This paper describes a method of the breathing detection based on the sensing of acoustic signals in trachea. Parameters of the breathing, detection inspiration and expiration and apnoea pause are possible to determine from these signals. This method is simple and easy to use, portable and provides an accurate measurement and seems to be well suited for use as a modern breathing monitor. Monitoring of Respiration is important for monitoring respiration towards observation quality sleeping or The Sudden Infant Death Syndrome (SIDS).
Organism needs the energy for an arrangement of the
all vital function. Energy is released by the oxidation of
the energy matter (saccharides, lipids and proteins).
Water and carbon dioxide are evolved too. Continuous
supply of the oxygen and removing carbon dioxide are
necessary by the oxidative processes. Therefore, the
respiration falls into basic vital functions. In medicine
this function is needed to sense and to detect its
parameters. This article describes a solution of
respiration diagnostics. Concretely it is thought the
external respiration (pulmonary).
The organism is possible to imagine as the biological
system generating biological signals that transfer
information of the biological system. These signals are
nearly always continuous. Bio signals are possible to
divide according to origin into electrical, magnetic,
acoustic, chemical, mechanical, optical, impedance,
thermal, radiological and ultrasonic [5, 6].
The respiration can be detected in various ways. One
of methods is the detection of the gas flow (Fleish
pneumotachometer). Further, the respiration is possible
to detect from the EEG signal. The method based on
the imaging of the acoustic signals appears as very
Typical lung volumes and mechanism of the
respiration are measured by the functional examine of
lungs. One of the ways of study regulation respiration
cycle is monitoring of breathing paradigm – depending
among basic quantities of ventilation: respiration
frequency, minute ventilation, inspiration and
expiration time, apnoea pause, the respiration volume.
Further parameters are: pressures and flow rates of
respiration gases, lung plasticity, depending between
the flow and the volume, resistance of airways [1, 2].
Characterization of Breathing
The breathing cycle is divided into four different
successive phases: inspiratory phase, inspiratory pause,
expiratory phase and expiratory pause. The breathing
cycle is defined here as starting with the onset of
inspiration at the moment when the air inflow starts.
When the airflow stops, the inspiratory phase ends and
the inspiratory pause begins and lasts until the air
begins to flow out from the lungs and the expiratory
phase starts. The expiratory phase is followed by the
expiratory pause, which lasts until the end of the
breathing cycle .
Method of Monitoring
Bioacoustic method is based on measuring acoustic
signals originating in the trachea (Fig. 1). The
microphone is used to measure these acoustic signals.
The acquired signal of the microphone m(t) is
consisted of different sources. Additional signals are
added to the desired signal a(t). The equation (1)
describes the relationship between the signal m(t) and
the signal components. Constituent components are
divided in four categories originating from: a) the
airflow in the trachea, b) disturbances at the interface
between the microphone and skin, c) internal
components from the body without relation to the
airflow and d) external components generated by the events in the environment where the measurements are
m(t) ... signal observed by the microphone
a(t) ... vibration airflow in the trachea
y(t) ... disturbances from the interface between the
microphone and skin
i1(t) ... internal disturbances from the blood flow
i2(t) ... other internal disturbances, e.g. from vessel
e1(t) ... external continuous disturbances
e2(t) ... external transient disturbances
The measure system in Fig. 2 was used for finding
the information about the measured acoustic signal.
Measure system is composed of four parts: microphone
part, signal preprocessing, interface and PC.
The microphone part converts acoustic signal into
electrical signal. Microphone 1 senses breathing signal
originated in trachea – signal a(t). This microphone
also senses additional signals added to the desired
signal a(t) – external signals. The microphone 2 is used
only for sensing external signals. Signals from both
microphones are subtracted to cancellation of external
signals. These signals from microphones are amplified
by signal preprocessing block and converted to digital
form by interface. Processing of signals is provided in
PC by software LabView. Fig. 3 shows microphone
part with MEMS microphone and its electrical scheme.
In Fig. 4 is illustrated algorithm for processing
signals of microphones. Signals from both
microphones are converted to digital form by DAQ
device. These signals are subtracted to cancellation of
external signals. Level adjustment block serve for
equalization of both signal level from microphones.
Afterwards it follows filtration of signal in band 200Hz
– 800Hz. This eliminates the remaining spurious
signals. FFT block pursues conversion signal from time
domain to frequency domain (spectrum). Finally,
signal in time domain and its spectrum are displayed.
