hearing 2 Flashcards
(51 cards)
Sensorineural hearing loss and causes
More common and most serious form of hearing loss
* Due to damage of hair cells or auditory nerve
Mammalian hair cells don’t regenerate → damage is permanent
Causes:
Ingestion of ototoxic drugs, e.g., neomycin destroys stereocilia!, cancer drugs, antibiotics
Traumatic injury, e.g., fracture of temporal bone
Tumors (especially in cochlear nerve - inflammation prevents APs transmission, e.g., acoustic neurinomas)
Diseases, e.g., rubella
Most common! Exposure to extensive environmental noise (i.e., noise-induced hearing loss NHL or NHIL)
- Causes injury to hair cells and damage to the transductional mechanisms
- Degree depends on amount and duration
- Sounds > 80 dB are potentially dangerous; those over 120 dB are considered painful (sports games, concerts, earbuds for many hours)
Ex: ppl who lived in small/quiet town and moved to a big city, after 5 years hearing got way worse than those who stayed.
Hereditary Hearing loss Factors, and Aging
Mutations in >150 different genes cause certain types of hereditary hearing loss
Usher syndrome (vision loss) and Waardenburg syndrome (multi colored eyes)
Presbycusis and causes
hearing loss that occurs gradually due to effects of aging
>2000 auditory nerve cells die each decade! Start w/35 000
Loss initially occurs with high-frequency sounds and progresses to the point where ordinary conversation becomes difficult to hear
Worse for men
Causes
Sensorineural hearing loss due to loss of IHCs (loss of hair cells)
Conductive loss due to abnormalities of ossicular function (calcification of ossicles)
diagnosis of hearing condition
Otolaryngologists (ear-nose-throat doctors, ENTs): clinical diagnosis of auditory disorders
Audiologists: evaluate hearing function
Can use bone conduction test (Rinne test)
Apply a vibrating tuning fork to a bone on the skull behind the ear
Normal: sound is louder in air than on bone (pinna help amplify loud sound in air)
Conductive hearing loss: sound is louder when touching bone (Why? - vibrations bypass canal+ossicles and go directly to bone)
Sensorineural hearing loss: sound isn’t heard at all (if hair cells/cochlear nerve doesn’t work, person won’t hear sound regardless)
Hearing aids and their components and problem
Battery-operated miniature amplifiers that can fit into the auditory canal or can be placed behind the ear
Components:
Small microphone to collect/detect the sound
An electronic amplifier (louder)
Small speaker to deliver/project the sound to the ear
Most effective if cochlear function is not impaired
Problem: don’t want to amplify ALL sounds - ppl with hearing loss can still hear low frequency sounds, don’t want painfully loud sounds (e.g., background noise, loud sounds)
When we amplify all sounds, difference between all sounds is hard to detect
Psychoacoustics
The branch of psychophysics that studies the psychological correlates of the physical dimensions of acoustics in order to understand how the auditory system operates.
- The perceptual experiences of sound intensity and frequency are referred to as loudness and pitch, respectively
Special features of hearing:
- Can locate sounds in the environment
- Can identify sounds among noise
Absolute sensitivity
Knowing this can help understand how sound interacts with auditory structures, and how sound is perceived at different frequencies
- Useful for creating hearing standards for clinical comparison
Audibility threshold and 2 perceptual tests for it
the lowest sound pressure level that can reliably be detected at a given frequency
Perceptual Tests:
Minimum audible field (MAF) threshold: person sits in an enclosed room, speaker in front of them. Start at low intensity and slowly inc. intensity
Advantages: naturalistic to how we hear
Disadvantages: moving head can affect threshold, sound can be reflected/absorbed by room and walls
Minimum audible pressure (MAP) threshold: sounds presented through headphones
Advantages: control sound pressure value that reaches ear
Disadvantages: less naturalistic
Absolute threshold depends strongly on
frequency: Plot shows different psychometric functions for sounds of four different frequencies.Typically use pure tones.
Ear is most sensitive to: 1500 Hz frequency (we can hear it at lowest sound intensity)
Audibility threshold of 100 Hz frequency: 30 dB spl
The audibility curve (minimal audibility curve, MAC): and terminal threshold
If we plot the 4 different sound levels on a graph of sound intensity vs. frequency,
Lowest detection thresholds are between 2000 and 6000 Hz
These frequencies are enhanced by the physical properties of the ear canal
Thresholds increase for frequencies above and below middle range
threshold: upper limit of auditory function
- Area between MAC and terminal threshold is the dynamic range of human hearing
What are ultrasonic and infrasonic sounds?
