RESEARCHES ON HEARING THROUGH THE MEDIUM 
OF THE TEETH AND CRANIAL BONES. . 
By CHARLES HERMON THOMAS M.D. . /\ , 
Rend before the Philadelphia County Medical Society, December 17, 1879. 
[.REVISED AND REPRINTED FROM THE PHILADELPHIA MEDICAL TIMES, FEBRUARY28, r88o.] 
TN presenting the results of my investiga- 
tions and experiments on the means 
and methods of transmitting vocal sounds 
to the internal ear by the way of the teeth 
and cranial bones, I shall but briefly de- 
scribe the two instruments which have 
been exhibited here to-night, and which 
have received so large a measure of public 
attention during the past few weeks. 
The audiphone* resembles in form a 
Japanese fan, and is composed of vul- 
canite. When in use it is slightly bowed 
by cords connecting the handle and the 
upper edge, which edge is to be held in 
contact with the eye-teeth of the hearer, 
the convex surface being turned towards 
the source of sound. The dentaphonef 
is of the same model as a telephonic 
mouth-piece, its diaphragm being perfo- 
rated at the centre for the attachment of 
a string, which serves to connect it with a 
small bit-piece, the latter to be held be- 
tween the teeth of the hearer, who with 
one hand extends the mouth-piece with 
its opening towards and close to the face 
of the speaker, keeping the string in a 
state of tension. 
It will be seen that the principles upon 
which their action is founded are not new 
to us ; indeed, are very generally known ; 
but the remarkable results obtained by the 
application of these principles in the audi- 
phone and dentaphonej have fallen as a 
surprise alike upon the general public and 
the medical profession. 
Hearing through the medium of the 
teeth and cranial bones is certainly not a 
novelty; on the contrary, the knowledge 
that sound may be thus conveyed has long 
been made use of for a variety of practical 
purposes. The application of the tuning- 
fork by aural surgeons in diagnosis is an 
instance familiar to all present. A patient 
of mine, whose hearing was much impaired 
in early life by scarlatina, informs me that 
in order to learn if his watch was running 
he has for many years been in the habit 
of placing it between his teeth, and of 
testing his clock also by applying his teeth 
to the mantel on which it stands, being 
otherwise unable to hear either of them. 
I he same patient in his youth was accus- 
tomed to listen for the approach of a rail- 
road train by placing his teeth upon the 
track; in this manner he was able to hear 
it some time before those who were about 
him, and who had normal hearing. A 
deaf friend enjoys music by placing one 
end of his cane against the piano and the 
other against his teeth. Another patient, 
an experienced engineer in charge of a sta- 
tionary engine, has always practised listen- 
ing to the sounds of its valves by placing 
one end of a stick upon the part nearest 
them and the other against his teeth, mean 
while stopping his ears with his thumbs, 
he having been taught this method as a 
part of his trade many years ago. 
It has also been known that sound-waves 
produced in the air are communicated to 
contiguous bodies, causing them to vibrate 
in unison, though with varying degrees of 
intensity according to the density, elas- 
ticity, and form of such bodies. Further- 
more, it was well known that vocal sounds 
were no exception to this rule, and that 
these, as well as others, could be trans- 
mitted, under suitable conditions, as longi- 
tudinal vibrations through long distances, 
as witness the familiar examples of the so- 
called “lovers’ telegraph” and boys talk- 
ing through a fence-rail. All this being 
known, it appears that a happy accident 
was all that was-required to show that the 
complex vibrations of articulate speech as 
reproduced in solids might be conveyed 
through the teeth and cranial' bones, and 
thus be rendered audible, as were the 
* Patented September 23, 1879. 
f Patented November 18, 1879. 
J The names " Audiphone” and “ Dentaphone” being 
claimed as trade-marks, and being otherwise objectionable, I 
propose the name Oiteolhone, is a general term to be used for 
all appliances—including the above—intended to aid hearing 
by conveying articulate sounds to the ears through the medium 
of the cranial bones; the teeth not being essential factors. 2 
simpler sounds before mentioned ; and, at 
the same time, to furnish a suitable meavfs 
to accomplish the object.* 
My knowledge of the before-mentioned 
instruments and the results claimed for them 
led me to undertake a series of observations 
and experiments with a view (i) to de- 
monstrate the principles upon which their 
action is founded ; (2) to determine the 
practical value and range of use of these 
instruments; (3) to devise other and more 
convenient and less conspicuous forms of 
mechanism which might be substituted for 
them; (4) to improve the quality and in- 
crease the volume of the sound conveyed ; 
(5) to discover new physiological and 
pathological facts relating to the functions 
of vocalization and hearing; and (6) to 
throw open to professional, and so to pub- 
lic, use the results gained, thus supplying 
data for further investigation and invention 
freed as far as possible from the restrictions 
which patents impose. 
