null null

United States Patent [19]
[11]
[45]
Deno
Patent Number:
Date of Patent:
4,598,417
Jul. 1, 1986
[54] ELECTRONIC STETHOSCOPE
Assistant Examiner—Danita R. Byrd
[75]
[73]
[21]
[22]
Inventor:
Assignee:
Appl. No.:
Filed:
Attorney, Agent, or Firm-Scully, Scott, Murphy &
[51]
Int. Cl.4 ...................... .. A61B 7/04; H03F 21/00;
Norman S. Deno, State College, Pa.
Presser
Research Corporation, NY.
641,130
Aug. 15, 1984
[57]
of chestpiece. The subject electronic stethoscope uti
H04R 3/04; HO4R 19/01
[52]
ABSTRACI‘
An electronic stethoscope for reproducing at a user’s
ear as exactly as possible the sound pressure signals
originating from a conventional diaphragm or bell type
lizes a signal processing approach which relies upon
U.S. Cl. ................................ .. 381/67; 179/111 E;
acousto-electronic feedback to provide an error or ad
181/131; 381/95; 381/96; 381/121
justment signal to ampli?er gain control circuits. A
[58]
Field of Search ................... .. 381/67, 95, 96, 121;
[56]
128/715, 696; 181/126, 131; 179/111 E
References Cited
pickup microphone detects audible sounds from a pa
tient and produces an output signal representative
thereof, and a variable gain ampli?er ampli?es the out
U.S. PATENT DOCUMENTS
3,321,041
3,525,810
5/1967
8/1970
3,764,748 10/1973
3,989,895 11/1976
3,989,904 11/1976
4,220,160 9/ 1980
put signal of the pickup microphone. The ampli?er
drives an acoustic transducer in a headset which con
Bowen, Jr. ........................ .. 181/131
Adler .
Branch et a1. .
O’Daniel, Jr. ...................... .. 381/67
Rohrer et a1. .
Kimball et a1. .
verts the ampli?ed signal to audible sounds for audible
detection by the user. A feedback microphone is placed
near the acoustic transducer to detect the audible
sounds produced thereby, and provides a feedback out
put signal. A control circuit compares the feedback
output signal with the output signal produced by the
4,548,082 10/ 1985 Engcbretson et al. .
FOREIGN PATENT DOCUMENTS
36230
2141141
9/ 1981
2/1973
European Pat. Off. ............ .. 381/96
Fed. Rep. of Germany ...... .. 381/96
622207
8/1978
U.S.S.R. .............................. .. 381/95
pickup microphone, and in dependence thereon con
trols the transfer function of the variable gain ampli?er,
such that the audible output of the acoustic transducer
is substantially the same as the audible input to the
pickup microphone.
Primary Examiner-Gene Z. Rubinson
10 Claims, 7 Drawing Figures
r.
:OPTIONAL
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‘USER’
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CORRECTION
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HEAD
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OUTPUT FOR
MULTIPLE HEADPHONES
' U.S.,Patent
Jul. 1,1986
Sheet 2 Of4
MST
FIGBQ
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FIGBD
4,598,417
U.S. Patent Jul‘. 1, 1986
-
Sheet v3 of4
4,598,417
l5-4O Hz BANDPASS
R43
40-100 HZ BANDPASS
R54
R55
R44
WM
IOO- 1000 Hz BANDPASS
R76
lJRSQ
'
L23
U.S. Patent Jul. 1,1986‘
Sheet4of4
4,598,417
'S
031
-
|_--_
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‘
OFFSET
'
R82
Q6
032
ADJUSTMENT
A
R90
ADJUSTABLE
VOLUME
B. AS
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ADJUSTMENT
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R91
I I
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l I
'
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R89
Res
UNIVERSAL EAR MUFF
HEARING PROTECTOR
__/FoAM CUSHION
__,/—ELECTRET MICROPHONE
q SPEAKER ELEMENT
.THREE-WIRE HEADPHONE CORD
FIGES
Q
7
1
4,598,417
2
only the sum of the heart sounds, which is then ampli
tied and presented to a conventional loudspeaker, head
ELECTRONIC STETI-IOSCOPE
phone, audio system, phone line, tape recorder, or radio
transmitter with suitable bandwidth, for aural interpre
tation. This electronic stethoscope is typical of other
prior art electronic stethoscopes in that the ampli?er is
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an elec
tronic stethoscope, and more particularly pertains to an
electronic stethoscope which produces an audible out
simply a high ?delity electrical ampli?er which does
not exactly reproduce at the user’s ear the original
sound signals from the patient, and also introduces dis
put response having frequency components and charac
teristics which accurately match those of the initially 10 tortions thereto in accordance with the transfer charac
detected auscultatory sounds.
teristics of the ampli?er, an acoustic to electrical trans
2. Discussion of the Prior Art
ducer, and an electrical to acoustic transducer.