Infrasonic: below frequency range, low frequency sound. (ex: some animals can detect thunderstorms/earthquake)
Ultrasonic: above frequency range (higher than 20,000 Hz)
Why do we “feel” low frequency sounds (e.g., bass)?
Sound waves can be very long (metres long)
Body is physically within one long sound wave (period of compression)
Equal loudness curves are obtained by:
asking listeners to equate the loudness of sounds with different frequencies (Always start at 1000 Hz)
How loud does a 200 Hz tone need to be to sound as loud as a 1 kHz, 40 dB tone? Roughly 55 dbSPL
What do the two orange tick marks mean? Sounds are equally loud, (900 Hz at 60 db sounds same as 200 Hz at 40 db)
What do the purple tick marks mean? On diff curves, same sound intensity but perceived at diff loudness
Curves demonstrate that sound pressure level ≠ loudness
Experimental factors that affect absolute sensitivity
- Method of data collection
MAF thresholds < MAP thresholds
Accounted for by resonance properties of outer & middle ear - Monaural or binaural sound
Monaural stimulation → thresholds are higher by 6 dB Binaural summation - Masking
The presence of white noise or a small band of noise (the masker) around the frequency of interest increases threshold
Harmonic sounds
are the most common in our environment
Harmonic spectrum
he spectrum of a complex sound in which energy is an integer multiple of the fundamental frequency
* The lowest frequency of a harmonic spectrum is the fundamental frequency
* For harmonic complexes, the perceived pitch is determined by the fundamental frequency
* The harmonics (overtones) add to the perceived richness of the sound
Ex: a speaker produces a vowel sound with a fundamental frequency of 250 Hz. Her vocal cords will produce the greatest energy at 250 Hz, less energy at 500 Hz (2nd harmonic), even less at 750 (3rd harmonic) and so on
The Missing Fundamental Effect
- Play entire harmonic series → subjects hear a single pitch corresponding to the fundamental frequency
- When the fundamental frequency is removed, the pitch listeners hear still corresponds to the fundamental frequency!
How can we hear a sound when there is no stimulation at the corresponding region of the basilar membrane?
Frequency theory
* All harmonics of a fundamental have in common fluctuations in sound pressure at regular intervals that correspond to the fundamental frequency
* Neurons in cochlear nerve and cochlear nucleus fire action potentials once per cycle (thus encoding fundamental frequency)
Fluctuations in Sound Pressure:
Even when the fundamental frequency is missing, all harmonics share a pattern of sound pressure fluctuations at a rate corresponding to the fundamental frequency.
Example: If harmonics at 200 Hz, 300 Hz, 400 Hz, etc. are played, the combined waveform still fluctuates at 100 Hz intervals.
Phase-Locking & Neural Firing
Neurons in the cochlear nerve and cochlear nucleus fire action potentials in sync with these fluctuations. This means they fire once per cycle of the missing fundamental frequency.
Even though the basilar membrane is not directly stimulated at 100 Hz, the timing of neural signals encodes the missing fundamental.
Higher Brain Processing: The auditory cortex interprets this neural firing pattern and reconstructs the missing pitch. This is why listeners still hear the fundamental frequency even though it is not physically present
Timbre
is the perceptual quality that allows us to distinguish two musical instruments, even
when they’re playing the same note at the same intensity
* Everything other than loudness and pitch
* Differences in timbre are accounted for by differences in frequency spectra, i.e., the relative strength of harmonics in the spectrum
(different musical instruments play the same note but we can tell them apart, fundamental frequencies are the same and the intensities can be too)
Attack and decay:
An important quality of complex sound is the way it begins and ends. How it changes over time.
- Attack: the way a sound begins (onset) E.g., how quickly does it reach max intensity?
- Decay: the way a sound ends (offset). Depends on how long it takes for the vibrating object creating the sound to dissipate energy and stop moving. How long does it take to dissipate and stop vibrating.
Ex: violin = slower attack and slower decay
X axis: time, y axis: amplitude
Frequency spectra
accurately represent tone/sounds that don’t vary over time.
Spectrogram
the visual representation of a sound as it varies with time. Spectrograms are used extensively in the fields of music, linguistics, and speech processing
- For sounds that do change over time, we can plot frequency on the y-axis, time on the x-axis, and amplitude by the colour on the graph
Colour = intensity (red is more intense, blue is less)
Bands of acoustic energy on the spectrogram - formants
**it’s not just the timbre (energy at each harmonic) that allows us to differentiate between the different musical instruments - it’s also the attack and delay
Auditory Localization/Interaural time difference typical result:
the person sitting in the middle is best at locating the sound at + or - 90 degrees (directly to the left or right), and sometimes when someone claps in front of them, they would point to the person behind them.
When sound arrives at your ears at different times it allows
you to locate the sound in space
