Advancements of Electronic Stethoscope: A review

HSH Perera

Deparment of Electrical,Electronic and Telecommunication Engineering, Faculty of Engineering,

General Sir John Kotelawala Defence University, Sri Lanka
< hshperera99@gmail.com>

Abstract— Auscultation is one of the most popular auscultation method is widely practiced in the world using
method of disease diagnosis and stethoscope has been an conventional stethoscope due to its easiness to detect
integral device of medical examination which is used to health problems.
listen to internal body sounds. But the traditional
Conventional stethoscope inherits many limitations . In 1816, French physician René Laennec invented the
Hence, electronic stethoscope provides much more medical stethoscope by placing a wooden hollow cylinder
advanced and modern solutions for those limitations, and between patient’s chest and physician’s ear to detect
it increases the accuracy of different internal body sounds auscultation of heart and lungs (Swarup and Makaryus,
recognition, becoming an aid for medical professionals to 2018). After adding many improvements for Laennec’s
make a proper disease diagnosis. Electronic stethoscope stethoscope, conventional stethoscope which is also
amplifies the auscultation sound captured at the chest known as traditional acoustic stethoscope was introduced
piece and convert it to an electrical signal which is then as a noninvasive acoustic diagnostic tool which is simple,
transmitted through an advanced designed circuitry to light weight, easily available, affordable. Hence, it is
apply further signal processing techniques. The main widely used by doctors, and it has become the sign of the
objective of this review paper is to analyze the existing medical professionals today. Conventional stethoscope
medical stethoscopes and the advancements that has been mainly includes a chest piece which is connected by a
done using modern technology to improve electronic split “Y” hollow flexible tube to the earpieces [Figure 1].
stethoscope by adding various features including noise The flexible PVC ear tubes basically isolate the
reduction techniques and real time visualization auscultation sound and transfer it to the physician’s ears
techniques. Further, the paper discusses about how with a minimum quality loss (www.linkedin.com, n.d.).
wireless data transmission techniques have been included Chest piece which is known as the head of the stethoscope
in electronic stethoscope making it is possible to provide consists of a diaphragm - to transmit higher frequency
graphical analysis of the sound signal and the advanced components and bell-for lower frequency sounds
mathematical techniques applied to make denoising and transmission (Landge, Kidambi, Singhal and Basha, 2018).
feature extraction accurately. It further reviews about the But it further got many difficulties since most of the
alternations and modifications that have been possible in internal body sounds including cardiac sounds contain low
electronic stethoscope enhancing the quality of healthcare frequency components between 10-200 Hz (Swarup and
sector. Makaryus, 2018). Moreover, the sound quality of the
conventional stethoscope is very low specially when there
Keywords— Auscultation, electronic stethoscope, real is a thick chest wall. And due to variations of hearing
time visualization techniques, feature extraction sensitivity and disturbances in the measuring environment,
sometimes physicians may not be able to recognize low
frequency features of cardiac and pulmonary sounds.
I. INTRODUCTION Artifacts and background noises, leakages in tubing can
In a rapidly developing world, technology has affected
also disturb in feature recognition of cardiac sounds. Since
almost all the fields in the world over the years. In the
there is no technique for noise reduction and further
field of medicine, there has been many advancements with analysis in conventional stethoscope, it can lead doctors
the arrival of new technologies. Electronic stethoscope is
for wrong diseases diagnosis due to low accuracy.
one such improved medical device which make
auscultation process easier and more accurate.
Stethoscope plays a vital role in the field of medicine to Electronic stethoscope has been developed with many
hear internal sounds of the body mainly cardiac and more advancements to overcome most of those difficulties
respiratory sounds. Internal body sound auscultation is and limitations of conventional stethoscope and it can be
one of the basic ways to assess the diseases diagnosis. used for auscultation process for disease diagnosis with
Modern technology has introduced many newer methods high accuracy. Electronic stethoscopes can be used in
such as ultrasound imaging and doppler techniques for most of the clinical scenarios including listening to
diseases diagnosis. But despite other methods of detecting internal cardiac and pulmonary sounds, to detect abnorma
most of the diseases including cardiac diseases, changes in breathing sounds, to detect air or fluid around
lungs etc. Further, it increases the accuracy of heart II. METHODOLOGY
mummer recognition and cardiac disease diagnosis as well. Literature review plays an integral role in academic field
The key component of the electronic stethoscope over which study and analyse the knowledge on different areas.
conventional stethoscope is it converts the acoustic sounds It is also a better source of a general summary of an
into electrical signal and an advanced analysis can be interested topic. The Research topic was initialized, and
done. Electronic stethoscope combined with sound further reading was done to obtain broader knowledge on
amplification, filters and other noise reduction techniques area of study. First, by searching key words in different
improves the sound quality making auscultation process data bases like Google scholar, ResearchGate etc.,
more accurate. A wireless data transmission can be done journals, review papers and research articles published
by using Bluetooth connection module in the recorder. recently related to topic were collected. Abstract of
Thus, the recorded data can be transmitted to wireless sources were checked in order to omit duplicates and to
mobile terminal such as mobile phone or any kind of a obtain only the relevant articles. Due to limitations of the
computer for further analysis. This can be used to for conventional stethoscope, many studies have been
remote patient monitoring and can obtain a real time conducted to overcome those difficulties providing
graphical analysis of cardiac sounds captured at chest innovative solutions through modern technology, resulting
piece. Moreover, enhancements can be added for digitally advanced cost effective electronic stethoscopes with
record the captured data and play back options with higher accuracy. Hence, the sources were studied in depth
different speeds. Hence, due to its high efficiency and to provide an efficient literature review on advancements
accuracy, electronic stethoscope can be developed as a of an electronic stethoscope.
main medical device to detect cardiac sounds.

