d.
When performing the work, the authors modeled and tested different types of sensors used to register respiratory sounds and heart sounds on the basis of various sensing devices. In addition, they analyzed the properties of acoustic sensors and their suitability for recording the acoustic signals of the human bronchopulmonary and cardiovascular systems.
The analysis of the data obtained allows making the following conclusions and providing further recommendations.
It is preferable to detect breath sounds by means of MEMS microphones and highly sensitive electret microphones. At the same time, it is necessary to take into account the following aspects:
1) For high-quality perception of breath sounds, microphones should have a phonendoscope-resembling design: it implies the presence of a funnel with a sound-amplifying membrane. The funnel cavity should be necessarily connected with external space by means of a small tunnel. The quality of recorded sounds significantly depends on the diameter and length of this tunnel.
2) A microphone installed in the focus of the sound funnel should have a vibration- and sound-proof attachment. This requirement is also relevant to an electric cable attached to the microphone.
3) For high-quality detection of signals, an amplifier of the condenser or analog MEMS microphone should have a low level of noises, no more than 4-6 nV/[square root of Hz].
4) The analysis of digital MEMS microphones connected to a microcontroller by means of the I2S protocol demonstrated good results. However, this scheme necessarily requires software for active noise cancellation.
It is preferable to detect heart sounds by means of piezoelectric contact microphones and accelerometers. It is connected with the fact that the first heart sound has the frequency of 16-18 Hz, which lies outside of the frequency range of electret microphones. Piezoelectric sensors and accelerometers register such vibrations without any problems. The use of piezoelectric contact sensors also has its own peculiarities:
1) Input resistance of the piezoelectric sensor's amplifier should amounts to few meg-aohms. Input current should be at the level of nanoamperes. It makes provisions for the use of amplifiers with field transistors.
2) Noise characteristics of amplifiers with field transistors are marked by a high level of low-frequency flicker noises prevailing in the operating range. Therefore, acceptable results can be obtained by means of JFET transistors with the level of noise of no more than 1 dB, e.g., 2SK170, 2SK209, 2SK2394, or 2SK3557, and chosen couples for formation of bipolar circuits (2SK2145 or 2SK3320). Speaking about low-noise operating amplifiers, it is advisable to use ADA4004-1 and AD8671.
3) The construction of acoustic sensors implies hard contact with patients' skin; therefore, it is necessary to use medical stainless steel under the 18/10 brand or analogues as a metal membrane the piezoceramic element is attached to.
4) Hard contact between the sensor and patient's skin significantly lowers down requirements to the sensor's acoustic design. The only condition is a need for relatively strong attachment of the piezoelement's membrane to its body. On the one hand, it creates additional conditions supporting high-quality contact with the skin. On the other hand, it reduces the level of external noises and interference.
These activities were performed within the framework of State Contract No. 14.579.21.0019 concluded between the Ministry of Education and Science of the Russian Federation and LLC Diagnostika + on the subject "Development of the device for 24-h monitoring over mother and fetus' state of health during pregnancy by means of controlling parameters of the cardiovascular system based on acoustic data". In addition, the authors express their heartfelt gratitude to National Research Tomsk State University for the provision of premises and necessary equipment to conduct independent research for the purpose of choosing the type of sensors that allow detecting acoustic signals of the human bronchopulmonary system.