The inherent promise embodied in an electronic
SUMMARY OF THE INVENTION
stethoscope is that it will match the tonal qualities of a
standard ?exible tubing stethoscope while reducing 15 Accordingly, it is a primary object of the present
background and tubing noise, increasing the user’s com
invention to provide an electronic stethoscope which
fort, removing the risk of transmitting ear canal infec
reproduces at the user’s ear as exactly as possible the
tions, allowing tape recording and conferencing among
sound pressure signals originating from any conven
multiple users and compensating for a user’s hearing
tional diaphragm- or bell-type chestpiece.
loss. However, past commercial attempts to produce an 20
A further object of the subject invention is the provi
electronic stethoscope have received little enthusiasm
sion of a Dynamically Altering Transfer Audio
from the medical community because these units had
(DATA) electronic stethoscope which implements an
signi?cantly different frequency response characteris
innovative electronic and acoustic design to achieve the
tics when compared to standard rubber tubing stetho
full potential forseen for electronic stethoscopes while
scopes, and also introduced objectionable signal distor 25 overcoming the past problems associated therewith.
tion and background noise. To gain medical acceptance,
This unit is preferably a pocket-sized instrument with
an electronic stethoscope should sound the same to
comfortable headphones and a microphone pickup cap
physicians who are trained and practiced in recognizing
sule which is designed to attach to any conventional
audio cues which are transmitted by an auscultation
bell or diaphragm chestpiece. The DATA stethoscope
is preferably free of volume and tonal adjustments, and
provides a full required dynamic range without intro
ducing harmonic distortion or background “hash”
instrument. This requirement to recreate the sound of a
given standard stethoscope is a taxing one from an engi
neering point of view.
The realistic reproduction of clinically important
noise, while also reducing the ambient room noise
audible signals pushes currently available audio equip
ment to the limit by requiring high level, distortion-free 35 reaching the physician. Advantageously, the DATA
stethoscope can also have a calibrated (known milli
volts per sound pressure level) output port, to which
response down to the subsonic range. An electronic
stethoscope should have a wide dynamic range with a
another DATA stethoscope or recording equipment
low noise level to satisfy the wide dynamic range, keen
discrimination, and high sensitivity of the human ear.
The audible portion of the frequency spectrum of heart
can be connected.
sounds has been reported to be 40-500 Hz, and for
Korotkoff sounds, the range is 20-300 Hz. The greatest
energy of these signals is contained in the lowest fre
quencies, with resting heart sound, pressure levels re
ported to be 80 dB-SPL (0 dB-SPL at 0.0002 dyne/cmz) 45
at 20 Hz.
Kimball, et al. U.S. Pat. No. 4,220,160 discloses, for
example, an electronic stethoscope in which detected
audible heart sounds at sonic and subsonic frequencies
The subject invention utilizes a signal processing
approach which relies upon acousto-electronic feed
back to provide an error or adjustment signal to ampli
tier gain control circuits, thus ensuring that the signal
heard by the listener is as exact a reproduction of the
ausculated signal as possible. This acousto-electronic
feedback approach preferably utilizes pickup and feed
back microphones having as closely identical character
istics as possible.
In accordance with the teachings herein, the present
are detected and converted into somewhat correspond 50 invention provides an electronic stethoscope which
ing electrical signals, which are then transposed in fre
includes a pickup microphone for detecting audible
quency to a range more easily detectable by the human
ear. The new frequency range can also be suitable for
' sounds from a patient and for producing an output sig
nal representative thereof. An automatic variable gain
ampli?er is coupled to amplify the output signal of the
transmission over conventional phone lines, for discrim
ination of low intensity or brief heart sounds, and for the 55 pickup microphone. The ampli?er drives an acoustic
transducer which converts the ampli?ed signal to audi
display of the heart sounds on conventional visual re
cording devices such as, cardiographs, storage oscillo
ble sounds for audible detection by a person utilizing the
electronic stethoscope. A feedback microphone is
scopes, and chart recorders. The transposition of the
heart sound frequency components involves the addi
placed near the acoustic transducer to detect the audible
tion of a constant frequency component to all of the 60 sounds produced thereby, and provides a feedback out
heart sound frequency components in such a manner as
to preserve the characteristics of the heart sound fre
quency components. The transposer circuit employs a
put signal.
