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SPECIAL ARTICLES

CAN PHYSICS HELP THE DEAF CHILD ?*

BY PHYLLIS M. TOOKEY KERRIDGE,M.Sc., Ph.D., M.R.C.S.

LECTURER IN DEPARTMENT OF PHYSIOLOGY AND BIOCHEMISTRY,UNIVERSITY COLLEGE, LONDON ; DEMONSTRATOR IN DEPART-

MENT OF INDUSTRIAL PHYSIOLOGY, LONDON SCHOOLOF HYGIENE AND TROPICAL MEDICINE

THE most urgent need of the deaf child is goodspeech. How can physical science best help to teachhim it, if he has none and hears none ? Would itbe best to translate speech into terms which can beappreciated by one of his perfect senses, such as

sight or feeling Many devices have been elaboratedon these principles, but they have not been taken upvery widely. This, I think, is because they are notsimple enough, and because they involve re-educationof the teacher as a preliminary to education of thechild. Education through the ear is comparativelysimple from the teachers’ point of view. Moreover,the amazing naturalness of the voice of the childwho has a trace of hearing for everyday noises, whichcannot yet be taught to the child who never hearsthe human voice in his ordinary life, alone shouldwarrant the experiment of using magnified sounds inthe education of the deaf.

I shall therefore confine myself to discussion of

physical help through hearing and this will be con-sidered under the headings of :—

1. Tests of hearing.2. Hearing " aids."3. Methods of using hearing aids for teaching purposes.

There have been no " amazing new discoveries," asthe newspapers say, in any of these subjects, whichare each centuries old, but comparatively rapidprogress has been made in the last twenty years,following on the advances in the physics of acoustics.

Tests of HearingAccurate testing of hearing is an essential bedrock

on which to build. The deafness of many peoplevaries from day to day, and psychological difficultiesof testing vary from one person to another. Theseuncertainties remain, but the end-result is as reliableas possible only if the testing apparatus itself isaccurate and the test is easy. When testing is donewith the view to using sound-magnifying apparatussubsequently it t is s obvious sthat the voice test is inadequatefor the deafer children, as a voiceis not loud enough. It is not a

very accurate test in any case.

Nevertheless, it will probably longremain a preliminary test for theminor degrees of deafness. Testsshould also be made over the wholeof the audible range of pitch, as

uniform deafness is rare, and manychildren hear low notes compara-tively well, and these can be usedfor educational purposes. Accurate

testing both for air and bone con-

* A lecture given to the National Collegeof Teachers of the Deaf at the Educa-tional Conference at University Collegeon Jan. 5th. The term " deaf " is definedby Section 69 of the Education Act, 1921,as " too deaf to be taugnt in a class ofhearing children in an elementary school,"and it is used in this sense throughoutthe lecture.

duction is usually done now either by the use of cali-brated tuning-forks, or with a pure tone audiometer.The latter is a large and somewhat expensive instru-ment but is very much easier to use with children thantuning-forks, chiefly because the speed of the testcan be varied to suit the temperament of the child.The gramophone audiometer is useful for detectingand measuring deafness in the " hard-of-hearing "but is not often used for the deaf child, since its userequires understanding of heard words.The easiest way to express the results of hearing

tests is by means of a graph. The form shown(Fig. 1) is that usually associated with a tone audio-meter test but could equally well be used for tuning-fork tests. The amount of hearing loss is expressedvertically in decibels, but could be in any unit youliked, provided you knew what it was and could

explain it to other people. The pitch of the testingnote is represented horizontally. The note havinga pitch, or frequency, of 64 cycles per second corre-sponds to the C two octaves below the middle Cof the piano, if the piano note were purified of thosecomplicating harmonics or overtones which giveit its pleasing and characteristic sound. It is alsothe pitch of the lowest string of a cello. The C4marked 2048 corresponds to the pitch of the highest Con the piano. The octaves above are not essentialfor the understanding of speech, except s, sh, ch, v,th, and f, which require frequencies up to 6000, butthe high frequencies transmit those harmonics of

speech which enable us to distinguish one person’svoice from another’s.The lines named on the graph indicate the average

hearing of normal persons in a quiet room by air con-duction, by bone conduction, and that which corre-sponds to complete deafness. It will be seen thatone has more hearing to lose in the middle part,which is also the region most essential for speechinterpretation. It will also be noticed that even thenormal person hears very much worse through thebone than by air conduction. Hence a deaf personmay have quite unharmed bone conduction and

considerably damaged air conduction, and yet his

hearing by air may be better than by bone. I stressthis particularly because badly worded advertise-ments at the present time are bringing hopeunjustifiably to many deaf people. There are peoplewho can hear better and more distinctly through

FIG. 1

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X:- - --— Air conduction left ear. 8---- -8 - Bone conduction left ear.

