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Biomechanics of Human Hearing

Hipper

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#22
Lol! Definitely an old design. No new work on mammal designs from the big guy upstairs for decades now, as far as I know.
There is evidence to the contrary. See this report from London in 1944:

164839.jpg


"It's ridiculous to say these new flying bombs have affected people in any way!"
 
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AudioStudies

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Thread Starter #23
Sound Localization

Introduction

Sound localization is the ability of humans and animals to discern the location (origin) of a sound; ascertaining both direction and distance. Effective sound localization can help animals to catch their prey or help humans avoid accidents. Ineffective sound localization can affect human ability to safely interact in various environments. Clearly, if a person cannot determine the direction of an approaching vehicle it could lead to fatal consequences.

The mechanisms of sound localization of mammalian auditory systems have been studied extensively; thereby determining several cues contribute to the ability:
  • Time and intensity differences between each ear;
  • Spectral information;
  • Timing analysis;
  • Correlation analysis; and
  • Pattern matching.
Sound localization is based on interaural differences: differences in the sounds that arrive at the two ears; attributable to either the time of arrival or the intensity of the sound, or on monaural spectral cues. Spectral cues are the pattern of sound filtering caused by the angle of incidence of sound waves with the external ear, and are frequency-dependent.

The cues are analyzed in particular brainstem pathways and integrated as cortical representation. Interaural time differences (ITDs) and interaural level differences (ILDs) are the principal cues for horizontal (left-right) sound localization. Spectral cues are used for both vertical and back-front localization; and are attributable to the direction-dependent filtering properties of the external ears.

In reverberant environments, humans are exposed to both direct sound and reflected sound. In these cases, the human auditory system assigns greater precedence to the direct sound; a phenomenon known as the precedence effect (Wallach 1949)

Human adults are better at localizing sounds in the horizontal (left-right) dimension than in the vertical dimension and better at localizing sounds in front of them, than behind. Sound localization reflects the extent to which an organism has developed a spatial-hearing “map,” that is a perceptual representation of where sounds are located with respect to the head. The accuracy of sound localization in adult humans depends on factors such as the task, instructions to listeners, stimulus frequency content and duration, and response options.

Sound localization has been studied for decades in adult humans wherein the focus has been the extent to which listeners can either identify the exact position of a sound or discriminate changes in the location of sounds. Thus there are two different aspects to the study of sound localization:
  • absolute localization (localization acuity): the ability to judge the absolute position of a sound source in three-dimensional space; acuity is measured as the difference between the actual and the judged angular positions of the chosen loudspeaker relative to the listener’s head.
  • relative localization: the ability to detect a shift in the absolute position of the sound source. Relative localization is quantified by the minimum audible angle (MAA); defined as the smallest detectable shift in angular location of the sound source (Mills 1958).
In human adults, the accuracy of sound localization depends on many factors, including the spectral content of the sound stimuli and the location of the source. Adults can localize noises better than tones, with respect to both absolute and relative sound localization.

Human adults are better at relative than at absolute localization. Absolute localization acuity for broadband noises in the horizontal dimension is 5° to 10°: whereas the MAA (relative localization) can be as small as 1° to 2° for pure tones or noise.
 
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AudioStudies

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Thread Starter #24
Inner Ear Processing

Introduction

The inner ear consists of a thin diaphragm (oval window bordering the middle ear) and a maze of tubes and passages (labyrinth) that contain the vestibular (organ of balance) and the cochlea, a transducer that converts the mechanical sound energy to electrical impulses. Humans have a coiled (snail shaped) form of cochlea as do other mammals. The outer hair cells (OHC) of a mammalian cochlea provide enhanced sensitivity and frequency resolution; in contrast to other animals.

The function of the inner ear is to transduce mechanical vibrations received from the middle ear into nerve impulses (electrical signals) that can be perceived by the human brain. It is within the course of this transduction process that humans perceive pitch (the human interpretation of frequency) and loudness (the human interpretation of sound intensity).

The oval window (fenestra ovalis) is a connective tissue membrane between the middle ear and the inner ear. The oval window connects the tiny bones of the middle ear to the scala vestibule (upper part of the cochlea).

The middle ear transmits motion of the eardrum to the inner ear, thereby increasing the pressure on the connective tissue of the oval window. This pressure is transmitted through the stapes (bone in the middle ear) which presses against the oval window to the cochlea of the inner ear.

The cochlea within the inner ear, shaped like a snail shell, has two and a half turns and incorporates the membranous labyrinth, a collection of fluid filled tubes and chambers containing receptors for the human senses of equilibrium (balance) and hearing. The endolymph is the fluid within the membranous labyrinth.

