**1. Introduction**

Acoustic signals, which can be detected by human auditory organ, are acoustic (pressure) waves propagating in a material medium, such as gas, liquid, or solid. Acoustic waves cannot propagate in vacuum, contrary to light waves. Acoustic waves are in fact pressure disturbances

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

that propagate in air and are characterized by longitudinal (compressional) particle move‐ ments. Initially, acoustic waves enter the ear pinna and ear canal (outer ear). Then, acoustic waves travel through a sequence of elements in the auditory pathway, such as middle ear and inner ear, where they are converted into electrical impulses transmitted directly into the cen‐ tral nervous system. The most important element in the auditory pathway is the inner ear with the cochlea. In turn, the main constituent of the cochlea is the organ of Corti located between the basilar membrane (BM) and tectorial membrane (TM). The organ of Corti contains a large number of sensory cells such as the outer hair cells (OHCs) and inner hair cells (IHCs). It is generally assumed that the process of power amplification of input acoustic signals occurs in the cochlea and is accompanied by sharpening of its frequency selectivity. High sensitivity of the cochlea enables hearing of low‐level acoustic signals. On the other hand, high selectivity allows for frequency discrimination between two tones with nearly the same frequency.

The inner ear is one of the most complex sensory elements of the human body. It receives acoustic stimuli of various amplitudes and frequencies, carrying information from the exter‐ nal world. It is a highly nonlinear element presenting remarkable properties, such as:


None of the technical devices built up to date by humans can even approach the above char‐ acteristics. This would be impossible if an input acoustic signal were processed in a passive manner. Achievement of such an amazing performance requires that some active processes take place in the cochlea, i.e., an extra energy has to be added to the input acoustic signal, in order to amplify the power of the received acoustic signal and enhance the frequency selectiv‐ ity (narrow bandwidth) of the cochlea characteristics.

For nearly 2000 years humans tried to elucidate the nature of the physical processes occurring in the human hearing organ (cochlea). So far, there is no complete theory and understanding of the physical phenomena occurring in the cochlea.

Modeling of physical processes occurring in the cochlea is indeed a very complex task. There are still many exciting and unresolved research problems related to the complicated electrical and mechanical phenomena responsible for the mechanism of hearing. For example, a fasci‐ nating area of research is reception and perception of acoustic signals generated by musical instruments. Do aesthetic impressions, offered by music, depend on proper operation of the human auditory organ (cochlea)? Does music have healing properties for autistic children with a perfect pitch?

Accurate knowledge of the mechanism of hearing may allow construction of an artificial cochlea and determination of a possible correlation between the cochlea characteristics and musical skills. Is the construction of the human hearing organ (cochlea) significantly different for musi‐ cal geniuses (Bach, Mozart, Chopin, etc.) and those individuals only moderately gifted in music?

The mechanism of hearing is not yet fully understood, even though it was, and is the subject of intense research activities in many renowned scientific centers around the world (in USA, Japan, France, Germany, Switzerland, etc.). In particular, the following features of the cochlea are not yet explained:

**1.** power amplification,

at

), such

that propagate in air and are characterized by longitudinal (compressional) particle move‐ ments. Initially, acoustic waves enter the ear pinna and ear canal (outer ear). Then, acoustic waves travel through a sequence of elements in the auditory pathway, such as middle ear and inner ear, where they are converted into electrical impulses transmitted directly into the cen‐ tral nervous system. The most important element in the auditory pathway is the inner ear with the cochlea. In turn, the main constituent of the cochlea is the organ of Corti located between the basilar membrane (BM) and tectorial membrane (TM). The organ of Corti contains a large number of sensory cells such as the outer hair cells (OHCs) and inner hair cells (IHCs). It is generally assumed that the process of power amplification of input acoustic signals occurs in the cochlea and is accompanied by sharpening of its frequency selectivity. High sensitivity of the cochlea enables hearing of low‐level acoustic signals. On the other hand, high selectivity allows for frequency discrimination between two tones with nearly the same frequency.

