- Email: [email protected]
Free field pure tone audiometry as a comparative system for the assessment and fitting of hearing aids
FREE FIELD PURE TONE AUDIOMETRY AS A COMPARATIVE SYSTEM FOR THE ASSESSMENT AND FITTING OF HEARING AIDS Monty
Shulberg
ABSTRACT In this brief report it is shown that without the use of language and/or complex tones, it has been possible to determine whether or not the potential of residual hearing is being fully utilized by the hearina impaired child or adult
with or without an aid. Furthermore, because the system is one of direct and immediate comparison it can be used to confirm if the initialselection of a specific hearing prosthesis is potentially correct.
INTRODUCTION
chart biologically based with an indicated optimum ‘target’ area (within 15 db of our zero) for the threshold and profile. The chart was also roomcompensated at 125 Hz and 500 Hz to produce a more linear picture which we felt would help to show up deviations more effectively.
AND
BACKGROUND
This paper could just as well be entitled ‘Predictive audiometry’. The system was developed out of necessity, because we needed to establish a common testing factor for the 2000 children and adults we see every year who come to us from almost every country in the world. This precluded any possibility of a speech and language test as even if we have a common language many of the severely deaf children we aid for the first time have neither understanding/ discrimination of language nor the concept that sound (as we know it) could be meaningful. COMMON
TESTING
FACTOR
In view of this, our search for a practical easily duplicated common testing factor eventually led us to look at the ‘free-field pure tone’. It seemed reasonable to suppose that if a ‘test sample’ of normally hearing children and adults could ‘just hear’ a free field threshold of pure tones over a frequency span of 250 Hz to 4 KHz then we could presume that this threshold and profile was a valid target and objective (as subsequent results have confirmed). ESTABLISHING
THE
METHOD
OF
TESTING
We therefore set about establishing our free field pure tone threshold and profile by using a Kamplex TA155 audiometer to feed a series of non-warbling pure tones (over the frequency range of 250 Hz to 4 KHz) into a Philips N2400 cassette/amp. which in turn was linked to wide range high output speakers. With the audiometer on Zero and our ‘test sample’ seated at approximately 5ft from the speakers we adjusted the overriding volume control on the cassette/amp. until Threshold was reached. The
optimum
target
With the level of free field tones now established we were able to produce our first Free Field audiogram Cubex Hearing Centre, 324 Gray’s Inn Road, London WCl, UK. A ‘communications’ paper presented at the 9th Conference of the Biological Engineering Society, on the 9th April, 1979, at the University of Southampton.
0141-5425/80/030197-04 $02.00 0 1980 IPC Business Press
Making
the comparison
We still did not know at this stage the sound pressure levels of the tones we were emitting in free field but we now had a system of comparison where we could observe the changes in thresholds and profiles from unaided to aided, using two different sets of aids (see F@re 1 a) which were fitted bin-auraIIy. However, we did take SPL readings of the aids we were fitting by using an acoustic test box (ref. HClOOO) to give us digital readouts of the gain and maximum outputs over the frequencies response of the aids was affecting the aided free field response and so we extended our chart accordingly, (See Figure lb). We finally completed our chart by making provision for the pure tone audiogram (See Figure Ic) where it is possible to carry this out satisfactorily. General
observations
As can be seen in Fzjpre 1 the overall picture is one of confirmation: the free field unaided (line A) confirms in ‘general’ terms the pure tone audiogram, and the free field aided thresholds and profiles (models B and C) have changed according to the aids used. However, in these initial stages we are only looking for preliminary shifts, as cognition of the given signals will be dependent to some extent on the listening experience, which in turn may have been limited by the previous (poor) use of the residual hearing. THE
REASON
FOR
PURE
TONE
TESTING
Our present reluctance to use speech (in any form) to determine the choice or change of aid is based on the fact that speech is a mixture of complex tones, wide band noise and transients. Intensities and frequencies of ‘speech sounds’ change continually and rapidly. Furthermore individual voices and accents differ to blur the boundaries of
