Auditory handicap of hearing impairment and the limited benefit of hearing aids Reinier Plomp Institutefor PerceptionTNO, Soesterberg, The Netherlands,and Facultyof Medicine,Free University, Amsterdam, The Netherlands
(Received29 April 1977;revised31 August 1977)
The aim of this articleis to promotea better understanding of hearingimpairmentas a communicative handicap,primarilyin noisyenvironments, and to explainby meansof a quantitative modelthe essentially limited applicabilityof hearingaids.After data on the prevalenceof hearingimpairmentand of auditory handicaphavebeenreviewed,it is explainedthat everyhearinglossfor speechcan be interpretedas the, sum of a lossclassA (attenuation),characterized by a reductionof the levelsof both speechsignaland noise,and a lossD (distortion),comparablewith a decrease in speech-to-noise ratio. On the average,the hearinglossof classD (hearinglossin noise)appearsto be about one-third(in decibels)of the total hearingloss(A+D, hearinglossin quiet). A hearingaid can compensate for class-Ahearinglosses, givingdifficultiesprimarilyin quiet, but not for class-Dhearinglosses,givingdifficultiesprimarilyin noise.The latterclassrepresents the first stageof auditoryhandicap,beginning at an averagehearingloss of about 24 dB.
PACS numbers: 43.66.Sr, 43.70.Dn, 43.66.Ts
INTRODUCTION
Our insights into why hearing-impaired people appear to be so seriously handicapped in everyday listening situations seem to be very scanty. This lack of knowledge particularly manifests itself in the uncritical way in which hearing aids are assumed to be of benefit. Since
most
conductive
defects
in the transmission
chain
up to the cochlea can nowadays be successfully rehabilitated by means of surgery, the great majority of the remaining inoperable cases are sensorineural hearing impairments. Although it is generally recognized that electronic amplification cannot compensate satisfactorily for these losses, it is remarkable how much hearingaid prescribers expect from careful selection and fitting
followed by good training. On the other hand, many hearing impaired appear to be rather disappointed about their hearing aids. In view of these diverging opinions on the merits and
benefits of hearing aids, it seems worthwhile to present a critical review on the various aspects of sensorineural hearing impairment as far as speech reception, both in quiet and against a background of interfering sounds, is concerned. Although many references are given, no attempt at completeness has been made. The aim of this article is to promote a better understanding of hearing impairment as a communicative handicap and to ex-
results of several studies along this line are available. As early as 1935-1936 the United States Health Service conducted a nationwide investigation into the state of
health including impaired hearing (Beasley, 1940). More
than
700 000 households
were
visited
and data on
2.5 million people were collected. Any type of hearing impairment was classified as belonging to one of the following five groups (literal quotations):
(1) Partial deafness, stage 1: The individual has difficulty in understanding speech in church, at the theater, or in group conversation, but can hear speech at close range without any artificial assistance.
(2) Partial deafness, stage 2: The individual has difficulty hearing direct conversation at close range, but can hear satisfactorily over the telephone or can hear loudly spoken speech.
(3) Partial deafness, stage 3: The individual has difficulty hearing over the telephone at ordinary intensities, but can hear amplified speech by means of hearing aids, trumpets, or other means of amplification.
(4) Total deafness for speech: The individual cannot hear speech under any circumstances, but acquired the hearing defect after learning to speak language by ordinary
means.
plain the essentially limited applicability of the hearing
(5) Deaf mute: The individual was born deaf or ac-
aid.
I.
PREVALENCE
OF AUDITORY
OF HEARING
IMPAIRMENT
AND
HANDICAP
means.
The importance of the problem under study does not only depend upon the degree to which subjects with sen-
sorineural hearing losses are handicapped, but also upon the percentage of population involved. We start with the latter question.