component to provide sum and difference frequency
that the audible output of the acoustic transducer is
substantially the same as the audible input to the pickup
microphone. In this arrangement, the transfer function
A control circuit compares the feedback output sig
nal with the output signal produced by the pickup mi
voltage multiplier circuit for multiplying the heart
crophone, and in dependence thereon controls the
sound frequency components with a constant frequency 65 transfer function of the variable gain ampli?er, such
components of the heart sounds. The sum and differ
ence frequency components are then ?ltered to produce
3
4,598,417
of the ampli?er is controlled such that the feedback
output signal of the feedback microphone is substan
tially equal to the output signal of the pickup micro
phone, which thereby compensates for the distortion
characteristics of the microphones and headphone. The
pickup microphone and the feedback microphone are
substantially identical microphones, and preferably are
condenser electret microphones. In this arrangement,
conceptually since the microphones are as identical as
possible with identical distortion characteristics, then
by controlling the ampli?er transfer characteristics such
that the microphone output signals are as identical as
possible, the audible output of the acoustic transducer is
driven to be as identical as possible to the audible input
to the pickup microphone. In this arrangement, the
headphones are preferably suspended planar diaphragm
headphones which have superior harmonic and reso
nance characteristics.
In the preferred embodiment, the transfer character
istic of the ampli?er is varied by providing ampli?ca
tion through a plurality of different bandpass ampli?ers.
In one preferred embodiment, a low frequency ampli
?er is provided with a relatively high gain factor, a high
frequency ampli?er is provided with a relatively low
gain factor, and an intermediate frequency ampli?er has
a gain factor somewhere between the high and low gain
factors, such that the low frequency portion of the sig
4
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing objects and advantages of the present
invention for an electronic stethoscope may be more
readily understood by one skilled in the art with refer
ence being had to the following detailed description of
a preferred embodiments thereof taken in conjunction
with the accompanying drawings wherein like elements
are designated by identical reference numerals through
out the several views, and in which:
FIG. 1 is a block diagram of an exemplary embodi
ment of a embodiment of an electronic stethoscope
constructed pursuant to the teachings of the present
invention;
-
FIG. 2 illustrates one exemplary embodiment of a
coupler assembly for mounting an acoustical pickup
microphone relative to a conventional chestpiece cou
pler;
FIGS. 30 and 3b illustrate exemplary schematics of
preampli?er circuits for use with the present invention;
FIG. 4 is a schematic illustration of three adjacent
bandpass ampli?er circuits having automatic gain con
trol circuits for varying the transfer characteristics of
the ampli?ers;
FIG. 5 is an exemplary schematic circuit of a sum
ming and power ampli?er circuit which is coupled to I
the output of the variable gain ampli?er of FIG. 4; and
FIG. 6 illustrates one exemplary embodiment of a
headphone for use with the present invention.
‘»='1;'nal spectrum, which is very important for medical anal
30
‘ ysis, is provided with the greatest signal energy. In the
DETAILED DESCRIPTION OF THE
disclosed embodiment, the low frequency ampli?er has
DRAWINGS
' a bandpass from 15 to 40 hertz with a gain factor of
Referring to the drawings in detail, FIG. 1 illustrates
approximately 8 dB, the intermediate frequency ampli
a block diagram of a preferred embodiment of an elec
?er has a bandpass from 40 to 100 hertz with a gain 35 tronic stethoscope pursuant to the present invention in
' factor of approximately 6 dB, and the high frequency
which a pickup acoustical microphone transducer 10 is
ampli?er has a bandpass from 100 to 1000 hertz with a
mounted relative to a chestpiece to detect auscultatory
. gain factor of approximately 0 dB.
sounds from a patient through a conventional dia
The embodiment disclosed herein was designed to be
phragm or bell-type chestpiece. The output from the
....utilized with a conventional diaphragm or bell type 40 transducer 10 is directed through a preampli?er 12 to a
_.,i-.chestpiece for detecting ausculatory sounds. However,
control circuit variable gain ampli?er 14, the transfer
~.the.teachings herein have broad applicability for the
characteristics of which are controlled as described
detection of many different types of sounds.
hereinbelow. The variable gain ampli?er 14 is coupled
to a power ampli?er 15 which then drives a headphone
Moreover, in accordance with another advantageous
feature of the present invention, a compensating circuit 45 acoustic transducer 16 of a headset. The headphone
acoustic transducer 16 is preferably a suspended planar
is provided to enable the transfer function of the vari
diaphragm type of headphone. A feedback microphone
able gain ampli?er to be varied to compensate for the
18 is placed in the headset near the acoustic transducer
particular hearing loss characteristics of a particular
16 to detect its audible output, and provides a feedback
person, which could be a signi?cant feature. Since the
output signal to a further preampli?er 20 which is cou
response characteristics of the DATA stethoscope can
pled as a second input to the control circuit 14.
