oring of cardiac activity (in both adults and fetuses during their intrauterine growth), it is more preferable to record phonocardiograms within a low-frequency band (40-90 Hz), which allows avoiding the overlapping of breath sounds with heart sounds down to the limit. Thus, on the basis of the required frequency characteristics and the analysis of the modern market of heart noise and tone registration devices, the authors have chosen a piezoelectric converter [4,11]. The required coherence of resistances was provided by the electronic part of the sensor designed by the authors. The sensor was fastened with an elastic band at the standard points of hearts auscultation. The signal was recorded by analogy with the recording of bronchopulmonary noises. The sensor has allowed obtaining the recording of the human cardiovascular system tones (Figure 2) with a certain number of extraneous noises concerned with the activity of the human respiratory system (its low-frequency constituents) and ambient noises.
The structure used provides the quality of the signal in the required frequency band that is sufficient for the future discrimination of heart sounds. The sensor of such a type can be successfully used to register heart sounds in adults and fetuses (during their intrauterine growth). However, such peculiarities of fetus heartbeat registration as low amplitude, source movement, and a significantly noisy signal determine the need to modernize the sensor and its loading, so that it can be used for fetus heartbeat registration in the future.
In order to eliminate noises from the acoustic signal obtained, the authors used the designed and software-based Type 1 Cherbyshev low-frequency filter, and its performance result is presented in Figure 3.
The further mathematic treatment of the human heart sounds obtained will allow calculating the heart rate (both in real time and in a delayed mode) and performing the noninvasive diagnostics of the patient's cardiovascular system by analyzing the first and second heart sounds derived.
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:
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