Hearing: Physiology and Psychoacoustics - Flashcards
Hearing: Physiology and Psychoacoustics
9.1 The Function of Hearing
Awareness of Surroundings: Hearing helps individuals remain cognizant of their environment.
Object Identification: It allows for the recognition of objects based on the unique sounds they produce.
9.2 What Is Sound?
Creation of Sound: Sounds are generated when objects vibrate.
These object vibrations cause molecules in the surrounding medium (e.g., air, water) to vibrate, leading to pressure changes in that medium.
Sound Wave Speed: Sound waves travel at a specific speed, which varies depending on the medium.
Air: Approximately 340 meters/second (m/s).
Water: Approximately 1,500 meters/second (m/s).
Physical Qualities of Sound Waves:
Amplitude or Intensity: The magnitude of displacement (increase or decrease) of a sound pressure wave.
Perceived psychologically as Loudness.
Frequency: For sound, this is the number of times per second that a pattern of pressure change repeats.
Perceived psychologically as Pitch.
Psychological Aspects of Sound:
Loudness: The psychological characteristic of sound directly related to its perceived intensity (amplitude).
Pitch: The psychological characteristic of sound primarily related to its perceived frequency.
Low-frequency sounds are perceived as low pitches.
High-frequency sounds are perceived as high pitches.
Timbre: The psychological sensation that allows a listener to distinguish between two sounds of the same loudness and pitch, indicating their dissimilarity (e.g., the difference between a guitar and a piano playing the same note).
Units of Measurement for Sound:
Decibels (dB): The unit of measure for the physical intensity of sound (sound pressure level).
20 \mu Pa (0.0002 dyne/cm^2) is the reference pressure for sound waves in air, defined as 0 dB.
Each 10:1 sound pressure ratio corresponds to 20 dB.
A 100:1 ratio equals 40 dB.
Humans perceive a 10-fold increase in acoustic power as double the loudness.
Equation for sound pressure level (SPL): Lp = 20 \log{10} \left( \frac{p}{p0} \right) where p is the sound pressure and p0 is the reference pressure.
Decibel Scale is logarithmic; relatively small decibel changes can correspond to large physical changes (e.g., an increase of 6 dB corresponds to a doubling of sound pressure).
Hertz (Hz): The unit of measure for frequency.
1 Hz equals 1 cycle per second.
Human hearing range: from 20 Hz to 20,000 Hz (or 20 kHz).
Human vs. Animal Hearing Ranges:
Humans hear in the range of 20 Hz to 20,000 Hz.
Animals like dolphins, bats, and cats can hear much higher frequencies (ultrasound).
Animals like elephants can hear much lower frequencies (infrasound).
Types of Sound Waves:
Sine waves (Pure Tones): The simplest kind of sound, characterized by a waveform where variation as a function of time is a sine function.
Not common in everyday sounds due to their purity.
Complex Sounds: Most sounds in the world are complex.
All sound waves, however complex, can be described as a combination of sine waves using Fourier Analysis—a mathematical technique that decomposes a complex function into simpler, constituent sine and cosine waves.
Complex sounds are best described by a spectrum, which displays the energy present at each frequency in the sound.
Harmonic Spectrum: The spectrum of a complex sound where energy is at integer multiples of the fundamental frequency.
Fundamental Frequency: The lowest-frequency component of a complex periodic sound.
Typically produced by simple vibrating sources (e.g., guitar string, saxophone reed).
Spectrogram: A 3D display for sound analysis, plotting time on the horizontal axis, frequency on the vertical axis, and intensity by color or grayscale.
Waveform plots time vs. amplitude.
Spectrum plots frequency vs. intensity.
9.3 Basic Structure of the Mammalian Auditory System
Summary of Sound Transmission Through the Ear:
An air pressure wave is funneled by the pinna through the external ear canal to the tympanic membrane.
The tympanic membrane vibrates in time with the sound wave, vibrating the malleus, which vibrates the incus, which vibrates the stapes.
The stapes pushes and pulls on the oval window.
Movement of the oval window causes pressure bulges to move down the vestibular canal, which move the middle canal up and down.
This motion forces the tectorial membrane to shear across the organ of Corti, moving stereocilia atop hair cells.
Pivoting stereocilia initiate rapid depolarization, releasing neurotransmitters into synaptic clefts between hair cells and auditory nerve fibers.
Neurotransmitters initiate action potentials in auditory nerve fibers, carrying signals to the brain.
Outer Ear:
Pinna (Auricle): The visible part of the ear; collects sounds from the environment.
Auditory Canal (Ear Canal): Funnels sound waves from the pinna to the eardrum.
Its length and shape enhance certain sound frequencies (resonant frequencies).