be varied or selected, the inverse response characteris
The control circuit 14 compares the feedback output
tics of the user’s hearing loss response characteristics
signal
from feedback microphone 18 and preampli?er
can be selected in a special DATA stethoscope unit
20 with the output from the pickup microphone 10 and
such that the resultant sound heard by the user is sub
stantially the audible sound originating at the chest 55 preampli?er 12, and controls the transfer function of the
piece.
variable gain ampli?er 14 such that the audible output
Another advantageous feature of the present inven
tion is the provision of a trainer/conference, input/out
put port, which expands the medical utility of the elec
tronic stethoscope. The port enables a calibrated
(known millivolts per sound pressure level) tape record
ing of the auscultation signal, or sharing of the electrical
output signal with a student or colleague possessing
the audible input to the pickup microphone. In this
arrangement, the transfer function of the ampli?er is
controlled such that the feedback output signal of the
feedback microphone is substantially equal to the out
put signal of the pickup microphone, which compen
of the acoustic transducer is as identical as possible to
sates for the distortion characteristics of the micro
phones and headphones, including the distortion char
acteristics of the pickup microphone 10, the acoustic
another DATA stethoscope, or other persons in the 65
room could listen to the ausculated sounds by connect
transducer 16, and the feedback microphone 18. The
ing the trainer output to an audio ampli?er with speak
ers having a suf?cient bass response.
pickup microphone and the feedback microphone are
substantially identical microphones, and preferably are
5
4,598,417
condenser electret microphones. conceptually, since
the microphones are as identical as possible with identi
cal distortions, then by controlling the ampli?er 14
transfer characteristics such that the microphone 10, 18
6
erably a low noise electret cartridge 26 (Sony model
EC-l) excised from a commercially supplied case and
sealed in a cylindrical capsule 28, 1.4 cm in diameter and
5 cm long, into which a chestpiece 30 is also inserted.
The chestpiece coupler serves as an interface be
output signals are as identical as possible, the audible
output of the acoustic transducer 16 is driven to be as
tween standard auscultatory equipment and the elec
identical as possible to the audible input to the pickup
tronic stethoscope. Two simple user-oriented objectives
microphone 10.
for this coupler were to make it compatible with any
The electronic stethoscope includes a trainer/confer
auscultatory chestpiece, and to make it similar to a
ence input/output port 22, which greatly enhances or - O conventional stethoscope in feel and in ease of use. Two
expands the DATA stethoscope’s medical utility. The
performance criteria were that the coupler should have
port enables a calibrated (known millivolts per sound
good low frequency response and an inaudible noise
pressure level) tape recording of the ausculated input,
contribution. The chestpiece coupler which was de
or sharing of the signal with a student or colleague
signed with these considerations is a plastic tubing shell
possessing another similar electronic stethoscope. Fur 5 which houses a microphone cartridge and admits a
thermore, other persons and colleagues could listen to
3/16" outer diameter (o.d.) chestpiece stem, (FIG. 2).
the ausculated sounds by connecting the trainer output
The overall size of the chestpiece coupler is 2 5" long
to an audio ampli?er with associated speakers having a
by a" diameter. It is constructed from a 1” length of
suf?cient bass response.
3/16" inner diameter (i.d.) plastic tubing (Cole Parmer
One of the drawbacks to conventional acoustic 20 6408-45) which is ?xed inside a 2 5” length of %" i.d.
stethoscopes is that only one person at a time can listen
tubing (Tyson R-3603).
to them. Since it is very dif?cult to accurately describe
The microphone cartridge is mounted in the %" tub
the subjective experience of sound, it is dif?cult for new
ing i" from the end of the 3/16" tubing. The cartridge
students to learn auscultatory techniques if they must
and lead dressing take up the 1" remaining in the %"
match a teacher’s description with what they them 25 tube, and the output leads are set in a silicone sealant.
selves perceive. Similarly, physicians cannot always
The electrical cable is preferably a 3’ length of two-lead
reach a consensus on what malady they observed singly
shielded cable terminated by a subminiature phono
with an acoustic stethoscope, and the disparities in their
plug.
diagnoses are only aggravated by the need for imagina
In the disclosed embodiment, a Sony EC-l Electret
tive descriptions merely to communicate what they
Condenser Microphone was selected for the pickup
heard. Both of these situations can be improved with
microphone on the basis of its very low noise perfor
the DATA electronic stethoscope since it has a special
mance compared with other available microphones. A
port to allow sounds ausculated with one device to be
commercial microphone was disassembled, and the
transmitted to another.
