**14. Conclusions**

The new theory of the hearing processes, proposed by the author, links together the concepts emerged in former theories of hearing, such as a resonant theory, travelling wave theory, and Gold's theory of active amplification (cochlear amplifier). The new model of the phenomena occurring in the cochlea is a physical (physiological) model (not phenomenological), in which the role and oper‐ ation of individual elements of the actual cochlea are explained qualitatively and quantitatively.

To initiate the research on the mechanism of hearing the author was motivated by incomplete‐ ness of the existing models of the hearing process. In fact, the existing theories of hearing are phenomenological in nature and do not directly correspond to the physical components in the cochlea. Another stimulus was a potential importance for various possible applications, such as hearing aids, digital sound coding, perception of music, etc.

ity factor 12 has an effective frequency bandwidth of 80 Hz (8%), for example, from 960 to 1040 Hz. On the other hand, the active OHC resonator with the quality factor 120 has an effec‐ tive frequency bandwidth of 8 Hz (0.8%), for example, from 996 to 1004 Hz. The latter case represents a remarkable frequency selectivity (0.8%) enabling for frequency discrimination

**1.** Power amplification of the acoustic signal occurs in the circuit of the input electro‐ mechanical transistor formed by stereocilia of an outer hair cell (OHC), ion channels, bulk of the OHC, and the stria vascularis. Selectivity of the reception of acoustic sig‐ nals is realized by a parametric amplifier based on the nonlinear capacitance of the

**2.** Electromechanical transistor (based on the forward mechano‐electric transduction) supplies (pumps) the power to the nonlinear capacitance (parametric amplifier),

**3.** The parametric amplifier is realized with a reactive element (nonlinear capacitance), not on a resistive element (like laser amplifier or tunnel diode amplifier). It is worth noticing that a source of noise is mostly resistive elements. From that reason, the

**4.** By the inverse piezoelectric effect (electromotility), amplified acoustic (on electric side) signal is transformed into the mechanical side where it stimulates the tectorial

**5.** Both mechanisms (HB‐motility and electromotility) must operate simultaneously in order to achieve the power amplification and selectivity in the cochlear amplifier. **6.** TM stimulates mechanically stereocilia of the IHC. Thus, the mechanical energy from the TM is delivered to the IHC stereocilia and transformed by the IHC into electrical ion currents. These currents excite the afferent nerves, which generate a series of electrical impulses that are transmitted into the central nervous system.

The new theory of the hearing processes, proposed by the author, links together the concepts emerged in former theories of hearing, such as a resonant theory, travelling wave theory, and Gold's theory of active amplification (cochlear amplifier). The new model of the phenomena occurring in the cochlea is a physical (physiological) model (not phenomenological), in which the role and oper‐ ation of individual elements of the actual cochlea are explained qualitatively and quantitatively.

To initiate the research on the mechanism of hearing the author was motivated by incomplete‐ ness of the existing models of the hearing process. In fact, the existing theories of hearing are phenomenological in nature and do not directly correspond to the physical components in the

much narrower than one semitone in modern musical scales (6%).

**13.4. Consequences resulting from the author's model**

OHC and the piezoelectric phenomenon.

membrane (TM).

**14. Conclusions**

88 Advances in Clinical Audiology

producing a negative resistance in the resonant circuit.

parametric amplifier exhibits a very good noise characteristics.

Full physical model of the cochlea is described mathematically by nonlinear ordinary differential equations (ODE). Linearized physical model of the cochlea is described by linear ODE of Mathieu and Ince's type. In the established model of the cochlea new concepts from electronics (e.g., para‐ metric amplification and electromechanical transistor) are applied. This goes significantly beyond the range of methods and concepts used so far in the existing theories of the hearing.

The model proposed by the author explains the process of power amplification and frequency selectivity that take place in the cochlear amplifier. By contrast, existing models of the cochlear amplifier are incomplete and do not describe satisfactorily the physical processes that occur in the organ of Corti. Furthermore, in scope of the author's model, the phenomenon of an otoacoustic emission can be described.

The new model of a single OHC, developed by the author, conforms qualitatively to the exist‐ ing experimental data. A novel refined theory of the hearing processes, based on the piezo‐ electricity and parametric amplification, can open new horizons for research including deeper understanding of the physical phenomena underlying auditory processes, new phenomena in musical and speech acoustics, etc. This may create a possibility to design new musical instruments as well as new audio codecs that could outperform the existing MP3 codecs.

The author hopes that the results of this study (which relate to the validity of crucial electrical phenomena occurring in the cochlea) can also be useful in the construction and design of a new generation of hearing aids. Today, most hearing aids are based on power amplification of the input audio signals at specific frequencies without increasing the selectivity (the ability to distinguish between signals of different frequencies).

Therefore, the future hearing aids should be more closely based on the knowledge of com‐ plicated electrical and mechanical phenomena occurring in the cochlea. In these devices, in addition to the power amplification of the input acoustic signal, adequate selectivity of acous‐ tic signal reception (sharpening of the receiving characteristics of the receiver—the cochlear amplifier) should also be ensured.