III. LITERATURE REVIEW

There are different variety of electronic stethoscopes
A. Key Modules in Electronic Stethoscope
commercially available with different technical
Electronic stethoscope mainly consists of three modules:
advancements and specifications. Many researches have data acquisition; pre-processing; signal processing.
been done to improve the further efficiency and accuracy
adding more features to electronic stethoscope [Figure2].
1) Data Acquisition: Since internal body sounds contain
Thus, the review paper mainly focuses on how electronic low frequency component, amplification of the
stethoscopes are developed as a sophisticated medical auscultated sound below the threshold is required. In the
device for the auscultation process in disease diagnosis
first module, it includes sensor or a microphone on chest
with a higher efficiency. piece to capture the acoustic sounds and amplifiers to
amplify acoustic sound into audible level along with
transducers to identify cardiac sounds. Since the sound
signal can contain noise and artifacts, external filters are
added to exclude unwanted frequency components beyond
the required range. By adding a loudspeaker, it overcomes
the difficulties if there are issues with variations of
sensitivity of hearing enabling doctors to listen to internal
body sounds clearly in real time, but further analysis and
graphical visualization is absent.

Hence, a microcontroller or an analog to digital converter

is included to convert amplified and filtered acoustic
sound signal into a digital signal. It is essential to
determine the correct bit resolution and sampling
frequency for a better signal analysis. For a higher
accuracy to be obtained, an analog to digital converter
with a high bit resolution and high sampling frequency
should be obtained. Moreover, the air gap between the
diaphragm and microphone open up the possibility to
generate unwanted ambient noise between two diaphragm
surfaces which in turn can results a false electrical signal.
Figure 1 . Components of Conventional stethoscope To overcome such difficulties, a piezoelectric transducer
Source : (www.linkedin.com, n.d.) can be added which removes ambient noise of the signal.
In a piezoelectric transducer, piezoelectric crystals are
combined to diaphragm of the chest piece which generates
the electrical signal due to its deformation. Hence, output signal. The obtained signal is then normalized to a
electrical charges releases when a pressure is applied due certain range and segmented into cycles localizing the
to cardiac sounds including heartbeat or any other sound peaks which make possible to detect specific
pulmonary sounds. Therefore, the ambient noise present components of cardiac sounds clearly and to conduct
in captured sound at chest piece is filtered out before the further feature extraction techniques. Specially, since
amplification, enabling an optimal listening of the actual stethoscope is an integral medical device in cardiac
auscultation sound for medical professionals. 3M® disease diagnosis, advanced techniques for segmentation
Littmann electronic stethoscopes include piezoelectric and feature extraction procedures make it possible for
sensor which further amplify the sound signal up to 24 doctors to perform auscultation and diagnosis with ease.
times (Landge, Kidambi, Singhal and Basha, 2018). Hidden Markov Model (HMM) is a statistical Markov
Micro-electromechanical system (MEMS) piezoresistive model which is widely used for segmentation where both
electronic sensors also generate electrical signals with the specificity and Sensitivity are above 95% (Ghosh,
higher sensitivity and accuracy (Zhang et al, 2016). Nagarajan and Tripathy, 2020). Short Time Fourier
Electronic stethoscopes with those sensors show Transform, Continuous Wavelet Transform (CWT),
significant role is cardiac disease diagnosis with a higher
performance.