It may seem obvious that one should investigate the occurrence of auditory handicap by means of inquiries including a sufficiently large sample of population. The 533
J. Acoust.Soc. Am. 63(2), Feb. 1978
quired severe deafness sufficiently early in life to prevent him from learning speech through the usual
Similar inquiries, but on a much smaller scale, were
carried out in 1947 in Great Britain (Wilkins, 1950) and in 1950-1951 in Denmark (Bentzen and Jelnes, 1955). Whereas in these investigations the classification just quoted was adopted, this was not the case in more recent inquiries by the United States National Health Sur-
vey in 1962-63 and 1971 (NCHS, 1967, 1975a). Althongh it is usually overlooked, data obtained from
0001-4966/78/6302-0533500.80
¸ 1978 AcousticalSocietyof America
533
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534
534
Reinier Plomp: Auditory handicap and benefit of hearing aids
interviews are of onty timited vatue. Evidence witt be presented below showing that such data underestimate the percentage of population actually handicapped. In
evaluating the degree to which a person is handicapped, one cannot avoid taking the age of the subject into account.
Experimental
lOO
50
women
-
-
--.17
• 20
evidence indicating that. age affects the
evaluation of one's own hearing status has been pub-
lished by Merluzzi andHinchcliffe (1973). In a s[udyon auditory thresholds of a random sample of a rural population, each subject was asked the question: "Is your
hearing normal, or not a• good as it used to be?" Both
& 2
for the subjects who answered "yes" and for those who answered "no," the frequency distributions of hearing levels were determined. It appeared that the intersection of the two distribution curves rose substantially as a function of age. Accepting for this intersection the
o.5
average hearing loss at 500, 1000, and 2000 Hz (AHL) as a representative single measure of hearing staths, this average was found to shift from 5 dB for the 30-yr age group to 28 dB for the 70-yr age group. Apparently
ø10
subjects cannot avoid evaluating their hearing status with reference to their age, in a more or less comparative
0.2
20
i
age
FIG. 2.
i
i
40
I
60
i
•n years
As Fig. 1, but for female population.
manner.
We may assumethat the data from the inquiries referred to above have also been influenced by this behavior. This can be verified by comparing the age dependenceof the pe•rcentageof subjectively handicapped people with the age dependence of the percentage of subjects with AHL exceeding various critical values. This comparison can be performed rather easily because both percentage curves can be satisfactorily approximated by
lOO
-
-
50--
means of exponential curves characterized by the number of years required for doubling the percentage of auditory handicapped, or of AHL beyond some critical value. Although the absolute levels of the handicap
curves differ substantially for the various inquiries, the exponent appears to be rather invariant, corresponding to a doubling of the percentage of handicapped every 1314 yr. As will be shown below, the doubling time for AHL values that are not too low is less than 10 yr. This difference is so large that data obtained from inquiries should not be used for investigating how the percentage of the population handicapped by hearing impairment depends upon age. This percentage has to be derived from audiometric surveys.
Suchan audiometric survey, based on 6672 adults from 18 to 79 years of age, was carried out in 1959-
2o-
1962 by the U.S. National Health Survey (NCHS, 1965),
lO--
complemented with a similar
--
investigation for children
--
from 6 to 11 years of age in 1963-1965 (NCHS, 1970).
-
.%-
[The mostrecentinvestigationfor youthsfrom 12 to 17 years of age (NCHS, 1975b) does not give the percentage
-
distributionof AHL. ] Since,on the average, theAHL
2-
for the better ear appearsto be a goodpredictor of the hearing loss for speech (see Sec. IV) this AHL will be
-
considered
here.
0.5-
Figures I and 2 represent, as a functionof age, the
.