——--*- ,, " right" 0———o— ,, " right"Line at A indicates reading of audiometer when testing a person of normal hearing

with the D. -80904 bone conduction receiver.

their bone than through their whole ear, but theseare not a very great number. There are others whocan hear loudly through their bone but only con-fusedly. There is a factor in bone hearing which is atpresent more mysterious than the rest of the hearingmechanism.For those unaccustomed to thinking in decibels

the following figures may be helpful :-Decibels. Approximate noise equivalent.

0 .... Threshold of audibility.10 .... Country " quiet."

20-30 .... An ordinary room.40 .... A noisy room.50 .... A car noise.55 .... Ordinary conversation.60 .... Loud radio speech.70 .... Busy London traffic.80 .... A motor horn at 80 ft.90 .... A pneumatic drill at 20 ft.

100 .... An aeroplane engine at 10 ft.

The accompanying Figures show hearing curves

of children in the London deaf schools, which willserve as typical examples. Fortunately the shapes ofthe curves are not as numerous as the children, andthey fall into a few forms only, although the amountof deafness of each type may varyfrom a very little to almost com-pletion. In my experience, how-ever, complete deafness is veryrare. Testing with the loud notesof the tone audiometer, I have

only found 3 cases in 400 childrenin the London deaf schools inwhom I could not detect anyhearing anywhere in the scale.

Fig. lA shows the curves of a

child whose hearing by air conduc-tion is impaired about equally inboth ears to an extent varying from40 to 75 decibels below normal, thehearing loss being greatest in themiddle frequencies. He would not(and in fact does not) hear conver-sation. He would hear easily andwell with a speech - magnifyinginstrument. Notice that his hearingby bone conduction is quite normaland better than his air hearing.

Fig. Is shows a case where thedeafness for the high notes is much

greater than for the low. There is

impairment of bone conduction too,but the curves are not parallel. This

girl can hear conversation, as herdefect is not greater than 55 decibelsin the middle range. She is in a

partially-deaf class, and her head-mistress reported that her speechwas good but weak in s, andone can see why, for s needs the

frequencies near 6000, where herdeafness is greatest.

Fig. 10 shows the hearing of a

child of whom it would usuallyhave been said that she was stonedeaf from birth. Yet in fact shehas hearing over the whole range,but at too low a level to be of anyuse with unmagnified sounds. Noticethat her air conduction and boneconduction are identical, and thatthere is no difference between hertwo ears.

Fig. ID shows the patch of hearingof a girl who has been deaf from birth,as has her sister also. She has detect-able hearing in one octave only by air

conduction and over three octaves by bone conduction.She has no useful hearing at all in the speech range.

It is not possible in the present state of knowledgeto deduce exactly what is wrong with the ear froma medical point of view from a study of these graphs.Enough is known to be able to say that the inter-pretation is not simple, and research on the correlationbetween the pathology and the physical end-resultis being carefully and laboriously worked out inthe Johns Hopkins Hospital in America, and in thecourse of years we shall learn more from them.

Fortunately the temporary ignorance does not preventall progress in attempts at compensation by physicalmeans for the physical defect.