**Glossary of technical terms**

90 Advances in Clinical Audiology

**Anions** Negatively charged ions

**Cations** Positively charged ions

**Ampere (A)** The SI unit of electric current intensity. If the current flowing through

**Bandwidth** A difference between the maximum and minimum frequencies handled

contrast to unipolar FET transistors

pF (10‐12 F) to about 1 mF (10‐3

charge equal 1*e* = 1.602 × 10−19 coulombs

**Charge** A measure of the amount of electricity. The electric charge is measured

**Circuit theory** A set of techniques, definitions, and mathematical tools used to

**Conductance** A resistive electrical element defined as an inverse to the electrical resistance **Coulomb (C)** The SI unit of electric charge, defined as the charge carried by a

current and *t* is time **Current source** An active electrical element providing DC or AC electric current on its

**Decibel (dB)** A logarithmic unit used to express the ratio of two values of a physical

**Differential equation** An equation for an unknown function, e.g., *y*(*t*) containing sum of

terminals

log10(*<sup>P</sup>* 2 /*P* 1 ), where *P* 2 and *P* 1

**DC battery** A source of potential electrical energy **DC signal** A signal, which is constant in time

**Bipolar transistor** A transistor that uses both negative electrons and positive holes, in

**Capacitor** A passive electrical element, which can store electrical energy.

by a device

a cross‐section of a conductor has intensity of 1 A, then within 1 s the charge of 1 C flows through this cross‐section, namely: 1 A = 1 C/s,

Capacitance of the capacitor is measured in farads (F) to honor English scientist M. Faraday (1791–1867). 1 F = 1 C/1 V, where C stands for the coulomb and V for the volt. Typical capacitance may vary from about 1

in coulombs to honor French physicist C.A. Coulomb (1736–1806). 1 C = 1 As, where A stands for the ampere and s is the second. There are positive and negative charges. The electric charge is a discrete quantity. The smallest electric charge is carried by the electron. An electron has a

describe the flow of currents and voltage distribution in electrical networks composed of lumped passive and active elements. The theory includes Ohm's law, Kirchhoff's laws, and theorems (Thévenin, Norton), which are based on first physical principles, such as conservation of energy, conservation of electrical charge, etc.

constant current of intensity of 1 A that passes through a cross‐section of a conductor in one second. Therefore, charge *Q* = *It*, where *I* is the

quantity. For example, the device dynamics, measured in dB, equals 10

derivatives up to a certain order. For example, a differential equation describing harmonic oscillations contains derivatives up to order two

and minimum input power, which can be properly handled by the device

correspond, respectively, to the maximum

F)

where C stands for the coulomb and s for the second



**Glossary of technical terms**

92 Advances in Clinical Audiology

**Inductor** A passive electrical element, which can store magnetic energy.

**Inverse piezoelectric effect** An occurrence of a mechanical stress or deformation in the material

**Joule (J)** The SI unit of energy. 1 J = 1 Ws, where W stands for the watt and s for

**Kirchhoff's voltage law** It states that, an algebraic sum of all voltages on lumped elements in an

**Kirchhoff's current law** It states that, an algebraic sum of all electric currents flowing into and

**Longitudinal waves** Acoustic waves with particle vibrations parallel to the direction of

**Lumped element** A mechanical (spring, dashpot, mass) or electrical (capacitor, resistor,

**Mechanical displacement** A difference in the positions of a mechanical particle e.g., a particle that

meter, and s for the second

**Mechanical impedance** The ratio of the force applied to a mechanical element (spring,

**Mechanical side** The mechanical port of a multiport electromechanical device +

meter

**Nonlinear oscillators** An oscillator containing at least one nonlinear element

**Nanometer (nm)** One billionth part of the meter (1 nm = 10‐9 m) **Nanosiemens (nS)** One billionth part of the siemens (1 nS = 10‐9 S)

**Micrometer (µm)** One millionth part of the meter (10‐6 m) **Millivolt (mV)** One thousandth part of the volt (10‐3 V) **Noise** Any unwanted signal, random or coherent

**Mechanical stress** The force per unit area. Mechanical stress is measured in pascals (Pa)

**Nonlinear capacitance** An electrical capacitance, which value depends on voltage on its

(1623–1662). 1 Pa = 1 N/m<sup>2</sup>

adequately described by circuit theory

is stimulated to vibrations by an acoustic wave

mechanic elements connected to this port

the electric voltage on the nonlinear capacitance

the second

energy

Inductance of the inductor is measured in henrys (H) to honor American scientist J. Henry (1797–1878). 1 H = 1 Wb/1 A, where Wb

subjected to the electrical voltage. Inverse piezoelectric effect was predicted from thermodynamic considerations in 1881 by French

arbitrary closed loop in a circuit is always zero. The law was discovered in 1845 by German physicist G. Kirchhoff (1824–1887). Kirchhoff's Voltage Law is a direct consequence of the principle of conservation of

out of a node in an electric circuit equals zero. Kirchhoff's Current Law is a direct consequence of the principle of conservation of charge

inductor) element with no spatial dimensions. Lumped elements are

dashpot, mass) to the velocity at which the element moves. Mechanical impedance is measured in Ns/m, where N stands for Newton, m for

to honor French scientist, mathematician and philosopher B. Pascal

terminals. The constitutive equation for the nonlinear capacitance *C*(*u*) is given by *Q* = *C*(*u*)*u*, where *Q* stands for the electrical charge and *u* for

, where N stands for Newton and m for the

propagation (called sometimes compressional waves)

stands for the weber and A for the ampere

scientist and inventor G. Lippmann (1845–1921)