microphone cartridge was removed for use in the pres
The trainer/conference port is designed to meet three
ent invention. The microphone was mounted inside the
criteria. First, it provides a calibrated (known millivolts
direct-coupled, sealed case, and achieved a flat (+/—l
per sound pressure level) output from the primary
dB) frequence response down to 30 Hz, and unit to unit
stethoscope which obtains the auscultated signal. Se
variation was within 2 dB. This represents a consider
condly, it is constructed so that electronic stethoscopes
able improvement in low frequency response beyond
may serve interchangeably as the primary stethoscope 40 the commercial form of the microphone assembly.
or as the secondary stethoscope, with their respective
In the preferred embodiment, the headphones 16 are
roles established by a directionally dependent connec
preferably planar diaphragm speaker elements (Radio
tor. The directional dependance is established by the
Shack Realistic Model PRO-30) removed from their
interconnecting cable. The electronic stethoscope to
commercial headset and mounted in a hearing protector
which the source cable is connected becomes the source 45 headset (Bilson Universal Ear Muff) along with a sec
of acoustic information. The remaining connector is the
ond closely matched EC-l electret microphone 18,
listener connection. Thirdly, its presence must be trans
which provides ambient noise reduction and a well
parent to the primary user, whether or not a secondary
unit is attached.
sealed environment for the planar speakers to produce
low frequency sounds. These components and mount
Compensation for a user’s hearing loss using the elec
tronic stethoscope is another signi?cant feature of the
present invention. Since the response characteristics of
the stethoscope can be controlledly changed, the in
ings were chosen to maximize the low frequency capa
those frequencies are later ampli?ed to a greater extent 60
possible.
bilities, and the signal processing circuits described
herein are also designed and employed to remedy their
residual de?ciencies. This signal processing relies on
verse response characteristics of a user’s hearing loss
acousto-electronic feedback to provide the error signal
response characteristics can be selected in a compensat 55 to the gain control circuits, thus ensuring that the signal
ing circuit 24 such that the resultant sound heard by the
heard by the listener is as exact a reproduction of the
user is the same as the sound originating at the chest
auscultated signal as possible. The acousto-electronic
piece. For instance, the compensating circuit 24 can
feedback principle requires that the pickup and feed
selectively attentuate different frequencies such that
back microphones have as identical characteristics as
by the variable gain control circuit 14. The compensat
The second Sony EC-l microphone cartridge
ing circuit can be simply parallel bandpass ?lters feed
mounted in the headphones provides a feedback signal
ing a separately adjustable potentiometer for each band
which is a measure of the acoustic response realized by
pass, with the outputs of the separate potentiometers
the system and heard by the user. As will be described
being summed and combined again for an input to the 65 in more detail hereinbelow, this feedback is used to
variable gain circuit 14.
adjust the system’s gain so that the output of the feed
FIG. 2 illustrates a preferred embodiment of a chest
back microphone matches as closely as possible the
piece coupler in which a pickup microphone 10 is pref
output of the chestpiece microphone. In practice, the
7
4,598,417
output characteristics of the two microphones are
matched as closely as possible from commercially avail
able similar components.
The preampli?ers 12, 20 can be simply two matched,
high gain, low noise ampli?ers which provide gain for
the two microphones 10, 18 of matched sensitivity. The
two substantially identical circuits of FIG. 3, with the
individual electrical components and integrated circuits
given in the Table of Component Values herein, are
suitable for preferred embodiments of the present inven
tion.
In a preferred embodiment, the transfer characteristic
of the ampli?er section 14 is varied by providing ampli
?cation through a plurality of different, adjacent band
pass ampli?ers. In one preferred embodiment, a low
8
lected maximum gains of 8 dB for the l5-40 Hz section,
6 dB for the 40-100 Hz, and 0 dB for the 100-1000 Hz.
5
The overlay of the rolloff regions and the adjoining
bandpass sections results in the actual gain being higher
than the speci?ed gain. Examination of the headset
response shows that the bandpass regions and chosen
gains are well suited for equalizing the low frequency
response.
A very important feature of this electronic stetho
scope design is the use of acousto—electronic feedback to
regulate the headphone output so that it more nearly
matches the chestpiece coupler input, despite variations
in the frequency response of the chestpiece microphone
and the headphone due to resonances and attenuation.
A signi?cant feature of the automatic gain control cir
cuitry is that it corrects for the poor low frequency
frequency ampli?er is provided with, a relatively high
gain factor, a high frequency ampli?er is provided with
' response of headphones in the critical 20-50 Hz region.
a relatively low gain factor, and an intermediate fre
quency ampli?er has a gain factor somewhere between
The automatic gain control also adjusts for limited vari
ations in the quality of the seal achieved at the earcush
the high and low gain factors, such that the low fre
quency portion of the signal spectrum, which is very
important for medical analysis, is provided with the
greatest ampli?cation and signal energy. In the dis
closed embodiment, the low frequency ampli?er has a
20 ions of the headset.