Hearing AidsIt should be axiomatic that it is desirable that a

hearing aid should suit the patient. Much has alreadybeen written about prescribing hearing aids, like

spectacles, and making the aid exactly fit the defect.I do not know how often this has in fact been done,but in my experience, as far as total loudness isconcerned, it is not necessary, and is indeed resentedby the patient, who is hurt by the apparently excessivenoise. All that is required is to magnify sufficiently

FIG. 1 B

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to enable him to hear sounds thatare 30 or 40 decibels above thenormal threshold. He will thenbe able to hear conversation easily.He does not need to appreciatethe minutiae of the quiet of thecountryside. This is fortunate,for it means that the "stonedeaf " person, with 80 decibelsloss, can be made to hear with asmall instrument magnifying by40 decibels ; whereas if completematching of the aid to the defectwere essential, only the hard-of-

hearing cases with 40 decibels losscould be helped with a similarinstrument.Where matching is important is

not in total loudness, but that thatpart of the scale in which the personis deafest shall be magnified most,not the whole pitch range indis-

criminately, nor only that part which can be mosteasily magnified by a given microphone. Thecharacteristics of the microphone always deter-mines largely the degree of matching which is practical,but not that which is ideal.

Statements have been made that aids damage thehearing, and on the other hand that they improve it.I know of no scientific evidence that the former istrue and I do not see how suitable evidence on thepoint could be obtained. I think the impression hasbeen erroneously gained by the too long use ofunaccustomed instruments, which have producedfatigue of hearing, for which rest is a cure, and wiseuse at first a preventive measure. I do not considerthat the outlook in deafness is good. enough to with-hold instrumental help from cases on the grounds ofhypothetical future deterioration in the hearing.Similarly it is obvious that an instrument which

merely magnifies sound does not cure the deafness,but in some very deaf people exercise of the hearingdoes improve the perception to a certain degree.I will give two examples.The curves in Fig. 2 represent the hearing of a

boy of 13 who was born deaf, and without an aid onlyheard the hoot of a motor. The lower curve representshis hearing last March. Since August he has been wearingan individual portable aid during school hours. The boyand his parents said persistently that his hearing wasimproved, that although he did not wear the aid at homehe had heard the door-bell ring and a man cough, and he

FIG. 1 D

could distinguish between a voice singing and a band.on the wireless, and enjoyed the rhythm of the music..After he had used the instrument for three months Ire-tested his hearing and it will be seen from the upper curvethat it had in fact improved, and this must be attributedto the exercise of the ear or the attention.

Fig. 3 shows the curves of hearing of an adult whowas born deaf and had no hearing for speech, who hadused a new instrument for the first time for two hourson two successive days, using the right ear only. She wasre-tested at the end of the second period. The left ear-curve was identical with that of the previous day, whilethe right ear showed appreciable improvement, particularlyin her worst region.

The commercial hearing aids may be roughlydivided into those that are frauds and those whichare not. The latter may be subdivided into those-which suit the patient and those which do not.It is hoped to be able to protect the public in thefuture from the fraudulent variety by official testingof the others. The National Physical Laboratoryare able and willing to measure the amount of

magnification of sound by hearing aids over a widerange of pitches. The figures which would be thus.obtained would enable people with the necessaryknowledge to sort out the suitable from the unsuitable,with a much greater accuracy than is at presentpossible. They would also enable instrument makers to-evaluate alterations in design. Attempts are nowbeing made to cover the initial costs of launching

this service, so that it can be con-tinued at a price which will enableevery instrument sold to the publicto be tested. In this way it is.

hoped that the deaf public maybe prevented from exploitation-and both the medical professionand the instrument makers helpedto improve the instruments.A more scientific classification of

hearing aids is as follows :—

I. NON-ELECTRICAL INSTRUMENTS I

e.g., horns, auricles, speaking tubesII. ELECTRICAL INSTRUMENTS

(a) Carbon microphone instru-ments on the telephone principle, with !single or multiple microphones, and- !

air- or bone-conduction receivers.

(b) Instruments with valve ampli- Ifiers, and air- or bone-conduction I

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The shaded area represents the improvement in hearing after wearing an " aid "

for three months during school hours. The improvement occurred to aboutthe same amount in both ears, although only the left was used with the "aid."

receivers : (i) large instruments, suitable for permanentinstallation in school, church, cinema, or private room,with single or multiple receivers ; (ii) smaller portableinstruments.

Many examples of all these types are exhibited andit will be observed that every new name does notmean a new principle.

