Ch19 - technical audiology

what if we dont hear?

questions

• Why don’t we hear? (mechanisms)

• How to measure ? (audiometry)

• How to improve hearing? (hearing instruments

answer (technical devices)

• Audiometric equipment

• Hearing aids

• Cochlear implants

what are hearing thresholds?

0 dB HL (i.e., hearing level) is the normal hearing reference (frequency specific hearing threshold of normal human auditory system)

Deviations of this reference are called hearing threshold shifts

hearing loss → degradation of hearing sensitivity (often quantifies in terms of hearing threshold shift) BUT HTs dont give the whole picture (there is a loss of hearing ability in some dimension)

what are the categories of handicap?

• disease

• impairment

• disability

• handicap

what are hearing disorders (based on social classification)?

• hard to hear persons

• deafened persons

• deaf persons

info about hearing disabled population?

360 million people worldwide → disabling hearing loss ~5% of population

in FI → 740k with hearing degradation 14k new hearing devices fittings per year

occurrence increases with age (65 - 37%, 75 - 65%)

with aging population → 7fold increase in people in need of aural rehabilitiation

what are effects of hearing degradation?

• Early language acquisition

• Speech communication / Social impact

• Listening comfort

• Listening effort in communication

• Music perception

what are the classifications of impairments?

pure tone average (PTA) → common measure of hearing degradation

average of hearing threshold values at 500, 1000, 2000, 4000 Hz

• Mild: 20-40 dB HL

• Moderate: 40-70 dB HL

• Severe: 70-95 dB HL

• Profound: equal to or over 95 dB H

how is hearing loss related to dementia?

strong link between hearing loss and dementia

compared to a normal hearing person 

• mild x2

• moderate 3x

• severe 5x of developing dementia during next 10 years

risk decreased with hearing aid

what is the classification of impairment?

• Conductive hearing loss (External and middle ear problems)

• Sensorineural hearing loss (Inner ear and retrocochlear problems)

• Central hearing loss (Higher neural levels)

• Psychic hearing problems (No clear physiological reason)

how can conductive hearing loss originate?

• Blocked ear canal, tumor, or deformation

• Ear drum trauma

• Infection in the middle ear

• Mucous otitis media (glue ear)

• Otoclerosis (stiffening of ossicles)

• Malfunction of the eustachion tube

consequence → hearing threshold shift

how can sensorineural hearing loss originate?

• Excess noise exposure

• Age-related hearing loss (presbyacusis)

• Cancer, inborn hearing loss, head trauma

• Ototoxic substances

consequences

• Hearing threshold shift

• Decreased dynamic range

• Decreased frequency selectivity -> increased masking

• Tinnitus and hyperacusia

how can central hearing loss originate?

• Higher neural levels

• Problems in sound separation or speech analysis

• Slow vs. fast speech

• Problems in localization (spatial separation)

• Tinnitus

how can psychic hearing problems originate?

no clear physiological reason

what are the effects of hearing impairments (of age)?

which is the healthy and unhealthy one?

a) healthy organ of corti

b) unhealthy organ of corti

name these hair cell damage mechanisms?

what is recruitment?

abnormal growth of loudness: people with sensorineural (cochlear) hearing loss may not hear quiet sounds well, but loud sounds feel much louder, much faster, than in normal hearing

• Normal hearing: Loudness perception increases steadily with sound level.

• Conductive loss: Loudness grows the same way, but everything is shifted up—you need more volume everywhere.

• Cochlear (sensorineural) loss: Soft sounds are barely perceived, but above a threshold, loudness shoots up fast—loud sounds are almost as loud as with normal hearing. This steeper slope is recruitment.

example of sensorineural effects?

critical bands broaden → more energy to each critical band → increased masking effect (even 10-12dB)

decreased freq selectivity due to 

• IHC damage

• OHC partial damage

• OHC full damage

results in problems in sound source separation and speech intelligibility in noise/reverberation

speech communication problems 

largen signal to noise ratio needed

what is tinnitus?

sinusoidal tone, hum, broadband noise, pulsation

source can be at diff levels (basilar instability, neural phantom sound)

treatments available but no cure known (tinnitus masters, tinnitus retraining therapy TRT)

what is noise?

harmful or disturbing sound

harmfulness

• risk of hearing loss

disturbance

• eg. decrease in work efficiency

annoyance

• more subjective concept

noise can cause hearing loss, but there are other factors at play

• vibration

• smoking

• genetic effects

• often combined (more than their sum)

what are some ear protectors?