These researches involved the examina- 
tion and oft-repeated re-examination of 
many deaf and partially deaf persons, the 
construction of new mechanism, and the 
testing, in different forms and under varied 
conditions, of a great variety of materials. 
In all this labor I had the valuable as- 
sistance of Dr. Edward C. Kirk, who will 
aid me in the repetition of several ex- 
peri mentsf before you to-night. 
Before going farther, let me say that, 
though a number of novel results will here 
be shown, and some of them not without 
practical value, it is my opinion that oste- 
ophony as a subject worthy of study is not 
nearly exhausted, and that the future will 
develop other and far more useful applica- 
tions of the method than have yet been 
reached. 
The conclusions arrived at, briefly stated, 
are as follows: 
I. As has been indicated, both the audi- 
phone and dentaphone depend for their 
action upon the principle of acoustics that 
solids—in this case in the form of thin 
plates—vibrate in unison with the sound- 
waves produced in the air near them. In 
these instruments the vibrations are of suf- 
ficient force to be audible when conveyed 
to the internal ear through the medium of 
the teeth and cranial bones, i-ndependently 
of the ordinary channel of hearing,—the 
transmission being direct in the audiphone 
and indirect through the conducting string 
in the dentaphone. 
Though, within certain limits, sound- 
vibrations in solids are readily conducted 
in this manner,—becoming audible in all 
cases where the internal ear is intact,—the 
relatively slight vibrations produced in 
dense substances by sound-waves propa- 
gated in the air near them are not recog- 
nizable as sound, except where the normal 
exercise of the function of hearing is sus- 
pended by disease of the middle ear or 
obstruction of the external portion of that 
organ. Normal hearing so far exceeds 
that obtained by the means in question 
as to render the comparatively small ad- 
dition made through them inappreciable 
and as nothing in the presence of the 
greater sensation. 
The size of the sound-receiving sheet 
governs the distance at which the instru- 
ments will act, and the dimensions must 
be increased in proportion as the source 
of sound is removed. The audiphone is, 
therefore, much better adapted than the 
dentaphone for use at a distance, the lat- 
ter being only suited to transmit sounds 
emitted near its mouth-piece. The bow- 
ing strings of the former are not at all 
necessary to its perfect working, as its 
arched form may very conveniently be 
given to it by the hand, pressing its upper 
edge against the teeth. Not tension, but 
the arched form is the condition essential 
to its proper action, for this form is that 
best adapted to impart the impact of 
sound-wayes against its convexity, which 
is then expended as a thrust of the arch 
against the teeth, these forming one of its 
abutments. 
II. That these instruments are of great 
value in a considerable proportion of cases 
of deafness there is no reason to doubt, 
but there is no just ground for the public 
belief that with their aid the deaf are 
enabled to hear as well as those with ordi- 
nary hearing. On the contrary, they sup- 
ply, I am convinced, but a very small 
fraction of normal hearing,—much less than 
a hundredth part. Neither instrument adds 
in the slightest perceptible degree to the 
hearing of those with perfect ears, except 
when the latter are securely stopped. I 
have repeatedly tested both instruments 
* Miss Van B—n, a lady of high standing in a neighboring 
State, writes me as follows :“.... Seven years ago I was 
using a Japanese fan, and, happening to put it within my 
teeth, found, to my surprise, that the conversation of those 
around me was audible. Almost fearing it was not so, I 
made several experiments, and found it to be a fact. Any 
one that has been deprived of the sense of hearing may 
imagine nly delight and thankfulness to a kind Providence. 