Non-Electrical InstrumentsThe non-electrical instruments magnify the sound

by resonance just like the air in the body of a violinmagnifies the note emitted by the vibrating stringabove it. They also act as collectors of sound toconvey it right into the ear. You will have noticedthat the elderly gentleman who is becoming hard ofhearing puts his curved hand behind his ear toachieve the same object. This class of instrumentis excellent for the deafened adult if he will use them.But unfortunately they are rather large and unusualin appearance. Many ladies wear auricles undertheir hair very successfully, but they are very muchmore noticeable on a man. The horn type of aid hasbeen estimated to magnify sound by about 12 decibels,chiefly in the middle frequencies, so that a person whohad a loss of 60 decibels in the speech range would beraised to 48 decibels when he

would hear speech comfortablyinstead of with difficulty. Speechthrough a non-electrical aid ismore natural than that which canbe heard through any electricalaid which has been made up tothe present time. Most of thechildren in the deaf schools are

too deaf to be helped by a non-electrical aid, although some canbe reached in this way, as thoseof you who have used speakingtubes for residual hearing classesknow. Auricles would assistchildren in the partially-deaf schoolsor partially-deaf children in ordinaryschools, who for geographical or

other reasons are not attending aspecial school. But the partially-deaf child who can hear conver-

sation and is taught lip-readingdoes not need an apparatus nearlyas much as an adult with the

same amount of deafness, who willrarely give the necessary time andattention to acquire good lip-reading. The hard-of-hearing adult,particularly one with high note

deafness, who wants to hear naturalspeech through an inconspicuousinstrument, is very difficult to

help, particularly if he wantsdistant sounds magnified more

than near ones, as in a theatre.

Electrical Instruments

(a) CARBON MICROPHONE INSTRU-MENTS ON THE TELEPHONE PRINCIPLE’

Most of the electrical hearingaids on the market are of this

type. They have been the causeof such a lot of hope and disap-pointment to deaf people that Ishall deal with them at some

length. They consist essentiallyof a microphone, which is singleor multiple, an earpiece, a pocket

battery, a switch, and connecting flex. Some are

fitted with a variable resistance, by means ofwhich their magnification can be varied as

required. Minor adjustments of response can be madeby altering the kind of carbon in the microphone.These instruments magnify, mostly in the middle

frequencies, by about 20-60 decibels according to theirsize and other characteristics. Therefore they suitpeople best whose loss is greatest in the middle anddo not suit those with maximum high note loss sowell, as they cannot hear so intelligibly. Some adults.who have become deafened do not seem to mind this,probably because they hear enough to enable them toguess the rest. Others are more critical of the qualityof what they hear. The quality varies greatly fromone make of instrument to another and is one ofthe first things to listen for when testing an instrument.But even the best types sound more mechanical than.ordinary speech. One of the children put it veryaptly when she said that the instrument I had lenther made her teacher sound like a " movie star."

Bone-conduction instruments require specialmention merely to emphasise the points made earlier,that relatively more magnification is necessary as.

the threshold for bone conduction is lower, and that

FIG. 3

The shaded area shows the improvement in the hearing after the first two 2-hour-periods (on successive days). There was no improvement in the ear which didnot hear.

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intelligibility through bone varies greatly withindividuals. One of the attractions of the bone-conduction instruments is that the receiver wornbehind the ear is less noticeable than one in the ear.The same consideration has led some manufacturersto employ on the air-conduction instruments tinyearpieces with inferior characteristics. These mayseriously impair the qualities of the reception andnullify any improvement in the microphone.When the question of an instrument arises a deaf

person should always be advised to go to a dealerwho will let the patient take the instrument home onapproval for a reasonable fee. Many dealers allowthis, and the National Institute for the Deaf keeps alist of them expressly for the protection of the deaf.Personally I advise people to go where they can trymore than one make, so that they can compare onewith another under the same conditions. They shouldalways go with someone whose voice they knowwell, so that they can more easily evaluate the qualityof the instrument. When at home they should trylistening to general conversation, not merely to oneperson saying slow sentences. They should listenfor the amount of background noise ; that is to say,the noise which is inherent to the instrument whenthe battery is switched on, but nobody is speakinginto it. This may be considerable, but the deaf

person does not always notice it. They should trythe effect of moving about with the instrument on.No carbon microphone really likes being movedabout or tilted, but some are affected more thanothers, and it is advisable to test the instrumentunder the same conditions as those in which it will

actually be used. They should see how long thebattery lasts. Those who use these aids must makethemselves a battery allowance. The best thing todo is to come to an arrangement with somebodywho uses a bicycle lamp, for the batteries are usefulfor this purpose after they are useless for hearing.Inferior makes of battery are false economy. The

earpiece should be comfortable, and a larger one aswell as a small inconspicuous one should be tried.A new instrument should not be worn for too longat first, as the hearing gets fatigued and the deafnessmay be apparently increased.