Insulates and protects the tympanic membrane.
Tympanic Membrane (Eardrum): Vibrates in response to sound; border between outer and middle ear.
Middle Ear:
Ossicles: Three smallest bones in the body; amplify sound waves and transfer their energy from the tympanic membrane to the cochlea.
Malleus (Hammer): Vibrates from the eardrum.
Incus (Anvil): Connected to the malleus.
Stapes (Stirrup): Transmits vibrations to the oval window (border between middle and inner ear).
Amplification Mechanisms:
Lever Action: The ossicles act as a lever system.
Concentration of Energy: Energy from the larger tympanic membrane is concentrated onto the smaller oval window.
Acoustic Reflex: Muscles in the middle ear (tensor tympani, stapedius) tense in response to loud sounds, stiffening the ossicle chain and muffling pressure changes to protect the inner ear.
Inner Ear:
Location where fine changes in sound pressure are transduced into neural signals.
Cochlea: A spiral-shaped structure containing the organ of Corti, filled with watery fluids in three parallel canals.
Vestibular Canal (Scala Vestibuli): Extends from the oval window to the helicotrema (apex).
Closest to ossicles; pressure waves move through here first.
Tympanic Canal (Scala Tympani): Extends from the helicotrema to the round window (base).
Middle Canal (Scala Media): Sandwiched between the vestibular and tympanic canals; contains the cochlear partition.
Vestibular and tympanic canals are filled with perilymph.
Middle canal is filled with endolymph, which bathes the cochlear partition in nutrients and ions essential for hair cell activity.
Stria Vascularis: Specialized tissue in the middle canal, responsible for balancing charged ions in the endolymph.
Cochlear Membranes:
Reissner's Membrane: Separates the vestibular and middle canals.
Basilar Membrane: Forms the base of the cochlear partition, separating the middle and tympanic canals.
Organ of Corti: A structure on the basilar membrane composed of hair cells and dendrites of auditory nerve fibers.
Translates movements of the cochlear partition (via the basilar membrane) into neural signals.
Hair Cells: Sensory receptor cells supporting stereocilia.
Transduce mechanical movement in the cochlea into neural activity sent to the brainstem.
Arranged in four rows along the basilar membrane.
Stereocilia: Tips of hair cells that, when flexed, initiate the release of neurotransmitters.
Tectorial Membrane: A gelatinous flap, connected at one end, resting on top of the hair cells.
Inner Hair Cells: Convey almost all information about sound waves to the brain (using afferent fibers).
Outer Hair Cells: Receive input from the brain (using efferent fibers); involved in an elaborate feedback system.
These cells can make parts of the cochlear partition stiffer.
This increases the sensitivity and sharpens the tuning of inner hair cells to specific frequencies.
Auditory Nerve (AN) Fibers:
Place Coding: Responses of individual AN fibers to different frequencies are related to their distinct location along the basilar membrane.
Characteristic Frequency (CF): The frequency to which a particular AN fiber is most sensitive; response is clearest when sounds are faint.
Two-tone Suppression: A decrease in an AN fiber's firing rate to one tone when a second tone is presented simultaneously, especially if the second tone is lower in frequency.
Rate Saturation: The point at which a nerve fiber is firing as rapidly as possible, and further stimulation cannot increase the firing rate.
Increasing sound intensity widens an AN nerve fiber's frequency selectivity.
The auditory system relies on responses from approximately 14,000 AN fibers to determine frequency and perceive sound.
Temporal Code for Sound Frequency:
Phase Locking: The firing of a single neuron at one distinct point in the period (cycle) of a sound wave at a given frequency.
Temporal Code: Information about the particular frequency of an incoming sound wave is coded by the timing of neural firing relative to the sound wave's period.
Volley Principle: Multiple neurons can collectively provide a temporal code for frequency if each neuron fires at a distinct point in the sound wave's period, but not necessarily on every period themselves.
Auditory Pathways and Brain Structures:
Cochlear Nucleus: The first brainstem nucleus where afferent AN fibers synapse.
Superior Olive: Inputs from both ears converge here; crucial for sound localization.
Inferior Colliculus: Midbrain nucleus in the auditory pathway.
Medial Geniculate Nucleus (MGN): Part of the thalamus that relays auditory signals and receives input from the auditory cortex.
Primary Auditory Cortex (A1): The first area within the temporal lobes that processes acoustic information.
Secondary/Associational Auditory Areas:
Belt Area: Neurons project from A1 to this area, which responds to more complex sound characteristics.
Parabelt Area: Responds to even more complex sound characteristics and receives input from other senses.
Bilateral Representation: Signals from both cochleas reach both sides of the brain after only a single synapse (e.g., at the medial superior olives).