I use it constantly, and find it a great help ’’ 
f There were present two patients, upon whom the various 
appliances herein described were tested in the presence of 
the Society. The following experiment was also shown. A 
string one hundred feet long was stretched from the face of 
a guitar placed in the auditorium, there to serve as a sound- 
ing-box, to a tense diaphragm located in a distant room, 
where an assistant was posted, whose voice, as in the repe- 
tition of the alphabet, was made distinctly audible to every 
one in the hall through the volume of resonance developed 
in the guitar. On removing the guitar and placing the end 
of the string in the grasp of the teeth of a deaf lady, and the 
same sounds being repeated by the assistant, she was able to 
hear and instantly to duplicate them, though they could now 
be heard by no one else in the audience. 3 
with this question in view, choosing as 
subjects those receiving from them decided 
benefit in listening to conversation and 
otherwise, and have as yet found no case 
in which a loud-ticking watch was heard 
when held close to the centre of the sound- 
receiver. Furthermore, when the watch 
was placed upon a large Koenig resonance- 
box, thus multiplying the sound many 
times, and this placed so near to the dia- 
phragm as barely to escape contact with it, 
still no sound was heard. Considering the 
law of acoustics, that the intensity of sound 
is inversely as the square of the distance of 
the sonorous body from the point at which 
it is heard, and taking into account the 
distance at which the sound described 
should be heard by the normal ear, my 
estimate of the strength, or rather the 
weakness, of the sound conveyed by these 
instruments will be seen to be justified. 
It is important that this be taken into 
account, for large numbers of the partially 
deaf suffer such disappointment at their 
failure to hear in full that they undervalue 
or altogether disregard a positive gain of 
many times their usual hearing. The dif- 
ference between normal hearing and that 
derived through these means is hardly 
less marked than that between sunlight 
and candle-light; nevertheless, this very 
small fraction is of priceless value in many 
cases, for to those who practically hear 
nothing without them, who sit in acoustic 
darkness, the gain is all the difference be- 
tween nothing and something,—scarcely 
less than infinity. 
In view of certain strongly expressed 
statements which have obtained currency, 
the results to be derived from the use of 
the audiphone in deaf-muteism are likely 
to prove very disappointing. Repeated 
tests show that those who are able to hear 
with the aid of the audiphone hear their 
own voices perfectly without it; while 
those who are unable to hear their own 
voices without it can hear neither their 
own nor any other voice with it. It is, 
therefore, worthless to those who do not 
possess the faculty of self-hearing. 
Practice is required in using either instru- 
ment, especially in cases where hearing has 
long been abolished, and the suggestion 
which has been made, to read aloud to the 
patient while he follows the printed words 
with the eyes, is a good one. Besides 
this, practice with the watch or tuning-fork 
against the teeth, thus directing the atten- 
tion to receiving the sound through the new 
channel, will prove valuable as an educator 
of the disused sense. The rod osteophone 
—hereafter to be described—has a special 
value for the same purpose. 
This class of instruments should be in 
hands and under the special direction 
of physicians, among other reasons, be- 
cause caution is necessary to prevent their 
being relied on to the neglect of proper 
examination and treatment of the ears. 
Probably the most striking results to be 
found from their use would be in obstruc- 
tion of the external ear by impacted ceru- 
men 5 but manifestly it would be malprac- 
tice to make use of such means instead of 
effecting the removal of the offending ma- 
terial. 
It is perhaps too soon to decide the rela- 
tive merits of the ear-trumpet and the 
osteophone, but enough is known to render 
it probable that the osteophone will either 
substitute or supplement the former in many 
instances. And, certainly, it is far more 
agreeable for most persons to talk towards 
the surface of a curved sheet than into 
the cavity of a trumpet. Moreover, the 
distress and injury to the ears sometimes 
caused by trumpets are avoided altogether 
in the osteophones. As to artificial teeth, 
they limit to a great degree the value of 
most osteophorfes. 
III. The audiphone is open to the ob- 
jection that during its use it obscures to a 
certain extent the features of the user, and 
the dentaphone is held more or less in the 
line of vision. Both instruments are open 
to the still more serious objection that 
they each require the constant service of 
at least one hand during their use. With 
neither can the user hear when his hands 
are engaged in other employment. 
To remedy these defects I have made 
the following device. A large receiving 
diaphragm is attached, in an arched form, 
to a rod of wood or metal. The rod is 
bent in the form of a pipe-stem, one end 
of which is to be held firmly between the 
teeth, as a pipe is held (Fig. i), thus en- 
abling the user to listen to sounds about 
him, and, at the same 
time, leaving his hands 
free for other occupa- 
tion. The diaphragm 
being below and away 
from the face, it is 
comparatively incon- 
spicuous. 
Ornamental fans of 
almost all sorts may be 
utilized for occasional use, and, when coated 
with shellac and tipped with ivory or hard 
rubber, may be made to answer fairly well, 
though they will prove unsatisfactory for 
ordinary uses. Many other forms of in- 
struments have been found to answer the 
purpose of conveying vocal vibrations : 
thus, a piece of yellow pine wood, turned 
Fig. i. 4 
out into trumpet-shape, about two feet 
long, with an expanded end three inches, 
in diameter, may be used for conversation, 
the small end being applied to tire teeth 
of the deaf person and the opposite end 
held close to the mouth of the speaker. 