,
Gain control of 20 dB is achieved by using a single
JFET and some additional passive components in the
circuit shown in FIG. 4. JFETs Q3, Q4, and Q5, one for
each bandpass section, all share the same design and
bandpass from 15 to 40 hertz with a gain factor of ap 25 nearly the same component values. Using Q3 of the
proximately 8 dB, the intermediate frequency ampli?er
has a bandpass from 40 to 100 hertz with a gain factor of
1540 Hz bandpass section as an example, the JFET is
used as a Voltage Variable Resistor (VVR) with resis
approximately 6 dB, and the high frequency ampli?er
tor R40 in a current divider network. As the Gate-to
has a bandpass from 100 to 1000 hertz with a gain factor
Source Voltage (V GS) on Q3 is increased above about
of approximately 0 dB.
30 -2.0 V, the JFET begins to turn on and its Drain-to
Referring in particular to FIG. 4, the signal process
Source resistance decreases. Consequently, this VVR
ing task is shared by three parallel bandpass sections 32,
shunts some of the signal to ground that would other
34, and 36, which together cover the entire spectrum of
wise go through R40 into the summing junction of
interest of auscultated sounds. The gain of each particu
1015. As the VGS becomes more positive, more signal
lar bandpass section is regulated by an automatic gain 35 current (Is) is drawn away, decreasing the voltage gain
control circuit, respectively 33, 35 and 37. These band
pass sections compensate for variations in acoustic load
ing at the headphones and also for frequency distortion
caused by the pick-up microphone and the headset
speakers. The three bandpass sections also allow a
, greater emphasis on the low frequencies since this por
. tion of the signal spectrum encompasses both the great
achieved at the output of IC-15.
For the J FET to provide a linear change in resistance
with varying VGS and Drain-to-Source voltage (Vds)
bias, it must be operated with Vds near zero. In prac
tice, this turns out to be Vds of 0.2 V P-P. To keep Vds
in this range, it is necessary to form a voltage divider
with R38 and R40 to limit Vds to this value for maxi
mum signal levels (assumed to be 10.0 V P-P for 15-40
Hz, 7.4 V P-P for 40-100 Hz, and 3.6 V P-P for the
est auscultation signal energy (a typical auscultation
signal has its largest sound pressure levels at its lower
frequencies) and the weakest headphone response. The 45 100-1000 Hz bandpass). Linearity of the JFET is also
gain control circuits 33, 35 and 37 compare the ausculta
maintained by the AC feedback provided by C17 and
tory input level and the acoustic output level and adjust
R39.
the gain of each bandpass section to minimize the error.
The quiescent value of the gain control ampli?er is
A quiescent gain level is established by potentiometers
R35, R54 and R73 in the absence of an ausculated input
signal . Because of the low repetition rate of the aus
set by R35 to about —l.6 V. This level is a prudent
choice as it falls about in the middle of the gain versus
VGS curve, and it gives a value of RDS large enough to
culated signals, the gain control circuits have a 2 second
time constant. Each gain control section has a dynamic
prevent quiescent hash noise generated by the gain
range of 20 dB, which is suf?cient to correct the re
ming ampli?er (IO-15) is provided to attenuate any
control ampli?er from being audible. C31 on the sum
maining errors in the pickup microphone and the 55 noise above 1 kHz.
speaker response characteristics. Separate quiescent
The dynamic value for gain control is varied around
gain and variable gain control is used for each section so
the set point by the action of the automatic gain control
that the sum of all three sections yields a close approxi
level ampli?er. This ampli?er sums the oppositely
mation to a ?at frequency response for the entire elec
phased, half-wave recti?ed outputs of the chestpiece
tronic stethoscope system.
60 and feedback preampli?ers. Their overall amplitude
The bandpassing is implemented in FIG. 4 using sin
difference is thus obtained and, greatly ampli?ed, is
gle stage two-pole ?lters, of which the con?guration for
used to charge a large capacitor, C16, with a 2-second
IC-6 is a good example. Each chestpiece bandpass ?lter
time constant. As the feedback signal amplitude falls
has a matching feedback bandpass ?lter. This is neces
below the chestpiece input signal over time (this time
sary for computing a true error level between the chest 65 constant is dictated by the heart rate), more gain is
piece and feedback inputs within each bandpass.
called for by decreasing VGS.