Tonotopic Organization: An anatomical arrangement where neurons responding to different frequencies are organized in order of frequency.
Begins in the cochlea and is maintained through the primary auditory cortex (A1).
Suggests that frequency composition is a primary determinant of how sounds are heard.
9.4 Basic Operating Characteristics of the Auditory System
Psychoacoustics: The branch of psychophysics dedicated to studying the psychological correlates of the physical dimensions of acoustics to understand auditory system operation.
Loudness: Psychological perception related to intensity.
Pitch: Psychological perception related to frequency.
Audibility Threshold: The lowest sound pressure level that can be reliably detected at a given frequency.
Equal-Loudness Curve: A graph plotting sound pressure level (dB SPL) against frequency for which a listener perceives constant loudness.
Human hearing has the best absolute thresholds within a specific frequency range; frequencies outside this range require larger amplitudes to be perceived.
Temporal Integration: The process where a sound at a constant level is perceived as louder when it has a longer duration.
Occurs over an interval of 100 to approximately 200 milliseconds (ms).
A sound played for less than 100 ms will be perceived as quieter than the same sound played for 200 ms, but no further increase in loudness occurs beyond 200 ms.
Masking: The use of a second sound (often noise) to make the detection of another sound more difficult.
White Noise: Noise containing all audible frequencies in equal amounts (analogous to white light in vision).
Critical Bandwidth: The range of frequencies conveyed within a specific channel in the auditory system.
A target sound becomes increasingly difficult to detect when noise within its critical bandwidth increases.
Beyond this range, the noise is less effective at masking.
Real-World Example: Manatees and Boat Noise:
Manatees have excellent hearing underwater but are not sensitive to low-frequency sounds.
Boat engines primarily produce low-frequency sounds, especially when slowing down, making collisions a significant threat.
Solution: Develop high-frequency manatee alerting sounds, projected from boats, that fall within their peak hearing range.
9.5 Hearing Loss
Definition: An increasing need for higher sound levels to detect a sound, and subsequently, to understand and respond to it.
Highlights the distinction between sensation (detecting sound) and perception (understanding sound).
Congenital Hearing Loss: Hearing loss present at birth.
Can sometimes be addressed with a Cochlear Implant: Tiny flexible coils with miniature electrode contacts threaded through the round window to the cochlea apex.
A tiny microphone transmits radio signals to a receiver implanted in the scalp.
Signals activate electrodes at appropriate positions along the implant, bypassing damaged hair cells.
Acquired Hearing Loss: Hearing loss that appears later in life.
Causes:
Obstruction/Blockage: E.g., earplugs, excessive earwax (cerumen) buildup in the ear canal.
Conductive Hearing Loss: Problems with the bones of the middle ear.
Otosclerosis: A more serious type caused by abnormal growth of middle ear bones, which can often be remedied by surgery.
Sensorineural Hearing Loss: The most common and serious type, due to defects in the cochlea or auditory nerve.
Hair Cell Injury: Caused by infection, ototoxic antibiotics/cancer drugs, or excessive exposure to noise.
Damage to outer hair cells decreases and weakens the selectivity of AN responses.
Auditory Nerve Fiber Loss: Can occur due to aging.
Stria Vascularis Dysfunction: Inability of the stria vascularis to bathe the cochlear partition with nutrients and ions, leading to decreased hair cell activity.
Mammalian Limitation: Hair cells cannot be regenerated in mammals.
Non-Mammalian Regeneration: Other vertebrates (e.g., zebrafish, newts, lizards, parakeets, quails, chickens) can regenerate hair cells.
Noise-Induced Hearing Loss: Damage to hair cells from excessive noise exposure.
Volume: Sounds above approximately 120 dB can cause permanent damage immediately due to increased pressure on the tympanic membrane.
Duration: Prolonged exposure to loud sounds can also cause damage.
Temporary Threshold Shift (TTS): Swelling of hair cells, resulting in muffled sounds; can be temporary unless exposure is repeated.
Tinnitus: A 'ringing in the ears' sensation, often caused by extended exposure to loud sounds.
Hidden Hearing Loss: Damage specific to the synapses between AN fibers and hair cells, leading to reduced connectivity for information transfer in the auditory cortex.
Sensation may be intact, but perception is compromised.
Presbycusis: Age-related hearing loss, a natural consequence of aging.
Hearing Aids:
Early Devices: Simple horns.
Modern Electronic Aids: More sophisticated than just amplification.
Cannot amplify all sounds across the frequency range equally, as this would be painful and damaging.
Instead, they compress sound intensities into a range the user can hear, allowing normal speech loudness levels to be heard without reaching damaging levels.