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Патент USA US2131669

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Sept. 27, 1938.
Filed May 31, 1935
2 Sheets-Sheet l
FRE‘Ol/E/VC/ ‘
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‘ SePt- 27, 1938-
Filed May 31, 1935
2 Sheets-Sheet 2
Patented Sept. 21, 1938
Alexander Poliako?, London, England
Application May 31, 1935, Serial-No. 24,445
In Great Britain June 4, 1934
1 Claim. (Cl. 179-107)
The present invention relates to all sound re
producing devices in which head telephones and
not loudspeakers are used to radiate sound, such
as ordinary communication telephones, deaf-aid
invention to a thermionic ampli?er used for deaf
aid or ordinary speech frequency ampli?cation. .
Figure 3 shows sound output characteristics
of the circuit in Figure 2, and
5 ampli?ers, ?eld telephones, aircraft radio receiv
ers and similar installations.
Hitherto, the majority of deaf-aids, both simple
microteiephones and valve ampli?ers, utilized a
single telephone.
Attempts have been made with‘valve ampli
?ers to shape the response curve of the apparatus
to compensate partially the patient's “tonal deaf
ness" as shown by his audiometric curve. Such
a curve ‘is commonly obtained by means of a
valve oscillator provided with a calibrated at
tenuator .or by means of a set of calibrated tun
ing forks. In both cases the patient hears one
note at a time, so that the disturbing or masking
e?ect by low and middle notes on the high notes
in‘ a complex sound is left out of consideration.
' This effect is known, however, to be ‘large and
to increase rapidly with the sound intensity level;
curves illustrating the effect may be seen in
_ Harvey Fletcher, Speech and Hearing, 1929, part
3, ch. IV.
. U
in a complex sound by the other components of 10
such sound, by the use of two head telephones
one on'geach ear, the-low and/or middle fre
quency components in the sound output of one
of which telephones being su?iciently reduced in
intensity not to mask appreciably the high fre
quency components in the sound output of one
tact with that telephone. Since there isno mask
ing of high frequency components received by
one ear by low and middle frequency components
received by the other ear, a given acoustic output » 20
of high frequency components in a complex sound
thus produces far greater sensation than would _‘
be the case if the whole output were applied to one ear only, or were shared between the two.
cars without frequency discrimination between’ 25.
as 40 decibels greater loss at say 4,000 cycles than
at 512 cycles. Witha portable valve ampli?er
employing a carbon microphone and the usual
' 30 -magnetic telephone not only is it impossible to
compensate for that loss but even to obtain a
level response between these frequencies at a
reasonable ampli?cation level, since both the
microphone and the telephone have large high
note losses and the number of valves used must
necessarily be small.
In addition, the high notes although present
in the telephone of such an instrument will not
be perceived by the listener on account of their
40 being masked by the noise in the room and by the
low and middle notes in the sound particularly
in the region of the principal resonance of the
_ telephone
The present invention" substantially eliminates
the masking of high audio frequency components
A common case of deafness will have as much
Figure 4 is a circuit diagram ofa portion of
the ampli?er in which separate valves are used
to feed the level and high pass telephones.
An experimental verification of. the last state
ment has been carried out as follows:—-
Two thermionic ampli?ers were constructed
with common input from a high ?delity non-v
carbon microphone coupled to the ampli?ers by
means of a valve common to both.
One of the
ampli?ers had an approximately level frequency
response characteristic, while theother had a
steeply risingcharacteristic. Two identical mag.
netic telephones were connected in the output in
such a manner that, by manipulating a switch,
the-outputs of the‘ two ampli?ers, could be ‘either
combined and shared equally'by the two tele
phones, or the high-pass and level outputs were 40
delivered to different telephones.- _
Figure 1 shows the curve of the high-pass am
pli?er relative to the level ampli?er in terms of
Deaf aid ‘apparatus supplying each ear with a
high ?delity signal ampli?ed separately to'com
pensate accurately the audiometric curvesof the. in decibels, where V1. and V1; are respectively volt
two ears, which are usually di?erent, apart from
its prohibitive bulk will not be free from the ‘
masking effect and its attendant great diminution
in high note perception,
The invention is hereinafter described with ref —
erence to the accompanying drawings in which:
Figure 1 is a chart showing the curves of a
, high-pass ampli?er relative to a level ampli?er.
55 Figure 2 shows a simple way of applying the
ages applied to the ?rst and common grid for v ‘
equal outputs from the level and high-pass am
The output level of the high-pass ampli?er
is below that of the level ampli?er at all frequen
cies except 4,000 cycles at which the two outputs
are equal.
With a listener of normal hearing acuity, the 55
experiment consisted of making speech in the
room containing the microphone unintelligible than a pair of like telephones but the high notes
by a controlled noise radiated by a loudspeaker > will still _be strongly masked by the middle notes.
It is not suggested that isolation of high notes
in the same room, the two telephones sharing
equally the outputs of the two ampli?ers. On in one telephone should replace correction, that ’
switching over to an asymmetric output, that is is, shaping of the sound output frequency char 5
with one telephone connected to'the high-pass acteristic to suit approximately de?nite types of
deafness or special requirements of listeners of
normal hearing acuity situated in noisy sur
roundings. It is claimed however that with a
givenavailable high note sound output in a com 10
plex sound the application of the invention re
ampli?er only, intelligibility of speech was re
stored. Noise of different nature such as a single
note of SOD-1,000 cycles, imitation petrol engine
noise and ampli?ed noise picked up by a-micro
phone in a workshop were tried with the same re
sult. Further the use of one phone only con
sults in'a far greater sensation being produced by
nected to the high-pass ampli?er did not pro->
15 duce intelligible speech, while with both tele
phones connected to the level ampli?er only and
the masking noise increased to the point where
speech was just unintelligible, intelligibility was
‘not restored by adding the-output-of the high
20 pass ampli?er which on account of its low level
was entirely lost.
coil telephone in the “high pass” position or
30 be applied to the simplest microphone systems em
ploying ordinary magnetic telephone receivers.