The inner ear is one of the most complex sensory elements of the human body. It receives acoustic stimuli of various amplitudes and frequencies, carrying information from the exter‐

**2.** Possibility to amplify power of very weak input acoustic signals. The human ear can receive and distinguish acoustic signals with power slightly above the power of thermal noise in air (1.9 × 10−18 W). Power density (intensity) of these acoustic signals equals 10−12 W/m2

as roaring of jumbo‐jet engines, can exceeds trillion times the threshold of human hearing

**4.** Very high frequency selectivity. Humans (who have perfect pitch) are able to distinguish between musical sounds differing only by 0.2%, e.g., 1000 cycles per second (Hz) and 1002 Hz. It is noteworthy that the frequency difference between the tones generated by any two adjacent keys (semitone), in the contemporary piano tuned to an equal (well) tempered

None of the technical devices built up to date by humans can even approach the above char‐ acteristics. This would be impossible if an input acoustic signal were processed in a passive manner. Achievement of such an amazing performance requires that some active processes take place in the cochlea, i.e., an extra energy has to be added to the input acoustic signal, in order to amplify the power of the received acoustic signal and enhance the frequency selectiv‐

For nearly 2000 years humans tried to elucidate the nature of the physical processes occurring in the human hearing organ (cochlea). So far, there is no complete theory and understanding

Modeling of physical processes occurring in the cochlea is indeed a very complex task. There are still many exciting and unresolved research problems related to the complicated electrical

). Thus, the dynamic range of reception amounts to (1 W/10−12 W = 1012), 12

2 − 1) × 100% [4]. The frequency range of the human ear

nal world. It is a highly nonlinear element presenting remarkable properties, such as:

**3.** Very high dynamic range (120 dB). The power level of a very loud sound (1 W/m2

**1.** Ultra‐low power consumption (14 μW = 14 × 10−<sup>6</sup> W) [1].

12 √ \_

spans approximately 10 octaves, i.e., from ∼20 to ∼20,000 Hz.

ity (narrow bandwidth) of the cochlea characteristics.

of the physical phenomena occurring in the cochlea.

a frequency of 1000 Hz [2].

orders of magnitude, i.e., 120 dB [3].

scale, equals ∼6%, or exactly (

(10−12 W/m2

60 Advances in Clinical Audiology


A complete and accurate model of the physical processes occurring in the cochlear amplifier (CA) should explain the course of these aforementioned processes.

Full understanding of the physical mechanism of hearing may be of paramount importance for:


#### **1.1. What is the sensitivity?**

The sensitivity of a given device or system is defined as the lowest amplitude of the signal, which can be detected by the system. In case of the human auditory organ (cochlea) sensitiv‐ ity is defined as the lowest level of the input acoustic signal, for which the cochlear amplifier, treated as a receiver, produces an output electrical signal of an appropriate level, i.e., the signal with a satisfactory signal to noise ratio. In other words, sensitivity of the human hear‐ ing organ (cochlea) corresponds to a threshold acoustic signal, for which one can still hear with an acceptable quality and perception.

#### **1.2. Active processes in the cochlea**

Postmortem measurements of BM motion in the cochlea (passive cochlea) show that increase of the amplitude of sound gives rise to a linear increase in the amplitude of BM mechanical vibrations. However, passive cochlea is not able to explain the amazing amplitude sensi‐ tivity and frequency selectivity of the human hearing organ. It was found that with a pas‐ sive cochlea only very loud sound could be heard (low sensitivity) with a poor frequency selectivity.