Having pointed out the way to show the defectsof these instruments, it is only fair to add that theyare used with apparent satisfaction by a large numberof people with losses up to 80 decibels, and even,in my own experience, by persons who were borndeaf and who were said to be completely deaf, neverhaving heard speech before using the instrument.

They are simple, and could profitably be used ineducation. The advantage of an individual instru-ment which can be constantly worn and the usecontinued in adult life is self obvious. Through thegenerosity of one of the makers I have recently beenable to lend suitable instruments of this type to fivechildren who have been completely deaf to speechfrom birth. Gramophone reproductions of their

speech have been taken, together with that of threeother children who are not having aids and who willserve for comparison. None of the children are havingany special tuition in school. In a year’s time theirspeech will be again recorded, together with thecontrol children, and we shall be able to see whetherthe former have been helped by hearing their ownand other peoples’ voices. How much they willlearn to interpret what they hear without specialtuition only time will show. An intelligent adultof my acquaintance, who has just heard speech forthe first time with one of these instruments, is

going to begin shortly a course of special tuition

by a phonetics teacher in the interpretation of speechsounds and in the correlation between these and the

lip-reading she already knows well. We may hopeto gain a great deal of help from experiments of thiskind.

(b) INSTRUMENTS INVOLVING VALVE AMPLIFIERSLarge instruments have ample magnification for

the needs of the children with any hearing at all,and most of the instruments, by various devices,can be made to magnify that part of the pitch rangewhere the hearing is most deficient. This is a greatadvance and is particularly valuable to the groupwith maximum high note loss. The church andcinema instruments are on the same principle, butwithout the individual adjustments of tone, althoughthe volume can be varied. It is obvious that ifexpense is no object a much more complicated andrefined instrument can be made if weight is not

objectionable and the microphone will not be con-tinually shaken. These instruments fill a definiteneed in speech education, and for the imparting ofknowledge as has been already shown by Dr. and Mrs.Ewing at Manchester University.

Smaller instruments.-These are the newest comersto the hearing aid world, and in my opinion probablywill be the most important in the future. It is

possible to combine some of the refinements of thelarger valve instruments with a reasonable measureof size and weight. The smallest of them is at presentlarger and heavier than the largest carbon micro-phone telephone type, but the quality is better anddifferential pitch magnification much more possible.There are several types already on the market, andI have recently met two deaf physicists who havemade very good sets for themselves and who are

anxious to help other deaf people. But generalexperience with this type is at present small. For thedeaf child who has so little hearing that he will neverappreciate more than rhythm a simpler instrumentwill suffice, but for the others I shall look forwardto the day when they can all have an instrumentof this type adjusted to their special requirements,which they will be able to use for speech training -andunderstanding conversation, not only in the school-room but throughout their life.Methods of Using Hearing Aids for Teaching

PurposesThe problem which most teachers have to face is

how best to use the instrument with which they aresupplied. There is no great difficulty in this if

hearing curves are provided for the children andinstrument.s adjustable to suit the individual child.The instrument is used so that the child may hearthe teacher’s voice and his own attempts to imitateit in the ordinary speech lessons. It will be possibleto tell from the hearing curves which child has onlysuch a small patch of hearing that probably he willnever appreciate anything more than rhythm, andperhaps the difference between high and low notes.On an examination of the hearing curves of thechildren in one London school I estimated that84 per cent. should be able to appreciate amplifiedspeech and 15 per cent. rhythm and gross differencesof pitch only. It is lack of rhythm that makes thespeech of a deaf child so difficult for the unaccus-tomed person to understand, and inequalities of pitchwhich make it sound so unnatural, and the use of asound-magnifying instrument should help 99 percent. of deaf children to improve in these points.Nothing is going to replace good teaching, but it isreasonable to hope that these instruments may easethe very difficult task of the teacher of the deaf.