The output of the high-pass telephone alone
must not be confused with a frequency corrected
output, since in general the level of low and mid
dle frequency components will be below the
threshold of audibility so that the high-pass tele
quency response of the output of the ampli?er 15
feeding telephones T1 and T2 by means such as
an intervalve transformer described in U. S. Pat
ent No. ‘1,996,685. '
In practice it has been found that sufficient
low and middle frequency attenuation in the out 20
series condenser to meet the requirements of most
under as the “level” telephone and the other as
the “high pass” telephone. '
The invention is not dependent on the use of a
special receiver such as a condenser or a moving
even on the use of thermionic ampli?ers, and may
It has been found useful to control the fre
put of T2 may be produced by means of a simple
To facilitate description, the telephone repro
clucing the whole scale will be referred to here
the high notes.
deaf cases. Where space permits, however, a
proper ?xed or variable high-pass ?lter ‘may be
used, the construction'of which is common know 25
ledge. The use of separate thermionic valves to
feed T1 and T2 need only be adopted with cases
‘requiring a very large output of high notes. In
‘such cases the two output valves V2 and V2, Fig
ure 4, have their input from the valve V1, and the 30
high-pass output from the valve V2 to telephone
T2 may be obtained for example, by using two
intervalve transformers M1 and M2 with their
primaries in parallel through a small condenser
C2. The transformer M1 being of normal design, 35
phone alone will not usually provide intelligible feeds the grid of the level ouput valve V2 while
the transformer M2 is of the type described in'
As shown in Fig.2, two telephones T1 and T2 ‘ U. S. Patent No. 1,996,685, the response curve of
which is made more or less sharply rising with
40 which may be identical are connected in the out
frequency according to the setting of the asso
put, T1 direct, T2 through a condenser C1 the im
ciated potentiometer. The function of the small
pedance of which is high at low and middle fre
quencies compared with Z1 and Z2. If Z1‘and Z2 condenser C2 is to prevent the primary of M2 un
are the impedances of T1 and T2 respectively and duly lowering the impedance in the plate circuit
Z1=Z2, then the sound output of T2 will have a of the valve V1 for low and middle frequencies.
In some cases it is possible to do away with a 45
rising characteristic and will be at all frequen
?ltering arrangement by using receivers inthe
_ cies at a lower level than T1. ~If Z1 is made higher
than Z2 the sound outputiof T2 will be higher than high-pass position which by virtue of their elec
of T1 at high frequencies and lower at low and trical or mechanical construction have a natural
ly rising ‘response characteristic. A condenser
middle frequencies.
Curve I, Fig. 3, is the sound output of T1 with receiver for instance connected straight across a 50
magnetic receiver will in general reproduce high
T2 disconnected. Curve 2 is the sound output notes
of T1 with T2 in circuit, Z2 being slightly smaller
It has moreover been found that with impaired
than Z1. Curve 3 is the sound output of T2.
There is no. increase in the total acoustic output hearing the high-pass telephone should generally
on connecting T2 in addition to T1, but merely a be placed on the ear with better high note hear 55
transfer of half or more of the high frequency ing, which in the majority of cases is also the
output to the other ear.
The curves in Figure 3 should be contrasted
with those published for dual speaker combina
tions, the object of which is to reproduce a
greater range of frequencies than can be covered
by a single ‘speaker and in which the introduction
of the high note speaker greatly increases the
total high note output.
more sensitive ear at all frequencies.
The inven
tion has then the additional advantage of putting‘
more energy into the bad ear than the good ear
so that loud noises produce less distress than 60
would have been the case if .a single telephone
were used on the better ear.
So far mention has been made principally of
. vention does not consist merely in the use of two
deaf aid ampli?ers, the invention is however of
considerable value in sound reproducing systems 65
used by persons of normal hearing acuity.
The existence of large high note losses in the
ordinary communication telephone are of‘ course
better than the other. It is common in dual
'speaker'dombinations for the output of the two
units to be approximately equal at 1,000 cycles.
surroundings may be usefully regarded as su?er-~ 70
ing from fairly acute deafness since such high
notes as are transmitted will be largely masked by
In my invention the low and middle notes par
> ticularlythe latter are reduced in the high-pass
telephone to a low intensity level. Thus my in
70 telephones, one of which reproduces high notes ‘ well known. The user of a telephone in noisy
The headphone equivalent of such a pair of
speakers may have slightly higher intelligibility
the noise. - The same considerations apply in this
case as in deaf aid ampli?cation, and a high-pass 75
' 2,181,669
3 _'
telephone added to the output greatly enhances response of low, middle and high frequency com
ponents and‘ means for ‘altering the frequency
response in the second receiver to cause the low
I claim:
and middle frequencycomponents to be reduced
Sound reproducing apparatus for deaf aid am
to an extent that they do not mask the high fre- 5
pli?cation and communication in noisy surround
quency component of said second receiver.
ings, comprising two similar head telephone re
ceivers one for each ear and arranged in di?erent _
‘circuits, one receiver having a uniform frequency
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