Short review of devices for detection of human breath sounds and heart tones.
The key drawback of this method is the subjectivity of physicians' assessment of acoustic data. It is connected with the fact that diagnostic decisions depend on many hard-to-record and subjective factors. It is noteworthy that frequency characteristics of the stethoscope weaken signals with the frequency of over 120 Hz [2-4] and the human ear is characterized with low sensibility to low frequencies. In this connection, physicians' assessment of the bronchopulmonary and cardiovascular systems by means of auscultation is subjective. Subjective perception can be explained in the following way. Up to 80% of adults have an ear for music, but only 12-15% of them have musical memory. An ear for music implies an ability to detect both individual musical elements and quality of musical sounds (pitch, volume, and structure) as well as functional relations between them in a piece of music (tonal memory, sense of rhythm). It has been found out that the range of frequencies detected by the female ear is significantly wider than that for the male ear. Females more clearly perceive high sounds and better distinguish tones and intonations. The acuity of female hearing remains high up to 38-40 years old, while males' sense of hearing starts going down at the age of 32-35 years. Hearing acuity of males and females becomes comparable at the age of 63-65 years, and it becomes approximately 2-2.5 times lower as compared to the level of 25 years of age. However, people maintain an ability to perceive high-frequency timbre components. The hearing acuity becomes approximately three times lower by the age of 65 years, people stop hearing sounds with the frequency of over 10-12 kHz. The changes indicated are connected with sclerotic processes in the internal ear and general reduction of the sensitivity of cortical representations of the analyzers in the brain .
Unfortunately, the height of physicians' professional strength and optimal combination of experience, 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 mi