In this manner a very good, volume of 
sound is conveyed. A tense string con- 
necting the upper teeth of the two about to 
converse also answers the same purpose. 
IY. The tone of the voice differs ma- 
terially when conveyed by different sub- 
stances. Where these are lacking in 
resonance (as in celluloid and binders’ 
board), flatness is the result, while others 
which are over-resonant or over-persistent 
in their vibrations (as vulcanite and ferro- 
type metal) yield ringing or confused 
sounds. The quality needed is that pos- 
sessed by good sounding-boards, of instantly 
responding to contiguous sounds and main- 
taining them during their continuance, and 
also of instantly ceasing to vibrate upon the 
cessation of thd causative sound. This 
right sort of elasticity of resonance, that 
capable of reproducing human voice-tones 
in their purity, is possessed to a high de- 
gree by fullers’ board (or press-board), 
which, when treated with shellac varnish 
and thoroughly dried, has proved not only 
far better than other paper or cardboards,* 
but also a great improvement upon the 
sheet metals or hard rubber; lacking the 
“reverberations” and “ roaring sounds” 
of the latter, as they are described by dif- 
ferent patients upon whom they have been 
tested. Besides, owing to its greater flexi- 
bility, it is less destructible than either 
these or the thin sheets of wood which 
otherwise answered the purpose, while its 
cost is comparatively trivial. 
A powerful osteophone, one that excels 
by many times in the volume of sound trans- 
mitted either audiphone or dentaphone, con- 
sists simply of a small rod of hard wood,—a 
convenient size being about two feet long 
which is placed (Fig. 2) against the teeth 
of the speaker, the other resting against or 
between the teeth of the person hard of 
hearing. If the speaker now articulates 
in a natural tone of voice, the vocal vi- 
brations will be transmitted in great vol- 
ume through the teeth and thence to the 
ears of the deaf person. It will also con- 
vey the voice distinctly when placed against 
the forehead or other portions of the 
skull of the hearer, and will convey per- 
fectly audible speech from the skull of 
one to that of the other. Again, in- 
stead of the speaker holding it against 
his teeth, he may place it against the 
upper part of his chest, when, upon using 
his voice, the sound will be conveyed as 
before. 
V. A physiological fact, not duly recog- 
nized or appreciated hitherto, so far as I 
am aware, is that sensible vibrations, pro- 
duced by and corresponding to those of 
the voice, are propagated in the hard palate 
and base of the skull of persons speaking 
in ordinary tones. These vibrations are 
found to be transmitted throughout the 
skull with varying degrees of intensity, ac- 
cording to the point of observation chosen. 
In some localities—as the forehead, the 
front teeth of the upper jaw, and the top 
of the head—which are specially vibratory, 
these are so powerful as to be readily felt 
by the hand of the observer pressed upon 
these parts of another person during the 
act of speech. They may also be evidenced 
as articulate sounds, and distinctly heard 
as such, byany one, through the use of the 
rod-osteophone, for, as has been observed, 
by its means the volume of sound con- 
sciously received is a perceptible addition 
to ordinary hearing. Through their agency 
the voice may be conveyed directly from 
the skull of the speaker to that of the deaf 
listener by simply bringing the heads them- 
selves in contact. The existence of these 
vibrations accounts for the further fact, 
before mentioned and which I have taken 
much pains to verify, that in certain cases 
of extreme deafness the patient hears his 
own voice with the utmost distinctness, 
even though speaking in low tones, when 
like sounds produced in his very ears 
would be totally inaudible. It seems 
evident, then, that auto-audition for deaf 
and well alike includes among its lead- 
ing factors the bone-vibrations spoken 
of; and the topographical relations of 
the larynx and the vault of the pharynx 
to that portion of the temporal bone 
which includes the internal ear supply 
good a priori grounds, if any such were 
needed, for reaching the same conclusion. 
Fig. 2. 
and a quarter of an inch thick,—one end of 
* Between the date of the delivery of this lecture (De- 
cember 17, 1879) and its present publication in the order of 
the Society’s proceedings the discovery of this application 
of cardboard has been claimed by others. 1 had previously 
discovered and demonstrated at this meeting the adaptability 
of this material, together with other varieties of paper, and 
especially of that known as fullers’ board, for diaphragms of 
osteophones. This was four weeks previous to the announce- 
ment, through the Associated Press of January 13, that the 
discovery had been made at Geneva, and also antedates any 
other like publication in America. 
1807 Chestnut Street.