crophones 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 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 