 knowledge, and professional skills fall on the age of 35-40 years--the period of time, when human sensory organs start losing their acuity. The use of up-to-date electronic achievements can compensate for age-related changes in the sensory organs to a certain extent. As opposed to standard stethoscopes, electronic stethoscopes allow not only recording acoustic signals produced by the human cardiovascular and bronchopulmonary systems but also conducting a mathematic analysis of these sounds in order to compose an objective conclusion on patients' state of health. It has restored interest in research on the human body's acoustic phenomena and led to the emergence of many scientific papers directed at the development of effective and sage methods for automated analysis of acoustic data of the human bronchopulmonary and cardiovascular systems and introduction of these methods into medical practice .
Acoustic sensors are used in medical equipment primarily to detect and record human bronchopulmonary sounds and heart sounds. At the same time, the type of acoustic detectors used to detect certain signals (heart sounds, pulmonary sounds) depends on the area of application of specific medical equipment.
Devices for electronic auscultation use practically all types of acoustic mechanoelectrical devices that transform acoustic vibrations into electrical signals. The most popular devices are various microphones, including piezoelectric sensors and acceleration sensors (accelerometers).
Dynamic microphones are one of the most common types of detectors. It is a light membrane based on a flexible hanger with attached copper wire coil. The coil is placed in a constant magnetic field. Sound vibrations detected by the mobile membrane are transferred to the wire coil. It produces synchronized magnetic vibrations that cause electromagnetic induction. The quality of generated signals significantly depends on the mechanical properties of the mobile system, uniformity of the magnetic field and other features of the system .
Dynamic microphones have good acoustic and electric characteristics: sensitivity from 1 mV/Pa, frequency range from 50-100 Hz to 10-15 kHz. Acoustic sensors based specifically on this type of microphones were used in the first experiments for registration of breath sounds . An undisputable benefit of this type of microphones is a very low level of noises primarily determined by heat noises of the wire used and noises produced by galvanic couples in the junction point.
However, dynamic microphones have several drawbacks: difficult and massive construction, relatively low sensitivity, difficulties with forming directional responses, etc. Therefore, nowadays acoustic sensors based on condenser (electrostatic) microphones are used significantly more widely. It consists of a thin metallized polymer film that plays a role of mobile condenser armature and massive immobile armature located at a close range (0.1-0.15 mm). The condenser carries constant voltage that is transformed into low-frequency signals, when the mobile armature vibrates under the influence of sounds. An alternative option of condenser microphones is an electret microphone. In this case, the immobile armature is a plate of polarized serum. Such a microphone does not require external power supply, since an electrostatic charge is formed on it during the process of production .
A wide range of detected frequencies, stable parameters, and high sensitivity can be attributed to the benefits of sensors based on electrostatic microphones. The sensitivity of up-to-date electrostatic microphones is from 1.5 mV/Pa for electret microphones and from 5 mV/Pa for microphones with phantom power supply (the higher this indicator, the higher the sensibility). The signal/noise proportion for the best models of electret microphones is -62 dB. As for microphones with external power supply, it is -110 dB. However, these microphones are characterized with high output resistance (dozen and hundred megaohms) and should be used with a low-ohm microphone cable. This problem can be solved