The problem of the stethoscope: an enquiry, experimental and philosophical, into the properties of this instrument and the physics of auscultation

Abstract

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.