J. Biomed. Engng. 1980, Vol 2, July
197
Hearing aids: M. Schulberg
CONVERTING
TO SOUND
PRESSURE LEVELS
Initially we were not concerned with the SPL’s of the Free Field tones we were emitting, but -because of the increased interest in our work and the questions it raised we decided to explore further. We learnt that if brief but characteristic elements of phonemes were actually sustained like pure tones then these elements would occur roughly within certain boundaries and they could be plotted in SPL’s. (See references). We therefore decided SPL’s to show:-
60 25
250
500
1000
2000
4000
6000
Frequency (Hz(z)
to produce
a new Chart in
(i) A ‘hatched’ section indicating the approximate area for the ‘sounds of speech at conversational level’. (This probably includes almost all of the ‘formant’ frequency bands of speech which distinguish the vowels and many consonants. (ii) The gain/output of the relevant (to be tested) on the same scale. CONFIRMING
sol
b
I
I
I
I
I
1IO
l/$---j z 3 t -0
40
,e
60
ZERO
Our next step was to see how our original biologically based chart related to the new SPL chart and we therefore measured the SPL outputs of the emitted test tones (with a Dawe’s 1405C sound level meter at approximately 5ft from the speakers) at ‘just hear’ level (and subsequently at all other levels) transferring the results to the new chart. We found that our biologically based ‘just hear’ levels fell directly within the SPL ‘hatched’ boundaries and as can be seen on the completed SPL Chart (Figure 2) the relationships between the unaided threshold and profile (line A) can be compared to the aided thresholds and profiles (lines B and C) with the relevant aids. Two further as follows:-
examples
of completed
SPL charts are
50
Patient Q.T. (See figure 3) - 19 year Singapore
$70 I
80 90 loo II0 c
THE BIOLOGICAL
aids
,201
I
I
I
I
I
I
I
I
Figure 1. (a) Free jield aided audiogram; l, unaided; l, f aided. (b_) Acoustic response of atd; n, model B; A, model C. .f70 dB SPL acoustic input *0 dB SPL = mm-= (c) Pure tone audiogram; 0, right ear; x, left ear.
the speech area. It is therefore difficult if not impossible to actually plot ‘speech sounds’ precisely in terms of sound pressure levels at particular frequencies. In addition, speech testing brings in more variables such as mother tongue, listening experience, cognitive and educational factors.
198
J. Biomed. Engng. 1980, Vol 2, July
old student
from
This chart is an interesting example of an aided threshold which improved steadily over an 18 month period which in turn related to the increasing ability to discriminate speech within of course the limitations of the previous lack of listening experience. Patient B.H. (See figure 4) - Aged
28 years
This chart is of a lady who had a long history of deafness (moderately severe), but in 1974 at the age of 24 years her hearing loss worsened. She was then fitted with a power body aid (line B) but its help was limited. She subsequently decided that the lack of help and the ‘distortion’ in the aid was more than she could bear and she reluctantly gave up wearing aids completely. In October 1977 we saw this patient for the first time and after preliminary tests we decided to fit her with a specific post-aural to produce an
Hearing aids: M. Schulberg
120
130 120
3
100
% 5
so
100
II0
so
3 3
%
70
g
70
60
i
z B _& f
IK
60
50
.F
p a v)
50
40
z
40
30
30
B B 2
110
120
100
110
120
130
110
so
100
80
2 I
70
g
70
60
j
tg
60
50
.g
n5
50
40
I”
fS
40
30
20
30
20
20
IO
20
IO
IO
0
IO
0
80
00
I
0 150
I
I
Frequency
I
I
so 00
0
I 80
140
70
8
S
130
2
120
60 50
E .s
2
110
40
.;
3
II0
30
a
100
20
2
.u ‘, 2 2
I
500 Frequency
1000
2 000
4 000
? 5 m 9
I40
2
120
so 80
: 1000
2000
4 000
0000
response (Hz)
Cubex test chart for patient Q.T. 0, Figure 3. unaided; A aided B, May 1976; C, November 1977 120
130
and profile as shown on line C. We found the initial ‘shifts’ exciting because of their proximity to the ‘hatched’ SPL indicating to us an unused potential. However, despite the fact that in the past the patient had good speech discrimination (aided), after 3 years of not using her ‘hearing’ she no longer had the ability to discriminate speech successfully and it was therefore made conditional that intensive auditory training was undertaken.
II0
120
100
II0
so
This was agreed and five months later the considerably improved thresholds can be seen (line D). Her speech therapist has commented ‘Speech discrimination without lip reading when wearing her aid is quite remarkable. I have been delivering material in normal running speech patterns at a rate of up to 140 wpm. She has graduated with honours’.