Another alternative for the purpose of transducer is a

capacitive MEMS. Despite other methods of transducers,
this system detects the change in the capacitance, which is
created due to acoustic pressure occurred by cardiac
sounds. Hence, the resultant electrical signal generated at
this system is directly proportional to the acoustic pressure
(Bassiachvili et al, 2008). Thus, in capacitive MEMS, the
capacitance change is converted into an electrical signal.
Compared to other types of sensors, MEMS sensor is
smaller in size and contains a higher temperature stability.
Thinklabs® One Digital stethoscope is a one good
commercially available example for an electronic
stethoscope where capacitive MEMS is included (Swarup
and Makaryus, 2018). Thus, the electronic stethoscopes
containing capacitive MEMS include an adaptive noise
canceller and can amplify the acoustic sound up to 100
times resulting a better amplification (Leng, San Tan,
Chai and Wang, 2015). In addition to that, Thinklabs®
One Digital stethoscopes contains advanced techniques
for specific cardiac sound extraction and for further
analysis of cardiac and pulmonary frequencies. Discrete Wavelet Transform etc are some more advanced
techniques that can be used in pre-processing module
2) Pre-Processing : Pre-processing module includes (Ghosh, Nagarajan and Tripathy, 2020).
Figure 2. Commercially available Electronic stethoscopes
denoising techniques and signal normalization techniques
Source : (Pinto et al., 2017)
along with segmentation to enhance the electrical signal
with more accuracy. This module is used to filter out
artifacts and other noises using digital filters which have 3) Signal Processing: The last module of the electronic
not been filtered by external filters used in data stethoscope is used for signal processing. In this module,
acquisition module. This in turn removes undesired feature extraction and a higher order classification of the
features and sounds in the cardiac sounds detected in data cardiac sounds can be obtained using advanced
acquisition module and increases the accuracy of the mathematical techniques. Hence it enables to extract
obtained acoustic signal. Different bandpass filters, High required features converting raw data into a parametric
pass and low pass filters have been used for linear filtering representation of interested sections for better cardiac
depending on different situations according to the sound classification. Feature classification provides a
requirement. It helps in extracting the desired signal better interface for medical professionals to conduct an
within the interested frequency band eliminating noise. accurate diagnostic decision making. Based on the
Thus, the techniques used in pre-processing module in performance of different heart sound classifiers, Support
turn increases the signal-noise ratio (SNR) of the final Vector Machine (SVM) with kernel function and
Artificial Neural Networks (ANN) can be considered as connection. In addition, small digital display can be added
good heart sound classifiers due to the high sensitivity and in the electronic stethoscope for data visualization. But,
accuracy of above 98% (Ghosh, Nagarajan and Tripathy, since it is not practical to include a large display, a clear
2020). Support Vector Machine (SVM) is a linear data- analysed graphical content and more information may not
based model which can be used to detect smaller murmurs be available on the display.
where a higher accuracy can be obtained with a reduced Due to variations in advanced technologies and techniques
amount of computing power (Ghosh, Nagarajan and used, a wide variety of high quality electronic
Tripathy, 2020). To detect heart sound abnormalities in stethoscopes are available today. L`ittmann 3M model, the
real-time, Artificial Neural Networks (ANN) shows Thinklabs One Digital, Welch Allyn Elite electronic
significant sensitivity and accuracy but when comparing stethoscope, Cardionics E-scope II, EcoScope, and
with SVM, ANN requires higher computational power ViScope etc are few of the electronic stethoscopes that are
and time (Ghosh, Nagarajan and Tripathy, 2020). Since commercially available today (Pinto et al., 2017). Among
almost all the physiological signals are non-stationary them, L`ittmann 3M model, the Thinklabs One Digital are
signals, feature extraction procedure is quite challenging, widely used electronic stethoscopes which contains bell
and it requires analysis in both time and frequency mode and diaphragm mode with higher order
domains to conduct a better analysis.Short-Time Fourier amplification and ambient noise reduction techniques.
transform (STFT), Discrete Wavelet Transform (DWT), Welch-Allyn® Elite is another advanced Electronic
Discrete Cosine Transform (DCT), Linear Frequency Stethoscope which contains a bell mode with a varying
Band Cepstral (LFBC), the Mel-Frequency Cepstrum frequency range. When a cardiac sound is to be detected,
Coefficients (MFCC) and linear predictive coding (LPC) 20 - 420 Hz frequency range is used and 350 - 1900 Hz
provides advanced and accurate feature extraction range in diaphragm mode is used to detect pulmonary
procedures used for cardiac sound analysis (Ghosh, diseases and abnormalities (Swarup and Makaryus, 2018).
Nagarajan and Tripathy, 2020). These advanced Cardionics E-scope II is another type which uses a
mathematical techniques make it possible to conduct an microphone and amplify the sound signal up to 30 times,
effective analysis of non-stationary biological signals but feature extraction techniques are absent. Bluetooth
captured at diaphragm becoming aid for doctors to based wireless data transmission modules are widely used
conduct advanced auscultation procedure. Thinklabs® in most of the commercially available electronic
stethoscopes. Some advanced digital stethoscopes have
the ability to connect with remote signal processing units
in devices by transmitting the signal wirelessly. A
smartphone, handheld PC or any type of a computer can
be connected as the wireless mobile terminal to capture
the data via Bluetooth through wireless data transmission
modules for real-time graphical visualization of the signal.