The 15-1000 Hz range is divided into three subranges
The output signal of the bandpass ampli?ers of FIG.
of 15-40 Hz, 40-100 Hz, and 100-1000 Hz, with se
4 is directed to a summing and power ampli?er circuit
9
4,598,417
10
as shown in FIG. 5. The output of the summing ampli
-continued
?er IC-lS (FIG. 5) is the completely processed chest
piece signal. The additional ampli?er stage provides the
high current drive required by the headphones. This
IC6-15
(l) 4741 Quad Op Amp
ampli?er should be suitable for a battery-powered in
Q3-5
2N4220A Low Noise, Gen Purp JFET
strument, have low quiescent power consumption,
Q6
IRF 520 N-Channel power HOSFET
being capable of driving an 8 ohm headset, have a flat
Q7
IRF 9520 P-Channel power MOSFET
D1-6
IN4148
frequency response over the 15-1000 Hz range, and
have inaudible noise and cross-over distortion. A pre
Chestpiece Preamp
Feedback Preamp
R1
R2
R4
R5
R6
R7
R15
R16
R18
R19
R20
R21
Table of Component Values
w
ferred power ampli?er, FIG. 5, is an AC-coupled input,
class AB power ampli?er with a DC-coupled output.
This simple ampli?er uses only two active components,
a pair of complementary power MOSFET’s (IFR 520
and IRF 9520). The bias network is a trimmed resistive
level to 0 Volts.
The ampli?er has 0 dB of gain in a no-load situation,
but is only down —2.5 dB for an 8 ohm load even be
yond the rated full scale output of 10 V P-P. This power
ampli?er exhibits a flat (+/- 1 dB) response with negli
gible distortion from 4 Hz to 100 kHz into 8 ohms.
Power consumption is preferably maintained as low
as possible so the electronic stethoscope is battery-pow
ered and suf?ciently portable and small to be placed in
a pocket. The strenuous power demands of the head
MM
ma.
R22,26
51.K
R41,45
68K
1160,64 150K
R24,28
270K
R43,47
' 270K
R25,29
R30,3l
R32
20 R33
270K
150K
75.K
75.K
R44,48
R49,50
R51
R52
270K
150K
75.K
75.K
1163,67 180K
1168,69 150K
R70
2.0 M
R71
75.K
R34
R35
R36
R37
R38
25 R39
R40
200K
500K Pot
510K
1.0 M
100K
1.0 M
2.0K
R53
R54
R55
R56
R57
RS8
R59
200K
500K Pot
510K
1.0 M
100K
1.0 M
3.0K
R72
R73
R74
R75
R76
R77
R78
mounted in a pocket-sized, 13.8X 8.3 ><2.8 cm case. The
R82
100K
R84,85
R86,88
100
500K
1187,89
1.0-M
R90
R91
R92
1.0 M Pot
500K Pot
100
total weight of the case and headphones was 780 grams,
response at low frequency, high intensity outputs ap
pears to be an inherent problem with dynamic-type
headphones. Fortunately, a new type of speaker ele
ment is available which, similar to electro-static speak
ers, uses a single thin planar diaphragm to produce
180K
200K
500K Pot
1.0 M
1.0 M
91.K
1.0 M
10.K
Sam
Chestpiece Preamp
and the battery pack had enough power for 10 hours of
Harmonic distortion that accompanys the headphone
R62,66
Emmi“
phones, however, necessitate a battery pack with a
moderately high current rating. In one constructed
embodiment of the present invention, the stethoscope
circuitry and a rechargeable battery pack were
operation before recharging.
9.1K
9.1K
2.0K
51.K
2.0K
200K
2%
con?guration (R86-92), which is adjusted so that the
ampli?er draws only 15 mA when quiescent. Another
trim potentiometer (R90) is used to set the output DC
9.1K
9.1K
2.0K
51.K
2.0K
200K
35 C1,3
10 uF
Feedback Preamp
C8,1O
10 uF
Deleted: C2,4-7,9,1l,30
BandPass
Cl2,14
C13,l5
C16
40 C17
1
.05 uF
.05 uF
BandPass 2
C18,20
.02 uF
Cl9,21
.02 uF
10. uf
.05 uF
C22
C23
10. uF
.05 uF
BandPass 3
C24,26 .001
C25,27 .015
C28
10.0
C29
0.05
uF
uF
uF
uF
Output Ampli?er
C31
C32,33
sound waves with very little harmonic distortion, even
in the 25-50 Hz range. These planar-type headphones
500 pF
.0068-.05 uF
are available as the Realistic PRO 30, by Radio Shack.
