My interest in Stethoscopy dates back to the
time when I first entered the class of Clinical
Medicine. I had previously, while on war service,
been interested in the propagation of sound in
sea water, and finding that acoustics played so
important a part in the diagnosis of pulmonary and
cardiac conditions, I naturally sought to understand both the nature of intrathoracic vibratory
disturbances and the mechanism of their conveyance to the ear.In those early days two great difficulties
confronted me. Firstly, the difficulty of recognising the various pulmonary signs even when
directed what to listen for; secondly, the difficulty of following the explanations tendered for
the mechanism of their propagation and production.By the time I had finished my classes the
former had been partly overcome, the latter remained. And so, immediately after graduation I
found myself engrossed in the literature of auscultation. The more I read the more I became impressed with the need for greater scientific precision, so many of the observations amounting to
little more than mere impressions.As it seemed logical that we should first
endeavour to understand the mechanism of our
instruments, I was ultimately led to undertake
the work of this thesis, not relying merely upon
observations and impressions, but taking as my
guide the concepts and propositions framed chiefly
by those great master minds in sound and sound
sensation, Lord Rayleigh and Helmholtz.It is submitted that the reasoning, mathematical
demonstrations, and experiments of the thesis
warrant the following conclusions.(1) That the apprliation of the exact nature
of things with reference to the vibratory
behaviour both of the stethoscope and
of the tissues is a very complex problem.(2) That these articles demonstrate what
actually is involved in the problem.(3) That the few problems treated in more
detail, e.g. the subject of resonance
point to the hopelessness of anything
like an exact mathematical or physical
interpretation of the behaviour of the
instrument.(4) That there is a great need for exact
physical concepts both as regards the
behaviour of the stethoscope and the
thoracic structures e.g. that we should
drop the term conduction (or at least
restrict it to its proper physical connotation) and try to visualise the
stethoscope and its contained air, as
well as the corporeal structures, as
systems or bodies performing molar
vibrations and not as media in which
wave propagation takes place.(5) That owing to the variable relationship
between sound sensation and the external
physical disturbances to which they are
due, and owing to the manner in which
sound sensation can be modified by education and personal bias in virtue of
the subjective phenomena of analysis and
attention, the exact comparison of any
two instruments cannot be effected; and
having so far no experimental means of
recording and measuring faint sounds it
cannot be done by purely objective
means.(6 ) That the phenomena of selective resonance especially as attributable to the
behaviour of the stethoscope is hardly
ever observed. That it is not to be ex^
pected, because in the first place, the
majority of auscultation sounds cannot be
supposed capable of representation by a
periodic function, i.e. that they partake
more of th6 character of noises, and in
the second place, £he only frequency at
which selective resonance might occur
would be that corresponding to very close
isochronism with the gravest natural tone
of the air enclosed in the chest piece.(7) That what is usually designated selective
resonance on the part of the instrument
is not a magnification of a vibration
emenating from the body surface, but that
it is an adventitious element- a superimposed tonal mass of sound generated by
the enclosed air being set in vibration
by irregular disturbances as well as regular
and that it is related only in intensity
to the forcing vibrations.(8) That true bone conduction in stethoscopy
even in the monaural pattern is largely
a misnomer. That so called bone conduction is really the transmission of a
vibratory motion to the apparatus of the
middle ear by the meatus walls and hence
that the subsequent path, as in normal
hearing as via. the stapes and not as
in true bone conduction by the vibration
of the cochlear walls.That diaphragms cause a modification in the
sound in virtue of their own intrinsic
properties and do not merely serve the
purpose of preventing the encroachment
of the soft parts on the lumen of the
chest piece.That in the binaural instrument with rubber
tubes transmission by the column of air
is alone important. That the vibrations
of the column of air depends on the
variation in pressure in the air in the
chest piece» That the variation in
pressure in the air in the chest piece
is effected both by the vibrations of the
body surface and by rapid variations in
the configuration of the chest piece, and
that the best idea of the behaviour of
this mass of air is to be arrived at by
combining the fundamental concepts on which
the theory of resonators is framed with
the theory of stream lines in the motion of
fluids.That the guiding considerations on which
a binaural stethoscope should be designed
are these»Consideration should first be given to
the acoustics of the instrument,
secondly to the ease with which it can be
used, thirdly to portability.The part in contact with the body surface
should not be metal but a bad conductor
of heat.The actual shape of the chest piece is not
directly a matter of importance.The interposition of a diaphragm means further
modification of the original physical disturbances.The larger the area of body surface covered
the louder the sounds.The thinner and lightly and more highly
elastic (in the physical sense) the walls
of the chest piece and the smaller its
inertia, fee greater will be the contribution to the variation in pressure in the
air it encloses.The adventitious rumbling effeot of the air
chamber should be eliminated either by making
the chamber small or by breaking the apace
up by perforated diaphragms.The exit from the chest piece should be placed where the energy of motion is greatest.The air chamber should be deep enough to
allow the encroachment of the soft parts
within the lumen without producing any
appreciable dimin^ion in the energy of motion
in the vicinity of the exit.Rubber conducting tubes are an accepted necessity
because of their flexibility. Though flexible
metal means louder signals.The ear pieces must fit exactly and without
the slightest discomfort into the meatus and
the normal to the exit should be in the
direction of the meatus. This can be provided
for by the proper curving of the head tubes
and their union by a spring of suitable tension.finally that advance in stethoscopy depends
upon the introduction of an accessory source
of energy controlled to act as a relay probably
through the medium of the oscillation valve
