An analog ear or analog cochlea is a model of the
ear or of the
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
(in the
inner ear
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
) based on an electrical, electronic or mechanical
analog. An analog ear is commonly described as an interconnection of
electrical elements such as
resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
s,
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
s, and
inductor
An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s; sometimes
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s and active
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
s are included.
Ear background
![Anatomy of the Human Ear en](https://upload.wikimedia.org/wikipedia/commons/d/d2/Anatomy_of_the_Human_Ear.svg)
The ear of the typical mammal consists of three parts. The
outer ear collects sounds like a horn and guides them to the
eardrum
In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear
The outer ear, external ear, or auris externa is the extern ...
. Vibrations of the drum are conveyed to the
inner ear
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
via a system of bones called
ossicles
The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ...
. These leverage the larger motions of the
eardrum
In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear
The outer ear, external ear, or auris externa is the extern ...
to the smaller vibrations of the
oval window
The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a membrane-covered opening from the middle ear to the cochlea of the inner ear.
Vibrations that contact the tympanic membrane travel through the three ossicles and into the in ...
. This window connects to the
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
which is a long dual channel arrangement consisting of two channels separated by the
basilar membrane. The structure, about 36 mm in length, is coiled to conserve space. The
oval window
The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a membrane-covered opening from the middle ear to the cochlea of the inner ear.
Vibrations that contact the tympanic membrane travel through the three ossicles and into the in ...
introduces sounds to the upper channel. The lower channel has a
round window but this is not driven by the bones of the middle ear. The far end of the structure has a hole between the two channels called the
helicotrema that equalizes slowly varying pressures in the two channels. A series of sensory
hair cells along the
basilar membrane respond to send neural pulses towards the brain.
Ear modeling
Models for the ear of a direct kind have been created, most notably by Nobel Laureate
Georg von Békésy. He used glass slides, razor blades, and an elastic membrane to represent the
helicotrema. He could measure vibrations along the
basilar membrane in response to different excitations frequencies. He found that the pattern of displacements for given frequency sine wave along the
basilar membrane rose somewhat gradually to a peak and thereafter fell. High frequencies favored shorter distances from the
oval window
The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a membrane-covered opening from the middle ear to the cochlea of the inner ear.
Vibrations that contact the tympanic membrane travel through the three ossicles and into the in ...
than did lower ones. Frequency values approximate a logarithmic distribution with distance.
Mechanical and electrical analogs
Early mechanical and electrical analog ears were recounted in the 1954 book ''Analog Methods in Computation and Simulation'':
A direct (mechanical) model uses the variables of air and water pressure, fluid velocity and viscosity, and displacement. An electrical analog model uses a different set of variables, namely, voltage and current. The outer and middle parts of the ear can be represented with a collection of coils, capacitors, and an ideal transformer to represent the leveraging effect of the
ossicles
The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ...
. This circuit terminates with a capacitor representing the
oval window
The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a membrane-covered opening from the middle ear to the cochlea of the inner ear.
Vibrations that contact the tympanic membrane travel through the three ossicles and into the in ...
. From there, the two channels are represented with a sequence of inductors and resistors for fluid flow within each channel with the two channels joined with a sequence of series resonant RLC circuits. Voltages across capacitances represent
basilar membrane displacements. Element values along the cochlea are tapered in a logarithmic fashion to represent lowering frequency responses with distance.
The pattern of voltages along the
basilar membrane can be viewed on an oscilloscope. Average values can be obtained with rectification and shown as patterns using a high speed commutator. The analog ear shows patterns that closely follow those observed by
Georg von Békésy on his more direct model.
The first relatively complete model was constructed in the early 1960s at the University of Arizona by two graduate students and their faculty mentor with support from the newly established Air Force
Bionics
Bionics or biologically inspired engineering is the application of biological methods and systems found in nature to the study and design of engineering systems and modern technology.
The word ''bionic'', coined by Jack E. Steele in August 1 ...
program. This work was first summarized in a report: "An Electronic Analog of the Ear", Technical Documentary Report No.
AMRL-
TDR-1963-60, June 1963, Biophysics Laboratory, 6570-th Aerospace Medical Research Laboratories, Aerospace Medical Division, Air Force Systems Command, by E. Glaesser, W. F. Caldwell, and J. L. Stewart. The report contains an extensive list of references. The work was also reported at
Bionics
Bionics or biologically inspired engineering is the application of biological methods and systems found in nature to the study and design of engineering systems and modern technology.
The word ''bionic'', coined by Jack E. Steele in August 1 ...
symposia.
John L. Stewart and Covox
Unlike models based on a series of active filters or represented with digital equations, an analog ear can incorporate nonlinearities that represent nonlinear actions of the
basilar membrane, perhaps caused by asymmetric motions of sensory cells resulting in asymmetric motions of the basilar membrane. Difference frequencies could be generated as are observed in the human. Some difference frequencies originating in the
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
can be observed in the
outer ear.
Neural signals responding to motions of the basilar membrane show responses in one direction as in rectification. At all but low frequencies, the neural measure averages over multiple cycles to give the equivalent of rectification followed by averaging (
low-pass filtering). Over the entire cochlea, response shows as a pattern that varies more slowly that the applied frequency but that does follow the envelope of the applied signal. Each group of cells can give rise to a semi-periodic wave that can be analyzed by neurons in the brain. The total pattern that arises from a sound can thus be thought of as a two-dimensional pattern in time with one axis being the distance along the basilar membrane and the other being distance along some sequence of neurons. These patterns, varying at rates less than lower audio frequencies, have shapes that can be identified much like patterns in vision. The concept of the "neural analyzer" as an extension of cochlear patterns is discussed in , "Speech Bandwidth Compression System", June 4, 1968 (filed in 1964).
It was found that the analog ear with its asymmetric overlapping bands was more reliable in identifying speech sounds than is a conventional
frequency spectrum. The second
formant is the most significant single measure. Speech sounds of interest include
whispered and
clipped speech.
Applications were made to animals and insects with appropriate ear models. Another study using the analog ear was "Simulating Mechanisms in Animal Echoranging", John L. Stewart and James M. Kasson.
Many reports, articles, and patents followed the research as cited in the reports listed here. The last full report employed a relatively early version of a computer program written in time-shared
BASIC
BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
.
Stewart also self-published several books, doing business as Santa Rita Technology, and later as
Covox, including ''The Analog Ear Story'' and ''The Analog Ear–brain System'' in 1964, and ''The Bionic Ear'' in 1979.
The research resulting from analog ear studies fueled the creation of special sounds for use in repelling birds and other pests. Sounds were synthesized to follow natural bird calls but were of a switching kind. The concept is similar to the use of a babble of human voices for jamming another person's communications. The "Av-Alarm" was the principal product. It was also adapted to the transonic and ultrasonic regions with a device called "Transonic".
The research also led to development of an early speech word recognizer that operated with 8-bit computers as well as later ones based on 16-bit processors. The product line was developed by Covox, Inc. with product names of "Speech Thing" and "Voice Master".
A number of U.S. (and foreign) patents on topics related to Stewart's analog ear were granted. In the order of filing dates starting in 1962, numbers are , , , , , , , , and .
Analog VLSI cochlea models
Several groups have built analog VLSI hearing chips in recent decades.
Notes
References
{{reflist
Biotechnology
Hearing
Audiology