#### *1.2.1. What are the active processes?*

In order to explain the fabulous properties of the human hearing organ (sensitivity and selec‐ tivity), the concept of the active element and the cochlear amplifier were introduced as early as in 1948 by Gold [5] and later extended by many researchers (e.g., Davis in [6]). The intro‐ duction of the active element served to explain the phenomenon of power amplification and sharpening the frequency characteristics that occur in the cochlea, see Refs. [5, 6]. In general, power amplification process requires that some extra energy is delivered from an external source to the system. The system with power amplification capability is called active in con‐ trast to a passive system, which can only dissipate (lose) the energy.

Properties of active elements (where active processes can occur) allow in a natural way to explain such features of human hearing organ as high sensitivity and selectivity (narrow bandwidth). An example of active elements found in classical electronics can be transistors, bipolar, as well as unipolar (field effect transistors). These elements operating in the amplifier circuit can amplify the power of the electrical input signals.

At present the existence of the cochlear amplifier is widely accepted in the literature, see Ref. [7]. However, the exact mechanism of the power amplification in the cochlea is still the subject of extensive research. It is also generally accepted that OHCs play a key role in the cochlear power amplification process [8]. Power amplification and sharpening of the fre‐ quency response occurs in the OHCs, see Ref. [6], that are located in the Cochlea. In fact, loss of the OHCs causes that the cochlear amplifier is not operating, and as a consequence hearing capabilities are lost.

#### *1.2.2. IHC operates as sensors*

Another type of sensing element is inner hair cells (IHCs) that are also located in the cochlea, between the BM and TM. The inner hair cells (IHCs) detect the mechanical signal, which was previously amplified by the OHCs. The IHC plays the role of sensor [9]. The IHC converts the input mechanical signal into an electrical signal and transmits the latter to the central nervous system via the afferent innervation. In this case the afferent innervation of the IHCs may be called an output circuit of the entire cochlear amplifier.

#### *1.2.3. OHC can operate as sensor and actuator*

signal with a satisfactory signal to noise ratio. In other words, sensitivity of the human hear‐ ing organ (cochlea) corresponds to a threshold acoustic signal, for which one can still hear

Postmortem measurements of BM motion in the cochlea (passive cochlea) show that increase of the amplitude of sound gives rise to a linear increase in the amplitude of BM mechanical vibrations. However, passive cochlea is not able to explain the amazing amplitude sensi‐ tivity and frequency selectivity of the human hearing organ. It was found that with a pas‐ sive cochlea only very loud sound could be heard (low sensitivity) with a poor frequency

In order to explain the fabulous properties of the human hearing organ (sensitivity and selec‐ tivity), the concept of the active element and the cochlear amplifier were introduced as early as in 1948 by Gold [5] and later extended by many researchers (e.g., Davis in [6]). The intro‐ duction of the active element served to explain the phenomenon of power amplification and sharpening the frequency characteristics that occur in the cochlea, see Refs. [5, 6]. In general, power amplification process requires that some extra energy is delivered from an external source to the system. The system with power amplification capability is called active in con‐

Properties of active elements (where active processes can occur) allow in a natural way to explain such features of human hearing organ as high sensitivity and selectivity (narrow bandwidth). An example of active elements found in classical electronics can be transistors, bipolar, as well as unipolar (field effect transistors). These elements operating in the amplifier

At present the existence of the cochlear amplifier is widely accepted in the literature, see Ref. [7]. However, the exact mechanism of the power amplification in the cochlea is still the subject of extensive research. It is also generally accepted that OHCs play a key role in the cochlear power amplification process [8]. Power amplification and sharpening of the fre‐ quency response occurs in the OHCs, see Ref. [6], that are located in the Cochlea. In fact, loss of the OHCs causes that the cochlear amplifier is not operating, and as a consequence hearing

Another type of sensing element is inner hair cells (IHCs) that are also located in the cochlea, between the BM and TM. The inner hair cells (IHCs) detect the mechanical signal, which was previously amplified by the OHCs. The IHC plays the role of sensor [9]. The IHC converts the input mechanical signal into an electrical signal and transmits the latter to the central nervous system via the afferent innervation. In this case the afferent innervation of the IHCs may be

trast to a passive system, which can only dissipate (lose) the energy.

circuit can amplify the power of the electrical input signals.

called an output circuit of the entire cochlear amplifier.

with an acceptable quality and perception.