0.2-
0.1
/
i
0
I
i
20
i
40
i
dB 1 I
i
60
80
age in years
FIG. 1. Percentage of male population with average hearing loss at 500, 1000, and 2000 Hz in the better ear (AHL) equal to or larger than 7, 17 ..... 77 dB, respectively, as a function of age. In this and all further diagrams hearing losses are given in dB ISO. (Data adapted from NCHS, 1965, Table 11 and NCHS, 1970, Table 26.)
percentage of subjects with AHL exceeding certain values for men and women, respectively. Plotted along a logarithmic percentage scale, most data points agree quite well with straight lines indicating that the percent-
age increases exponentiallywith age. The only 'substantial deviations are the higher AHL values for men below 50. We may assume that hearing loss due to exposure to high-level noise is the disturbing factor. Ex-
cludingthesecasesby Considering onlythe datapoints above1%, linear regressionlines for the averagedata
of menandwomenwere computed resultingin the lines
J. Acoust. Sac. Am., Vol. 63, No. 2, February 1978
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535
ReinierPlomp:Auditoryhandicap andbenefitof hearingaids
lOO
ß
50
-
20
c
•0
-
I
I
I
I
I
I
Up to now we have included all degrees of auditory
I•--I
'handicap,correspondingto stage 1 of Beasley's classification quoted above. It appears to be much more difficult to find out what percentage of population has difficulties in accordance with stage 2 and higher stages. The U.S. National Health Surveys in 1962-1963 and 1971 adopted a different classification in which the distinction between hearing difficulties in noise (including rever-
_ .
c•
535
beration; stage1) andin quiet (stage2 andhigher)was
5
not maintained. 2-
0
However, there is ample evidence
(e.g., Quist-Hanssen, 1967) that this is a meaningful distinction and that stage 2 corresponds to a substan-
, , //', :,. 20
40
60
80
ege •n yeors
FIG. 3. Average percentage of men and women with AHL-• 10, 15 ..... 80 dB, respectively, as a function of age. The lines are regression lines basedon average data points from Figs. 1 and 2. The slopes of these lines are expressed in the number of years for doubling the percentage.
of Fig. 3. The slopes of the lines are expressed in the number of years in which percentage is doubled. These diagrams indicate how hearing losses as a measure of auditory handicap increase with age, but they do not reveal which curve should be considered to represent the lower limit of auditory handicap in everyday
listening situations. To arrive at this, we have to "calibrate" the lines of Fig. 3 with the aid of the results of the inquiries mentioned earlier. Accepting the responses of the 20-yr age groups as representing the most reliable estimate of the percentage of auditory handicapped, Table I was obtained. The percentage includes all degrees of partial deafness except monaural hearing impairment and is the average for men and women. The last column gives the corresponding AHL, read from Fig. 3. With the exception of the earliest investigation, the riB-values do not differ very much, and we may conclude that young people begin to have hearing difficulties when their AHL is equal to about 24 dB.
tially greater AHL than stage 1. From the data available we may accept as a rough estimate that, on the average, subjects begin to have difficulties in direct one-to-one conversation at close range with AHL- 35 dB for the better ear. If stage 3 may be interpreted as
defining those people who are really hard of hearing and have difficulties
55 dB.
Subjectsmay be consideredto be totally deaf (stage 4) for AHL • ~ 90 dB (Erber, !974). Figures 1 and 2 give the percentage of hearing impaired as a function of age, but do not reveal what percentage of total population is handicapped. This can be determined by applying the percentage data of Figs. 1 and 2 on the age distribution of population. On the basis of estimates for the U.S. for 1 July, 1975, the curve of Fig. 4 was calculated. From the data points for men and women separately we see that above 40 dB AHL the percentage for men is slightly, but consistently higher than for women. For 20 dB 20, 40, and 60 dB, respectively, indicating that the
ages up to and including 50 contain only 21% of all subjects with AHL-> 20 dB, the ages up to and including 70 contain 59%, etc. The curves are based on the lines of Fig. 3.