RMS block calculates RMS value of signal. The RMS
signal is compared with level of service set for
detection of apnea.
The acoustic signal of the breathing occurs in
frequency band from 200Hz to 800Hz, see Fig. 5.
Therefore, the sampling frequency must be greater than
1600 Hz. The repeated breathing pattern and
differences between inspiration and expiration are
possible to read in the time behavior of the signal. The
smooth beginning and the abrupt ending are often
obvious during the inspiration. On the other hand, the
behavior of the expiration shows the abrupt beginning
and the smooth ending. Both phases are separated with
the inspiratory and expiratory pauses. Fig. 6 illustrates
breathing pattern – inspiration and expiration.
Recent advances in Micro-Electro–Mechanical
System (MEMS) technology enable complex
miniaturized systems for biomedical applications – e.g.
the integration of the Surface Acoustic Wave (SAW)
sensors with a MEMS microphone . The small size
and wireless operation shows wide range of its
potential patient monitoring applications including
monitoring breathing sounds in apnea patients,
monitoring chest sounds after cardiac surgery and also
monitoring chest sounds of the newborns by causing
minimal discomfort. Fig. 7 represents the concept of
the SAW based wireless acoustic sensor. Acoustic
signal from trachea are detected by microphone cause
the change of the impedance of the sensor
(microphone) connected across the output Interdigital
Transducer (IDT) and the amplitude and phase of the
reflected signal is as well as change and this is
transmitted by the antenna.
Another concept is depicted in Fig. 8. Sensory
system contains cheap commercially available MEMS
microphones. The detection of the breathing is based
on the pick-up of the acoustic signal by the sensor and
subsequent processing in PC. The communication
between sensor and PC is wireless. The input part
comprises the microphone which converts the acoustic
signal to the electric signal. After this signal is
amplified and digitalized. Subsequently, the signal is
coded and sent by the transmitter. The receiver gives
the information to PC where the breathing is analyzed.
Two microphones are used in this system. The
microphone 1 is used for sensing of the breathing –
signal a(t). This microphone also senses additional
signals added to the desired signal a(t) – external
signals. The microphone 2 is used only for sensing
external signals. Signals from both microphones are
added to cancellation of external signals. The power
part is used for the feeding sensor (battery, thermo or
Methods, based on the monitoring of the chest
motion, do not inconvenience patient with sensors
placed on the body. On the other hand, it
is necessary that the patient is at rest on bed. Acoustic
method is very interesting for the non-invasive
measurement of the breathing and for the possibility of
CMOS MEMS integration. Further, this method is
simple and easy to use, portable and provides an
accurate measurement and seems to be well suited for
the use as a modern breathing monitor. Parameters of
the breathing, the detection of inspiration and
expiration and the apnoea pause are possible to
determine from the time behavior.
This research has been supported by the
Czech Science Foundation project No. 102/09/1601
"Intelligent Micro and Nano Structures for
Microsensor Realized Using Nanotechnologies" and
partially by The Ministry of Interior of the Czech
Republic research program No. VG20102015015.
Department of Microlectronics
Faculty of Electrical Engineering
Czech Technical University in Prague
Technicka 2, CZ-166 27 Prague
Phone: +420 224 352 356
 Rozman J a kol. Elektronické přístroje v lékařství. Nakladatelství Academia, Praha 2006, ISBN 80-200-1308-3.
 Bronzino J. D. The Biomedical Engineering HandBook. Second Edition, CRC Press LLC, ISBN 0-8493-0461-X.
 Hult P., Wranne B, Ask P. A bioacoustic method for timing of the different phases of the breathing cycle and monitoring of breathing frequency. Medical Engineering & Physics 22, 2000, p. 425–433.
 Sezen A. S., Sivaramakrishnan S., Hur S., Rajamani R., Robbins W., Nelson B.J. Passive Wireless MEMS Microphones for Biomedical Applications, Journal of Biomechanical Engineering, Vol.127, Iss.6, p.1030, 2005, ISSN 01480731.
 Webser J. G. Measurement, Instrumentation, and Sensors HandBook, CRC Press LLC, ISBN 0-8493-2145-X.
 Kroutil J., Husak M., Laposa A. Monitorování dýchání, Slaboproudý obzor, 2010, vol. 67, no. 1, p. 19–25., ISSN 0037- 668X.