For the analysis of sound data obtained from electronic

stethoscope, some advanced software have been
developed which make diagnosis process easier and
accurate for doctors. Smartphone stethoscope apps have
been developed for real time graphical visualization of
auscultation sounds by connecting with electronic
stethoscopes. SensiCardiac, StethoCloud, Thinklabs,
Thinklabs and Mobile Stethoscope are some mobile phone
One Digital stethoscope is one such good example
apps that are already used in the field (Leng, San Tan,
available in the industry which contains specific cardiac
Chai and Wang, 2015) [Figure 3]. According to surveys
sound extraction techniques and advanced mathematical
and researches, SensiCardiac app is considered as the best
algorithms for pulmonary disease detection.
and accurate software to detect heart murmurs (Leng, San
Tan, Chai and Wang, 2015). It contains advance cardiac
Figure 3. Smartphone Sthethoscope apps :
sound classification and feature extraction techniques,
a. SensiCardiac,b.StethoCloud
hence it has a higher sensitivity to detect both pathological
Source : (Leng, San Tan, Chai and Wang, 2015)
murmurs and normal ones accurately. The Littmann Steth
B. Modifications Can be Added in Designing assist software included in Littmann M3200 model also
The electrical signal created by conversion of the audio analyses and converts the data and display a
signal can be recorded by adding a recorder along with a phonocardiogram or spectral graph as the output (Landge,
wireless data transmission module based on Bluetooth Kidambi, Singhal and Basha, 2018) [Figure 4]. Thinklabs
app also gives the ability to record the signal and real time
visualisation of PCG signal and contains screen signal patients frequently. To overcome this, digital stethoscopes
editing techniques as well. The software can be modified can be used. By giving instructions about the placement of
by adding various denoising techniques to remove diaphragm for the patients, doctors can listen and analyse
artifacts, play back options with different speeds and to internal body sounds in multiple times with advanced play
store and record data. Moreover, there are ongoing back and speed changing options.
When there are limited resources are available for disease
diagnosis procedure, and an ECG machines cannot be
accessed in emergency situations, electronic stethoscope
makes possible for doctor to detect cardiac murmurs and
supports to recognize cardiopulmonary pathological
features for accurate disease diagnosis compared to
conventional stethoscope.