At the present time we do not know in exact terms how speech discrimination is affected as you move outside the ‘hatched’ SPL boundaries, but we nevertheless feel that free field pure tone audiometry is a system which provides a valid target/ objective. We know that one of the most important prerequisites for the learning of language and speech is to utilize fully the potential of the residual hearing, and in view of this we feel that free field pure tone audiometry can be considered not only for the fitting or re-fitting of an aid but also to deter-
500 Frequency
response (Hz)
target
’
:,” 250
aided threshold
A valid
130
0000
Figure 2. Cubex test chart; showingfiee field unaided, l; and aided, n, A; profiles and acoustic response of aids; B, C.
(Hz)
I50
3 %
250
Frequency
(Hz)
30 30
20
20
IO
IO
0
0
I
I
I
I Frequency
I
(Hz)
3
150
% m E 2
140
70
1
I30 120
60 50
= .i
II0
40 30
.;
100 so 00
20 In
9
;, .o 5S 4
00
2
250
500 Frequency
1000
2000
4 000
0000
response (Hz)
Figure 4. Cubex test chart; for patient B.H. f unaided; n, A, aided C, November 1977; D, April 19 78.
J. Biomed.
Engng.
1980,
Vol2,
July
199
/fearing
aids:
M. S&u/berg
mine if the potential being used.
of the residual hearing is
The first tests Because of this the first free field tests we carry out on a patient are bin-aural as we want to know at this stage how the residual hearing is being used. However, monaural occlusion should be subsequently used (unazded and aided) to ensure that each ear is fitted according to the indicated requirement. It is also interesting to note that with asymmetrical losses an aided ‘cross-over’ effect can generally be observed. High frequency
losses
Occlusion is also essential when fitting severe high tone ‘ski-slopes’ and here we fit the earmould and the proposed aid (switched ofJ) to carry out comparative tests. One can then observe if the unaided thresholds and profiles are now affected before the selected aid/aids are switched on. Obviously this will also provide a useful guide to the amounts and benefits of venting. individual
requirements
There are many other situations where the tests will be a useful guide but I would stress that there is no simple formula. Individually there are large differences in requirements and the optimum gain and maximum output settings must relate directly to the usuable (comfortable listening) level. It is often apparent that one cannot continue to increase amplification of any one frequency in order to improve the threshold as one may just saturate the dormant potential, creating confusion, distortion and intolerance. SELECTING THE AID We are adding to our knowledge everytime we carry out tests and there is obviously much more to learn, but it is evident that audiometry in general has not yet produced a formula which actually selects the most suitable aid. Furthermore, because of the individual variable requirements we do not feel that a formula for the selection of an aid is possible and it is therefore the responsibility of the Audiologist to use his experience, skills and knowledge to make his selection, from the widest range possible.
adults using free field pure tone audiometry, and there are many important points that constantly emerge. For example in many instances one can predict the pure tone (headphone) audiogram from the unaided free field audiogram and also the likely threshold and profile of an aided audiogram. However we are currently extracting data on all these aspects and we hope to publish a further paper on this in the near future. COMFORTABLE LISTENING LEVELS With regard to ‘the usuable (comfortable listening) level’ for very young children this can generally be set according to subjective reaction and/or observation but as the ultimate objective of aid fitting is to raise conversational ‘speech sounds’ to a ‘hearing’ level, it is inevitable that unwanted background sounds are going to be raised proportionately unless one introduces a sophisticated compression system. It is not possible to pre-set a definite maximum output/pressure which is suitable in all conditions without some form of compromise but in general we feel that initially it is better to set for lower maximum outputs (relative to the pre-established gain requirements) until it is established that a higher maximum output is needed or can be tolerated.
CONCLUSIONS Finally, I would like to end by saying that if the subsequent results of aiding are dependent on the provision of good quality teaching, counselling and supportive help, then really good results can only be achieved when the initial aiding provides the optimum hearing threshold and profile to make the best use of the residual hearing. ‘Free Field pure tone audiometry - as a comparative system for the assessment and fitting of hearng aids’ has over the last four years proved itself to us time and time again. My hope now is that other Centres will investigate the system and use it to benefit their patients in the same way that we have.
PREDICTIVE AUDIOMETRY
REFERENCES
Over the last four years we have continually carried out numerous tests and follow ups on children and
Davis H. and Silverman S.R. (1960) ‘Hearing & Deafness’ Holt, Rinehart & Winston - New York - P.52 Fig. 2-6.