**1.2. Active processes in the cochlea**

62 Advances in Clinical Audiology

*1.2.1. What are the active processes?*

selectivity.

capabilities are lost.

*1.2.2. IHC operates as sensors*

The OHC is not only the mechano‐mechanical transducer. The OHC is both the mechano‐ electrical transducer and the electromechanical transducer. This can be attributed to two phenomena, i.e., the "forward mechano‐electrical transduction" and the inverse piezoelec‐ tric effect (electromotility), which play an essential role in the operation of the OHC and the cochlear amplifier as a whole [10]. An input acoustic signal entering the OHC is transferred to the electric side through the direct piezoelectric effect. There, on the electric side the signal is amplified.

#### *1.2.4. What is the piezoelectric effect (direct and inverse)?*

The direct piezoelectric effect is the ability of certain materials to generate an electric charge (voltage) in response to applied mechanical stress [11]. The inverse piezoelectric effect in turn is responsible for generating of mechanical deformations (stresses) induced by voltage applied to the material. Piezoelectric properties were found in certain solid media and biolog‐ ical materials, such as bones, ligaments, OHCs, and selected proteins. The piezoelectric effect, which is present in OHCs, is also termed in the literature as the electromotility or somatic motility. The piezoelectric effect is reversible, i.e., if the direct piezoelectric effect occurs in a material then the inverse piezoelectric effect will be present as well.

#### *1.2.5. Electric side and mechanical side*

A rectangular plate cut‐off from the piezoelectric material, with two parallel electrodes attached to the plate, forms the simplest piezoelectric transducer used in practice. The trans‐ ducer is in fact a three‐port device with one electrical and two mechanical ports. Application of an electrical signal to the electric port (two parallel electrodes) of the transducer will force the two parallel surfaces of the plate (two mechanical ports) to vibrate with the frequency equal to that of the electrical excitation. Conversely, application of a mechanical signal (force) to the mechanical port(s) will generate voltage in the electrical port with the frequency equal to that of the mechanical driving force. In this way, mechanical and electrical quantities are mutually interrelated and can be transformed one to each other via the piezoelectric effect.

#### *1.2.6. Importance of the electrical phenomena in the OHC*

The role of electrical phenomena occurring in the OHC is not merely auxiliary, but in the contrary, is essential. According to the author's analysis, the process of power amplification of the input acoustic signal (applied to one mechanical port of the OHC), as well as the process of sharpening the frequency characteristics (increased frequency selectivity), is carried out on the electrical side of the OHC. Consequently, sharpened electrical signal with amplified power is transferred back to both ports of the mechanical side of the OHC, through the inverse piezoelectric effect. Therefore, the OHC performs mechanical work on its both mechanical ports, i.e., on the BM and on TM. The mechanical energy supplied by the OHC to the TM is transferred to the corresponding IHC by the movement of its stereocilia. The IHC processes its input (in relation to the IHC) mechanical signal into an output electrical signal by opening ion channels (mechano‐electric transduction effect). These ionic currents affect the afferent nerve endings, where they are transformed into a series of electrical impulses that are trans‐ mitted into the central nervous system. It is assumed that there is no phenomenon of the power amplification in the IHC, which works as a passive sensor. Effectively, the IHC is a mechano‐electrical transducer that converts the mechanical signal received from the OHC, into a useful electrical signal, which is an "electrical image" of the received acoustic waves that we can hear.

In this chapter, the author emphasizes the crucial role of the electrical phenomena in the processes of power amplification of input acoustic signals and sharpening of the frequency characteristics of the cochlea. In the second part of this chapter (Sections 9–13), the results of the original author's research, i.e., new model and concept of the cochlear amplifier are presented.
