 by means of a buffer amplifier based on a field transistor or small electric lamp. This solution also has certain drawbacks: increased value of noises in the range of up to 100 Hz, if the field transistor is used .
Piezoelectric microphones are a bit less popular in phonography. The core of this microphone is a piezoelectric element impacted with sound waves (directly or through a membrane). As a result, it generates potentials that change together with changes in the sound pressure. It leads to generation of alternating electric signals on the opposite parts of the piezoelectric element. It is characterized with a simple structure, rather good frequency characteristics in the sphere of low frequencies; however, it is quite sensitive to temperature and moist.
Piezoelectric microphones used in medicine have a membrane structure. These microphones consist of a diffuser made of paper or a thin aluminum foil connected with the piezoelectric element. The sound pressure impacts the diffuser and, through it, the piezoelectric element. Such microphones are characterized with extremely high sensitivity (up to 50 mV/Pa). As for the drawbacks of piezoelectric sensors, these are a need for hard and high-quality contact with the patient's skin (it is achieved by means of special pastes and gels) and low mechanical durability. The latter is conditional to the fact that thin ceramic films are used in highly sensitive acoustic sensors. They can be of 100 [micro]m in thickness and less.
Some authors say about an increased level of noises of piezoelectric films. However, it can be reduced significantly, if advanced nanotechnologies are used to produce piezoelectric ceramic components.
At the beginning of the 1980s of the past century, the development of submicro- and nanotechnologies has made it possible to produce sensors based on the MEMS technology, including microphones. It is possible to single out the following benefits of MEMS microphones: MEMS microphones can be smaller than most compact microphones; they are less liable to vibration, temperature fluctuations and electromagnetic interference and more resistant to noises due to the absence of wires and installation of circuits close to the MEMS crystal.
Besides MEMS microphones, inexpensive, precise, and compact MEMS accelerometers can be used to detect acoustic signals. These devices became popular in the middle of the nineties of the past century. They contain one-, two-, or three-axis microsensors that can measure acceleration simultaneously on one or several coordination axes .
MEMS elements usually contain an electronic signal converter circuit that significantly simplifies the use of these devices. The converter of up-to-date MEMS accelerometers contain an electrometric amplifiers, 12- or 16-digit digital-to-analog converter and digital interface that usually uses the I2C serial data communication protocol. The ADXL103 MEMS accelerometer is widely used in smartphones and tablets. It costs six dollars and has the resolution of 1 mcg and operating range of frequencies from 5 Hz to 2.5 kHz. Its dimensions are 5 x 3 x 2 mm, and its weight is less than 2 g .
There are data on successful experiments on the use of up-to-date MEMS accelerometers to record breath sounds. The 12M1 MEMS accelerometer was used in adult volunteers. It successfully registered not only pulse fluctuations of the front chest wall but also breath sounds. Placing an accelerometer sensor on the front wall of the trachea allowed recording its vibrations within the range of 4.5 kHz in smokers whose length of smoking was over 5 yrs. The sounds recorded matched with pathologic side tones accompanying chronic bronchitis .
Based on the research findings and recommendations presented in Table 1 , the authors chose the following types of acoustic sensors for the initial research stage: electret, piezoelectric, and MEMS microphones. As opposed to other representatives of this group, the level of sensitivity of compact dynamic microphones did not allow recording weak acoustic signals.
During the experiments, it was revea