors and sometimes other medical staff use to listen to sounds produced by patient's body.
Before its invention, they listened to patient sounds directly by ear.
Stethoscopes through history
The first stethoscope, invented in the early XIX century, looked as tiny wooden trumpet and
it both enabled some level of sound amplification via collecting it over a wider area in
comparison to putting a bare ear onto a patient skin and also was less displeasing for patients,
especially ladies. Over time, the design improved in terms of both comfortability and the
level and clarity of sound reproduced. The two most significant improvements in stethoscope
design worth mentioning are the addition of flexible rubber tubings, which made stethoscopes
not simply much more comfortable but as well much safer for doctors to use, and the improvement
in the design of a so called "chestpiece" i.e. a metallic bell used to collect body sounds
through patient's skin when pressed onto it. Basic design of this valuable instrument hardly
ever changed from mid XIX century until the two last notable enhancements were made in the
mid XX century by Rappaport and Sprague and some years later by dr Litmann, both concerning
the mechanical and acoustical design of the chestpiece with the aim of providing improved sound
clarity and external noise reduction.
As of the first decade of XXI century, the most common type of this valuable instrument is still
a purely acoustic one. Despite today's stethoscopes can be in terms of the level of sophistication
compared to top quality musical instruments, they cannot overcome the traditional weakness of
all passive devices - there is hardly any gain in sound level they provide.
Electronic stethoscopes
As can be expected, all issues mentioned above can be relatively easily overcome using electronics.
The most obvious gain of processing signals electronically rather than purely acoustically is - gain.
But designing an electronic stethoscope is a bit more difficult than it might seam at first.
There are several factors that make this task a bit tricky, such as:
-
Signal levels of interest are rather low which necessitates the use of very low noise
preamplifiers. This is not too hard to achieve with today's components but does require
that we design circuits very carefully. In later text it will be explained how this can
be achieved without using extremely expensive top quality integrated operational amplifiers.
-
Frequency band of interest is significantly different from the usual 20 Hz - 20 kHz
audio band. The reason is simply the nature of sounds that human body produces.
Fortunately, the highest frequencies that need processing are not much higher than
1 KHz. Unfortunately, the lowest ones are even lower than 20 Hz. Since human hearing
is very insensitive to such low frequencies, such signals must have very large
amplitudes in order to be audible. This means that stethoscope needs a relatively
high power output amplifier despite relatively low subjective sound levels it produces.
-
A distinct problem is the choice of a sound sensor type that is optimal for the
purpose of collecting body sounds through skin and than efficiently converting them
into electrical signals without introducing distortions. As the device described here
is a toy, we will connect a standard electret microphone capsule to a standard chestpiece;
professional designs utilise very special purposefully made transducers, for example
small piezoceramic discs embedded in carefully shaped acoustic foam etc. This is currently
far out or reach of hobbysts as such proprietary parts can't even be purchased from producers
legally :)
So, making a high quality electronic stethoscope is not a trivial designing task. But if
everything works fine, electronic stethoscope becomes a much more powerfull instrument than
its acoustic predecessors. Not simply does it enable sound amplification but as well arbitrary
filtration, noise reduction, sound recording and reproduction, archieving and sending recordings
to consultants via Internet...
The rest of this article describes a working example of a simple audio frequency amplifier suited
to fit constraints as described on this page. The circuit provides a notable amplification of weak
sounds without introducing perceptable distortions. It can function as a standalone demonstrative
device which can introduce doctors to the world of electronic stethoscope sound processing.
More skilled hobby designers can consider it a frontend component of a more elaborate sound
processing system. Such system can be formed by adding analog or digital equalizer, a more powerful
output amplifier, a kind or a recording device such as a PC sound card etc.