200
J. Biomed.
Engng.
1980, Vol 2, July
Shulberg
ABSTRACT In this brief report it is shown that without the use of language and/or complex tones, it has been possible to determine whether or not the potential of residual hearing is being fully utilized by the hearina impaired child or adult
with or without an aid. Furthermore, because the system is one of direct and immediate comparison it can be used to confirm if the initialselection of a specific hearing prosthesis is potentially correct.
INTRODUCTION
chart biologically based with an indicated optimum ‘target’ area (within 15 db of our zero) for the threshold and profile. The chart was also roomcompensated at 125 Hz and 500 Hz to produce a more linear picture which we felt would help to show up deviations more effectively.
AND
BACKGROUND
This paper could just as well be entitled ‘Predictive audiometry’. The system was developed out of necessity, because we needed to establish a common testing factor for the 2000 children and adults we see every year who come to us from almost every country in the world. This precluded any possibility of a speech and language test as even if we have a common language many of the severely deaf children we aid for the first time have neither understanding/ discrimination of language nor the concept that sound (as we know it) could be meaningful. COMMON
TESTING
FACTOR
In view of this, our search for a practical easily duplicated common testing factor eventually led us to look at the ‘free-field pure tone’. It seemed reasonable to suppose that if a ‘test sample’ of normally hearing children and adults could ‘just hear’ a free field threshold of pure tones over a frequency span of 250 Hz to 4 KHz then we could presume that this threshold and profile was a valid target and objective (as subsequent results have confirmed). ESTABLISHING
THE
METHOD
OF
TESTING
We therefore set about establishing our free field pure tone threshold and profile by using a Kamplex TA155 audiometer to feed a series of non-warbling pure tones (over the frequency range of 250 Hz to 4 KHz) into a Philips N2400 cassette/amp. which in turn was linked to wide range high output speakers. With the audiometer on Zero and our ‘test sample’ seated at approximately 5ft from the speakers we adjusted the overriding volume control on the cassette/amp. until Threshold was reached. The
optimum
target
With the level of free field tones now established we were able to produce our first Free Field audiogram Cubex Hearing Centre, 324 Gray’s Inn Road, London WCl, UK. A ‘communications’ paper presented at the 9th Conference of the Biological Engineering Society, on the 9th April, 1979, at the University of Southampton.
0141-5425/80/030197-04 $02.00 0 1980 IPC Business Press
Making
the comparison
We still did not know at this stage the sound pressure levels of the tones we were emitting in free field but we now had a system of comparison where we could observe the changes in thresholds and profiles from unaided to aided, using two different sets of aids (see F@re 1 a) which were fitted bin-auraIIy. However, we did take SPL readings of the aids we were fitting by using an acoustic test box (ref. HClOOO) to give us digital readouts of the gain and maximum outputs over the frequencies response of the aids was affecting the aided free field response and so we extended our chart accordingly, (See Figure lb). We finally completed our chart by making provision for the pure tone audiogram (See Figure Ic) where it is possible to carry this out satisfactorily. General
observations
As can be seen in Fzjpre 1 the overall picture is one of confirmation: the free field unaided (line A) confirms in ‘general’ terms the pure tone audiogram, and the free field aided thresholds and profiles (models B and C) have changed according to the aids used. However, in these initial stages we are only looking for preliminary shifts, as cognition of the given signals will be dependent to some extent on the listening experience, which in turn may have been limited by the previous (poor) use of the residual hearing. THE
REASON
FOR
PURE
TONE
TESTING
Our present reluctance to use speech (in any form) to determine the choice or change of aid is based on the fact that speech is a mixture of complex tones, wide band noise and transients. Intensities and frequencies of ‘speech sounds’ change continually and rapidly. Furthermore individual voices and accents differ to blur the boundaries of
J. Biomed. Engng. 1980, Vol 2, July
197
Hearing aids: M. Schulberg
CONVERTING