The membranous labyrinth is found within the bony labyrinth in the inner ear and is of the same general composition. However, it is substantially smaller and partly separated from bony walls by the perilymph, a fluid that is essentially incompressible. The membranous labyrinth is separated into three perilymph-filled sections, by a membranous sac of triangular cross-section which run the length of the cochlea. The two outer sections are the scala vestibule, connected to the oval window, and the scala tympani connected to the round window. The three sections connect at the apex by a small opening (helicotrema) which serves as a pressure equalizing mechanism at frequencies well below the audible range.

The volume of the cochlea is approximately 0.2 mL, and within this volume up to 30,000 hair cells are present which transduce vibration into nerve impulses. Nerve fibers transmit signals both to the brain and from the brain.

The cochlea is continuous with the vestibular labyrinth, the organ of balance that serves as an accelerometer (both linear and angular) to enable the human brain to ascertain head position with respect to the surroundings, and have a relational sense of gravity and other forces.

Vibration of the foot plate of the stapes vibrates the perilymph in the bony cochlea. A counter opening is present in the labyrinth to enable fluid expansion when the stapes foot plate moves inwards. The counter opening is provided by the round window membrane beneath the oval window (in the inner wall of the middle ear). It is covered by a fibrous membrane which moves in synch (but opposite in phase) with the foot plate in the oval window.

The mammalian cochlea contains receptors that allow for transduction of mechanical waves into electrical signals. The basilar membrane is the essential mechanical element within the length of cochlea that serves as a mechanical analyzer, as it curls toward the center. The vibration patterns of the basilar membrane separate incoming sound into component frequencies that activate different cochlear regions. The basilar membrane is non-linear exhibiting less accurate behavior with lower frequencies.

Mass and stiffness properties of the basilar membrane vary in accordance with its length; it is thicker, narrower and taut where the cochlea is largest; and thinner, broader, and less taut near the apex of the whorl (where the cochlea is smallest).
 

Kal Rubinson

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#25
The mammalian cochlea contains receptors that allow for transduction of mechanical waves into electrical signals. The basilar membrane is the essential mechanical element within the length of cochlea that serves as a mechanical analyzer, as it curls toward the center. The vibration patterns of the basilar membrane separate incoming sound into component frequencies that activate different cochlear regions. The basilar membrane is non-linear exhibiting less accurate behavior with lower frequencies.
That is a superficial description of the "Place Principle" mechanism in which loci along the basilar membrane are mapped by frequency but the response is not so simply localized. Since there is a pressure input at the oval window, there will be a deflection of the membrane at the base of the cochlea due to the pressure difference between the scala vestibuli and the scala tympani. This deflection passes up the cochlea as a traveling wave along the basilar membrane. The frequency mapping is of the site along the length of the membrane where there is the maximal deflection and maximum response to a particular frequency. However, there is some deflection of the membrane and there are some hair-cell responses to most frequencies along most of the length of the membrane.

There is an additional mechanism, the "Volley Principle," in which the neurons which are driven by the hair-cells respond with action potentials which directly code frequency, i.e, a 60Hz tone would create a volley of APs at 60 per second. This would seem to be a nice and direct mode of coding but it is most obviously compromised by the neuron's ability to respond which begins to limit in the upper hundreds of Hz. (At that point, small groups of neurons may encode the frequency together.) Obviously, the volley principle is more effective for lower frequencies and that is complementary to the basilar membrane being "non-linear exhibiting less accurate behavior with lower frequencies."
 
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AudioStudies

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Thread Starter #27
Inner Ear Processing

Pitch Perception

Two early theories were offered in the literature to explain the perception of pitch that occurs in the human inner ear, the place theory (Helmholtz 1863) and the frequency theory (Rutherford 1886):
  • Place theory maintains that pitch perception depends on the vibration of various portions of the membrane formed by receptive cells; cells in each region of the membrane specialized for the detection of specific frequencies.
  • Frequency theory holds that pitch perception corresponds to the rate of vibration of all of the receptive cells along the basilar membrane. A frequency of 2000 Hz would cause the whole membrane to vibrate at a rate of 2000 Hz; subsequently the brain detects this frequency based on the rate of neuronal firing that matches the rate of vibration.
Extensive debate between the proponents of each theory has occurred since the origination of these theories in the 19th century. Many scientists believe that both theories are in part valid with respect to describing the mechanism underlying human pitch perception.

Place theory is accurate; however receptive cells along the inner membrane have no independent response (they vibrate together as the frequency theory contends). Sound waves travel along the membrane, peaking at a given region depending on the frequency.

The frequency theory was diluted by the discovery that receptor cells are incapable of firing at rates that reflect the higher frequencies of human hearing. In order to accommodate this weakness, scientists introduced the volley principle maintaining that different groups of receptive cells may fire in rapid succession. It was believed that this volley of impulses could generate the high frequencies that single receptive cells are incapable of generating.

Modern science maintains that sounds under 1000 Hz are translated into pitch through frequency coding. Sounds between 1000 and 5000 Hz are coded via a combination of frequency and place coding. Finally, for sounds over 5000 Hz pitch is coded via place only.