Since conventional stethoscope is a single purpose device,

it is difficult to use when teaching junior medical students.
To overcome those practical issues electronic stethoscopes
are used because the added loudspeaker enables multiple
listeners to hear internal body sounds multiple times and
the graphical representations of the signal can be used by
senior doctors to teach about even the very low frequency
researches to develop software by adding more complex
components of cardiac sounds (Legget et al., 2018).
mathematical algorithms and machine learning
approaches for cardiac feature recognition.
IV. DISCUSSION
The paper reviews about the limitations of the
Figure 4.Graphical representation of phonocardiogram using
conventional stethoscope and how electronic stethoscope
Littman Steth assist software
has overcome the drawbacks by using advanced modern
Source : (Landge, Kidambi, Singhal and Basha, 2018)
technology. It shows that the electronic stethoscope has
become an accurate, effective medical device that can be
used to conduct auscultation procedure for disease
C. Impact on Medical Sector
diagnosis. Thus, conventional stethoscope is already
When using traditional stethoscope for covid-19 patients
outdated to conduct auscultation process due to the high
in wards, doctors have to face many difficulties. But to
efficiency and accuracy of the electronic stethoscope
prevent intensive care conditions of covid-19 patients, it is
(Landge, Kidambi, Singhal and Basha, 2018). But, still,
essential to perform auscultation procedure for accurate most of the doctors are lack of modern and sophisticated
disease diagnosis. These types of electronic stethoscopes medical devices like electronic stethoscopes which make
can be very beneficial compared to Conventional easier for doctors to conduct an accurate clinical decision
stethoscope because doctors have the opportunity to making. Hence, it may cause auscultation procedure to be
analyse cardiac and respiratory sounds of the patient more complex, resulting wrong disease diagnosis due to
through real-time wireless mobile terminals easily (Jain et low accuracy. Since real-time visualization, graphical
al., 2021). Hence, medical professionals can listen to analysis and feature extraction can be done using
auscultation sound of covid-19 patients using a headset or electronic stethoscopes incorporated with wireless data
quality earphones and can obtain a visualization of cardiac transmission techniques like Bluetooth technology, it
sounds using smartphone. Moreover, since the advanced makes easier for doctors to conduct remote patient
electronic stethoscope are capable of transmitting captured monitoring marking a revolutionized point in telemedicine.
data wirelessly, it can be further developed for the use of It further makes it possible to detect even very low
remote patient monitoring in telemedicine allowing frequency components which cannot be detect by
specialized physicians to exam patients and analysis conventional stethoscope. Moreover, electronic
cardiac and pulmonary sounds in real time (Landge, stethoscope enables advanced noise reduction techniques
Kidambi, Singhal and Basha, 2018). Thus, this can in turn over conventional stethoscope and most of the researches
become a highlighted even in the field of telemedicine as approaches to enhance the feature extractions and noise
reduction techniques in pre-processing and signal
well.
processing modules providing modern solutions and
advancements in clinical disease diagnosis procedure
Moreover, when giving cancer treatments like Iodine131, through auscultation.
patients are isolated due to penetrating radiation. Thus,
auscultation procedure cannot be conduct by conventional V. CONCLUSION
stethoscope though it is required to monitor cancer
The review paper addresses the key advancements and Landge, K. et al. (2018) “Electronic stethoscopes: Brief review
features that can be included in electronic stethoscope to of clinical utility, evidence, and future implications,” Journal of
increase the efficiency of auscultation procedure for the practice of cardiovascular sciences, 4(2), p. 65.
accurate disease diagnosis. The paper discusses about the
designing process and the main modules of the electronic Legget, M.E., Toh, M., Meintjes, A., Fitzsimons, S., Gamble, G.
stethoscope. Already existing electronic stethoscopes are and Doughty, R.N. (2018). Digital devices for teaching cardiac
not deeply analysed and compared among others due to auscultation - a randomized pilot study. Medical Education
shortage of literature based on comparison of electronic Online, 23(1), p.1524688.
stethoscopes. Further improvements on amplification,
denoising, feature extraction and real-time visualization Leng, S., Tan, R., Chai, K., Wang, C., Ghista, D. and Zhong, L.,
techniques depicts that the modern technology has made 2015. The electronic stethoscope. [online] ResearchGate.
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Jun. 2021].

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ACKNOWLEDGEMENT
Jain, A., Sahu, R., Jain, A., Gaumnitz, T., Sethi, P. and Lodha, R. The author would like to thank Ms V Jayawardana for the
(2021). Development and validation of a low-cost electronic extended support.
stethoscope: DIY digital stethoscope. BMJ Innovations, [online]
p.bmjinnov. Available at: AUTHOR BIOGRAPHY

<https://innovations.bmj.com/content/early/2021/06/29/bmjinno
v-2021-000715 >[Accessed 17 June 2021]. Sithumini Perera is currently a BSc (Hons)
Biomedical Engineering undergraduate in the Department
of Electrical, Electronic and Telecommunication
Engineering of the Faculty of Engineering at General Sir
John Kotelawala Defence University.