TO SOUND
PRESSURE LEVELS
Initially we were not concerned with the SPL’s of the Free Field tones we were emitting, but -because of the increased interest in our work and the questions it raised we decided to explore further. We learnt that if brief but characteristic elements of phonemes were actually sustained like pure tones then these elements would occur roughly within certain boundaries and they could be plotted in SPL’s. (See references). We therefore decided SPL’s to show:-
60 25
250
500
1000
2000
4000
6000
Frequency (Hz(z)
to produce
a new Chart in
(i) A ‘hatched’ section indicating the approximate area for the ‘sounds of speech at conversational level’. (This probably includes almost all of the ‘formant’ frequency bands of speech which distinguish the vowels and many consonants. (ii) The gain/output of the relevant (to be tested) on the same scale. CONFIRMING
sol
b
I
I
I
I
I
1IO
l/$---j z 3 t -0
40
,e
60
ZERO
Our next step was to see how our original biologically based chart related to the new SPL chart and we therefore measured the SPL outputs of the emitted test tones (with a Dawe’s 1405C sound level meter at approximately 5ft from the speakers) at ‘just hear’ level (and subsequently at all other levels) transferring the results to the new chart. We found that our biologically based ‘just hear’ levels fell directly within the SPL ‘hatched’ boundaries and as can be seen on the completed SPL Chart (Figure 2) the relationships between the unaided threshold and profile (line A) can be compared to the aided thresholds and profiles (lines B and C) with the relevant aids. Two further as follows:-
examples
of completed
SPL charts are
50
Patient Q.T. (See figure 3) - 19 year Singapore
$70 I
80 90 loo II0 c
THE BIOLOGICAL
aids
,201
I
I
I
I
I
I
I
I
Figure 1. (a) Free jield aided audiogram; l, unaided; l, f aided. (b_) Acoustic response of atd; n, model B; A, model C. .f70 dB SPL acoustic input *0 dB SPL = mm-= (c) Pure tone audiogram; 0, right ear; x, left ear.
the speech area. It is therefore difficult if not impossible to actually plot ‘speech sounds’ precisely in terms of sound pressure levels at particular frequencies. In addition, speech testing brings in more variables such as mother tongue, listening experience, cognitive and educational factors.
198
J. Biomed. Engng. 1980, Vol 2, July
old student
from
This chart is an interesting example of an aided threshold which improved steadily over an 18 month period which in turn related to the increasing ability to discriminate speech within of course the limitations of the previous lack of listening experience. Patient B.H. (See figure 4) - Aged
28 years
This chart is of a lady who had a long history of deafness (moderately severe), but in 1974 at the age of 24 years her hearing loss worsened. She was then fitted with a power body aid (line B) but its help was limited. She subsequently decided that the lack of help and the ‘distortion’ in the aid was more than she could bear and she reluctantly gave up wearing aids completely. In October 1977 we saw this patient for the first time and after preliminary tests we decided to fit her with a specific post-aural to produce an
Hearing aids: M. Schulberg
120
130 120
3
100
% 5
so
100
II0
so
3 3
%
70
g
70
60
i
z B _& f
IK
60
50
.F
p a v)
50
40
z
40
30
30
B B 2
110
120
100
110
120
130
110
so
100
80
2 I
70
g
70
60
j
tg
60
50
.g
n5
50
40
I”
fS
40
30
20
30
20
20
IO
20
IO
IO
0
IO
0
80
00
I
0 150
I
I
Frequency
I
I
so 00
0
I 80
140
70
8
S
130
2
120
60 50
E .s
2
110
40
.;
3
II0
30
a
100
20
2
.u ‘, 2 2
I
500 Frequency
1000
2 000
4 000
? 5 m 9
I40
2
120
so 80
: 1000
2000
4 000
0000
response (Hz)
Cubex test chart for patient Q.T. 0, Figure 3. unaided; A aided B, May 1976; C, November 1977 120
130
and profile as shown on line C. We found the initial ‘shifts’ exciting because of their proximity to the ‘hatched’ SPL indicating to us an unused potential. However, despite the fact that in the past the patient had good speech discrimination (aided), after 3 years of not using her ‘hearing’ she no longer had the ability to discriminate speech successfully and it was therefore made conditional that intensive auditory training was undertaken.
II0
120
100
II0
so
This was agreed and five months later the considerably improved thresholds can be seen (line D). Her speech therapist has commented ‘Speech discrimination without lip reading when wearing her aid is quite remarkable. I have been delivering material in normal running speech patterns at a rate of up to 140 wpm. She has graduated with honours’.