J. Acoust. Soc. Am., Vol. 63, No. 2, February 1978
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ReinierPlomp:Auditory handicapand benefit of hearingaids
8O
- 24 dB and 3.4% for AHL >- 35 dB.
strongly suggest that the front-random ratio for the miniature directional microphones fitted in hearing aids is considerably smaller than predicted from theory and may be only 1-2 dB.
older
(6) Individual fitting of hearing-aid frequency responses. Although group averages of intellibigility scores are rather independentof the aid's frequency response (Knight, 1967), it appears that careful matching of the frequency response to the individual subject's hearing can result in a significant improvement of
speechintelligibility (Pascoe, 1975). It seems unlikely, however, that by such a procedure the threshold for speech in noise can be improved by more than a few decibels.
(5) More than 50% of all auditory handicappedare than 6?.
(6) Every hearing loss for speech (SILL) can be interpreted as the sum of SHLA(ttenuatton), characterized by a reduction of the levels of both speech signal and
noise, and SHLD(tstor•ton) , comparable with a reduction of speech level relative to noise. Up to SHLA+D (=hearing loss in quiet) ~ 40 dB, SHLD (=hearing loss in noise) is, on the average, about one-third of SHLA+D. SHLA manifests itself primarily in hearing difficulties in quiet, SHLD in hearing difficulties in noisy environments.
(?) The SHLD of 8 dB occurring for AHL = SHLA+ D= 24
(?) Compensationof the cat's distortion. Undoubtedly the best way to improve intelligibility
would be to neu-
tralize the ear's hearing loss of class D by some kind of processing of the speech signal in the hearing aid. The compensation for loudness recruitment in the pathological ear by means of amplitude compression is an attempt in this direction. A recent review of this was
given by Rintelmann (1972) who reported that a modest, but significant, improvement of the discrimination score in quiet has been observed. However, Blegvad
(1974) found that out of 42 patients 29 preferred linear amplification because of a smaller tendency to pick up background noises, which suggests that amplitude compression does not improve SRT in noise. More sophisticated signal-processing techniques tried out by Villchur (1973) may eventually lead to better results if the underlying presumption that recruitment is the
(main) cause of degraded speechperception is correct.
dB is numerically which
connected
equal to the number of decibels by discourse
can be attenuated
before
normal-hearing subjects become unable to understand speech in a typical everyday listening situation with one competing talker. This means that hearing difficulties appear to begin where this margin is nullified by hearing losses.
(8) The fact that the hearing aid does not compensate for SHLD, but only for SHLA, should be considered to be the main reason why so many hard of hearing are dissatisfied with their hearing aid.
(9) It can be proved that, irrespective of SHLA+D, hearing aids are of no benefit to the average hearingimpaired person listening to conversational speech at ambient-noise levels (average-speech spectrum) exceeding about 50 dBA.
(10) Assumingthat SHLD~« SHLA+D (Conclusion6) also holds for SHLA+D>40 dB, it follows that the maxi-
VIII.
CONCLUSIONS
The main results arrived at in the previous sections can be summarized
as follows:
(1) Inquiries appear to give an overly optimistic, and therefore unreliable, picture of the auditory handicap of elderly people compared with tone audiograms (average hearing loss for 500, 1000, and 2000 Hz in the better ear, AHL).
(2) Adopting the percentage of young adults complaining of hearing difficulties, being equal to 1%, as a base, we can conclude that people are auditory handicapped for AHL>-24 dB. For 24 dB--24 dB the percentage is doubled every 10 years resulting in 24% of auditory handicapped at the age of 65.
(4) The percentage of the total population with AHL exceeding some critical
value is approximately halved
mum useful acoustic gain of the hearing aid is equal to 2 ~SHLk=5 SHLA+D. Probably due to recruitment, etc., the average acoustic-gain settings in practice are increasingly lower for SHLA+D>60 dB.
(11) Only for SHLA+D> -- 35 dB are hearing aids generally recommended. This means that about 3.4% of
the population(Conclusion4)may benefit by wearing a hearing aid.
(12) Every 4-5 dB of noise reduction halves the percentage of auditory handicapped of any degree (~ 15%
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