Evaluation of the ability of a preferred embodiment
They had less perceptible harmonic distortion than
other tested dynamic headphones, but when measured,
of an electronic stethoscope as described herein to emu
late a standard stethoscope was carried out through
their response and harmonic distortion seemed similar
both objective acoustical measurements and subjective
to that of the dynamic types. In a preferred embodi
clinical evaluations. An operative embodiment of a
ment, the speaker elements were removed from the
PRO 30’s and they were mounted in a pair of supra 50 electronic stethoscope was given to several clinical
aural hearing protectors such as Bilsom Com?ex or
physicians and research physiologists for subjective
Bilsom Universal ESR hearing protectors. The tight
evaluation of its use for heart sounds and blood pressure
measurements. The subjective evaluations for users
seal provided by the hearing protectors improved the
experienced in the use of conventional stethoscopes
were generally very favorable. All users expressed the
opinion that the sound quality of the electronic stetho
low frequency response, and the reduction in ambient
noise served to further increase the signal to noise ratio.
The circuitry of the electronic stethoscope was de
signed to complement the frequency response of this
headset. The 8 and 6 dB quiescent gain provided by the
15-40 and 40-100 Hz bandpass sections offset the head
phone’s loss in these areas.
scope was equal to that of a conventional stethoscope.
Several users expressed very favorable opinions about
the comfort of the headsets, the conferencing feature
60 for multiple listeners, the elimination of motion-induced
noise by elimination of the conventional tubing, and
possibility for hearing loss compensation. All simulta
The following Table of Component Values lists par
ticular values for the circuit components of FIGS. 3, 4
and 5 in one designed embodiment thereof.
neous blood pressure measurements using the electronic
and conventional stethoscopes produced equal results.
65
Table of Component Values
ACTIVE DEVICES
IC1,4
(i) TL074 Quad Low Noise BiFET Op Amp
While several embodiments and variations of the
present invention for an electronic stethoscope are de
scribed in detail herein, it should be apparent that the
disclosure and teachings of the present invention will
11
4,598,417
12
suggest many alternative designs to those skilled in the
for the distortion characteristics of said pickup micro
art.
phone.
4. An electronic stethoscope as claimed in claim 1,
What is claimed is:
wherein said pickup microphone and said feedback
microphone are substantially identical microphones.
1. An electronic stethoscope, comprising:
a. a pickup microphone for detecting audible sounds
from a patient and for producing an output signal
5. An electronic stethoscope as claimed in claim 4,
said pickup and feedback microphones comprising con
representative thereof;
denser electret microphones.
b. a control circuit including a variable gain ampli?er
means coupled to the output signal of said pickup
said variable gain ampli?er means comprising a plural
microphone for amplifying the signal, said variable
gain ampli?er means having a dynamically control
lable transfer function;
ity of different bandpass, variable gain ampli?ers. .
7. An electronic stethoscope as claimed in claim 6,
said plurality of different bandpass ampli?ers compris
0. at least one acoustic transducer coupled to said
ing at least a low frequency ampli?er having a relatively
ampli?er for converting the ampli?ed signal to
high gain factor, a high frequency ampli?er having a
audible sounds for audible detection by a person
relatively low gain factor, and an intermediate fre
utilizing the electronic stethoscope;
quency ampli?er having a gain factor between said high
and low gain factors, such that the low frequency por
tion of the signal spectrum comprises the greatest signal
d. a feedback microphone for detecting the audible
sound produced by said acoustic transducer and for
providing a feedback output signal representative
thereof‘; and
20 energy.
8. An electronic stethoscope as claimed in claim 7,
c. said control circuit comparing said feedback output
signal with the output signal produced by said
pickup microphone to dynamically control the
transfer function of said variable gain ampli?er,
such that the audible output of the acoustic trans
ducer is substantially the same as the audible input
I
6. An electronic stethoscope as claimed in claim 1,
said low frequency ampli?er having a bandpass of sub
stantially 15 to 40 hertz and a gain factor of substantially
8 dB, said intermediate frequency ampli?er having a
25 bandpass of substantially 40 to 100 hertz and a gain
factor of substantially 6 dB, and said high frequency
ampli?er having a bandpass of substantially 100 to 1000
hertz and a gain factor of substantially 0 dB.
to the pickup microphone.
2. An electronic stethoscope as claimed in claim 1,
9. An electronic stethoscope as claimed in claim 1,
said control circuit including a compensating means to
said pickup microphone being coupled to a pick-up unit
for detecting ausculatory sounds from a patient, and
enable the transfer function of said variable gain ampli
said acoustic transducer being mounted in a headset.
3. An electronic stethoscope as claimed in claim 1,
wherein the transfer function of said variable gain am
?er means to be varied to compensate for the particular
hearing loss characteristics of a particular person.
10. An electronic stethoscope as claimed in claim 1,
pli?er is controlled such that the feedback output signal 35 further including an auxiliary output port to enable the
of said feedback microphone is substantially equal to the
output signal to be utilized for an auxiliary function.
*
*
it
*
it
output signal of said pickup microphone, to compensate
45
55
65

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