When pressure occurs at the oval window, a membrane deflection occurs at the base of the cochlea attributable to the pressure difference between the scala vestibuli and the scala tympani. This membrane deflection travels along the basilar membrane; and frequency mapping is of the site along the length of the membrane where there is the maximal deflection and thus maximum response to a particular frequency. However, there is some deflection of the membrane and there are some hair-cell responses to most audible frequencies along the majority of the length of the membrane.
 

goliardo

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#28
They also perform an impedance transformation for efficiently conveying the mechanical energy from air to fluid (endolymph) media.

Note that it is non-linear with poorer frequency/map resolution at lower frequencies. The compensation for this is that the lower frequencies can be directly coded into pulse-trains. That mechanism cannot work for higher frequencies because neurons cannot fire fast enough. Thus, the higher resolution of the basilar membrane mapping serves well there.
Frequency resolution is actually poorer at high frequencies, as measured by the wider bandwidth of 'auditory filters' (aka 'critical bands') at high frequencies than at lower frequencies. Better frequency resolution at low frequencies explains why it is possible to hear individual low-numbered harmonics (up to about 8th harmonic) of a harmonic complex tone, but it is not possible to do the same with high-numbered harmonics (8-10th harmonics and above).
 

goliardo

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#29
Inner Ear Processing

Pitch Perception

Two early theories were offered in the literature to explain the perception of pitch that occurs in the human inner ear, the place theory (Helmholtz 1863) and the frequency theory (Rutherford 1886):
  • Place theory maintains that pitch perception depends on the vibration of various portions of the membrane formed by receptive cells; cells in each region of the membrane specialized for the detection of specific frequencies.
  • Frequency theory holds that pitch perception corresponds to the rate of vibration of all of the receptive cells along the basilar membrane. A frequency of 2000 Hz would cause the whole membrane to vibrate at a rate of 2000 Hz; subsequently the brain detects this frequency based on the rate of neuronal firing that matches the rate of vibration.
Extensive debate between the proponents of each theory has occurred since the origination of these theories in the 19th century. Many scientists believe that both theories are in part valid with respect to describing the mechanism underlying human pitch perception.

Place theory is accurate; however receptive cells along the inner membrane have no independent response (they vibrate together as the frequency theory contends). Sound waves travel along the membrane, peaking at a given region depending on the frequency.

The frequency theory was diluted by the discovery that receptor cells are incapable of firing at rates that reflect the higher frequencies of human hearing. In order to accommodate this weakness, scientists introduced the volley principle maintaining that different groups of receptive cells may fire in rapid succession. It was believed that this volley of impulses could generate the high frequencies that single receptive cells are incapable of generating.

Modern science maintains that sounds under 1000 Hz are translated into pitch through frequency coding. Sounds between 1000 and 5000 Hz are coded via a combination of frequency and place coding. Finally, for sounds over 5000 Hz pitch is coded via place only.

When pressure occurs at the oval window, a membrane deflection occurs at the base of the cochlea attributable to the pressure difference between the scala vestibuli and the scala tympani. This membrane deflection travels along the basilar membrane; and frequency mapping is of the site along the length of the membrane where there is the maximal deflection and thus maximum response to a particular frequency. However, there is some deflection of the membrane and there are some hair-cell responses to most audible frequencies along the majority of the length of the membrane.
Pretty good summary about pitch coding for pure tones. However, current evidence seems to favour temporal (described as 'frequency') coding for sounds below about 5000 Hz, while above 5000 Hz phase-locking breaks down and pitch is likely processed with place cues.
Another thing I would add about this summary is that clear pitch percepts are confined to the frequency range that extends up to about 5000 Hz (matching the range of music). At higher frequency, pitch sensations become less clear.
 

Kal Rubinson

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#30
Frequency resolution is actually poorer at high frequencies, as measured by the wider bandwidth of 'auditory filters' (aka 'critical bands') at high frequencies than at lower frequencies. Better frequency resolution at low frequencies explains why it is possible to hear individual low-numbered harmonics (up to about 8th harmonic) of a harmonic complex tone, but it is not possible to do the same with high-numbered harmonics (8-10th harmonics and above).
Note that I did not say that functional frequency resolution is poorer at low frequencies, just that the resolution of the spatial map on the cochlea is poor. The better functional frequency resolution is due, mostly, to the effective employment of volley (or temporal or frequency) principles at those frequencies.
 

goliardo

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#31
Note that I did not say that functional frequency resolution is poorer at low frequencies, just that the resolution of the spatial map on the cochlea is poor. The better functional frequency resolution is due, mostly, to the effective employment of volley (or temporal or frequency) principles at those frequencies.
Oh, sorry. I thought that was what you meant by "poorer frequency/map resolution at lower frequencies".
 

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