At the present time we do not know in exact terms how speech discrimination is affected as you move outside the ‘hatched’ SPL boundaries, but we nevertheless feel that free field pure tone audiometry is a system which provides a valid target/ objective. We know that one of the most important prerequisites for the learning of language and speech is to utilize fully the potential of the residual hearing, and in view of this we feel that free field pure tone audiometry can be considered not only for the fitting or re-fitting of an aid but also to deter-
500 Frequency
response (Hz)
target
’
:,” 250
aided threshold
A valid
130
0000
Figure 2. Cubex test chart; showingfiee field unaided, l; and aided, n, A; profiles and acoustic response of aids; B, C.
(Hz)
I50
3 %
250
Frequency
(Hz)
30 30
20
20
IO
IO
0
0
I
I
I
I Frequency
I
(Hz)
3
150
% m E 2
140
70
1
I30 120
60 50
= .i
II0
40 30
.;
100 so 00
20 In
9
;, .o 5S 4
00
2
250
500 Frequency
1000
2000
4 000
0000
response (Hz)
Figure 4. Cubex test chart; for patient B.H. f unaided; n, A, aided C, November 1977; D, April 19 78.
J. Biomed.
Engng.
1980,
Vol2,
July
199
/fearing
aids:
M. S&u/berg
mine if the potential being used.
of the residual hearing is
The first tests Because of this the first free field tests we carry out on a patient are bin-aural as we want to know at this stage how the residual hearing is being used. However, monaural occlusion should be subsequently used (unazded and aided) to ensure that each ear is fitted according to the indicated requirement. It is also interesting to note that with asymmetrical losses an aided ‘cross-over’ effect can generally be observed. High frequency
losses
Occlusion is also essential when fitting severe high tone ‘ski-slopes’ and here we fit the earmould and the proposed aid (switched ofJ) to carry out comparative tests. One can then observe if the unaided thresholds and profiles are now affected before the selected aid/aids are switched on. Obviously this will also provide a useful guide to the amounts and benefits of venting. individual
requirements
There are many other situations where the tests will be a useful guide but I would stress that there is no simple formula. Individually there are large differences in requirements and the optimum gain and maximum output settings must relate directly to the usuable (comfortable listening) level. It is often apparent that one cannot continue to increase amplification of any one frequency in order to improve the threshold as one may just saturate the dormant potential, creating confusion, distortion and intolerance. SELECTING THE AID We are adding to our knowledge everytime we carry out tests and there is obviously much more to learn, but it is evident that audiometry in general has not yet produced a formula which actually selects the most suitable aid. Furthermore, because of the individual variable requirements we do not feel that a formula for the selection of an aid is possible and it is therefore the responsibility of the Audiologist to use his experience, skills and knowledge to make his selection, from the widest range possible.
adults using free field pure tone audiometry, and there are many important points that constantly emerge. For example in many instances one can predict the pure tone (headphone) audiogram from the unaided free field audiogram and also the likely threshold and profile of an aided audiogram. However we are currently extracting data on all these aspects and we hope to publish a further paper on this in the near future. COMFORTABLE LISTENING LEVELS With regard to ‘the usuable (comfortable listening) level’ for very young children this can generally be set according to subjective reaction and/or observation but as the ultimate objective of aid fitting is to raise conversational ‘speech sounds’ to a ‘hearing’ level, it is inevitable that unwanted background sounds are going to be raised proportionately unless one introduces a sophisticated compression system. It is not possible to pre-set a definite maximum output/pressure which is suitable in all conditions without some form of compromise but in general we feel that initially it is better to set for lower maximum outputs (relative to the pre-established gain requirements) until it is established that a higher maximum output is needed or can be tolerated.
CONCLUSIONS Finally, I would like to end by saying that if the subsequent results of aiding are dependent on the provision of good quality teaching, counselling and supportive help, then really good results can only be achieved when the initial aiding provides the optimum hearing threshold and profile to make the best use of the residual hearing. ‘Free Field pure tone audiometry - as a comparative system for the assessment and fitting of hearng aids’ has over the last four years proved itself to us time and time again. My hope now is that other Centres will investigate the system and use it to benefit their patients in the same way that we have.
PREDICTIVE AUDIOMETRY
REFERENCES
Over the last four years we have continually carried out numerous tests and follow ups on children and
Davis H. and Silverman S.R. (1960) ‘Hearing & Deafness’ Holt, Rinehart & Winston - New York - P.52 Fig. 2-6.
200
J. Biomed.
Engng.
1980, Vol 2, July