ear plugs

ear muffs

attenuation

what is pure tone audiometry?

main hearing test used to identify hearing threshold levels of an individual, determining of the degree, type and configuration of a hearing loss and thus providing a basis for diagnosis and management

audiometer - an electronic device used to measure a person's hearing ability by testing their hearing thresholds for different frequencies and volumes (patient indicates when they can hear them, with the results recorded on an audiogram)

calibrated headphones

what is speech audiometry?

testing of speech intellligibility (individuals ability to detect and understand spoken words)

signal: words or sentences → in silence or with background noise (masker)

measurements

• speech recognition threshold (SRT) → the minimum level at which speech can be correctly understood in noise

• percent-intelligibility

what is sound field audiometry?

Sound field audiometry is a procedure to assess the hearing sensitivity of a person in which acoustic signals are presented through one or more sound sources in a room (i.e. not through earphones)

overcomes problem with headphones

• Acoustic coupling btw headphone and ear is somewhat unpredictable

• Hearing aids dont generally have microphones in ear canal

• Listening scenario is more natural

• Spatial aspects of sound

• Real-world representative results?

however → more expensive and complex

what does this graph show?

SFA tests are typically used to evaluate hearing in situations that mimic real-life listening environments, especially for people who use hearing aids or cochlear implants

The test involves playing two types of audio: a "signal" (usually speech or a tone) and "noise" (background masking sound) (This is a competing sound, usually steady-state noise (like white noise or speech-shaped noise), used to simulate difficult listening conditions, making it harder to identify the signal)

what is ear drum impedance measurement?

Impedance audiometry is a quick, painless test that measures how well your eardrum and middle ear bones respond to sound and pressure changes

auditory dysfunction and diagnostics

what are the hearing aid types?

(a) behind-the-ear (BTE) hearing aid

(b) in-the-ear (ITE) hearing aid

(c) completely-in-the-canal (CIC) hearing aid

how is signal processing done in hearing aids?

Match the device with the individual needs of the user

Different processing in each frequency band

• Amplification

• Compression

• Limiting

Noise suppression

gain control graph shown

what are the two types of hearing aid control?

feedback control → input from the microphone is amplified by a variable amplifier, then sent to the earphone

feedforward control → input from the microphone is first analyzed by the gain control system, which determines the required amplification before it reaches the variable amplifier

what are the hearing aid output waveforms?

limited output → limiter prevents the signal from exceeding a set maximum amplitude. When the input signal surpasses this threshold (around "b"), the waveform is "clipped" at a certain level. The highest peaks are flattened.

compressor output → compressor reduces the difference between the quietest and loudest parts of the signal, but less aggressively than a limiter

what is a bone anchored hearing aid?

based on bone conduction. It is primarily suited for people who have

conductive hearing losses,

unilateral hearing loss,

single-sided deafness

and people with mixed hearing losses who cannot otherwise wear 'in the ear' or 'behind the ear' hearing aids

This type of hearing solution bypasses problems in the outer or middle ear by sending vibrations via the skull bone directly to the inner ear (bone conduction path)

Surgically implanted into the bone behind the ear

what are hearing aids for middle ear implants?

transducer is attached to the auditory ossicles or to the round window that vibrates the structures in the same manner that air-conducted sound would
One approach is to use an external sound processor and an inductive link, similarly to cochlear implants. The microphone can also be implanted under the skin or placed in the ear canal

other features in hearing aids?

Directional microphones → Fixed or adaptive beam

Noise cancellation

Wind noise cancellation

Feedback cancellation

Speech enhancement, blind source separation Binaural processing

Hearing aid + FM-transmitter

Pre-set modes for different situations

what is a cochlear implant?

an electronic device that stimulates the auditory nerve through electrodes placed in the cochlea of the inner ear, allowing some severely deaf people to perceive sounds.

600k units worldwide

for severe to profound hearing loss (for children optibally bilateral CI before language acquisition)

It consists of an external component with a microphone and processor, and an internal, surgically implanted component with a receiver and an electrode array that sends electrical impulses to the hearing nerve. Unlike a hearing aid, which amplifies sound, a cochlear implant converts sound into electrical signals to be interpreted by the brain

what is the hearing performance in cochlear implant?

Sound quality is significantly degraded

High individual variations on hearing performance Phone conversation usually OK

Bilateral implantation gives some spatial hearing

• ILD’s are available

• ITD’s are not generally available

• Envelope ITD’s may be used

• Better speech intelligibility in noise
  

what is sound processing in cochlear implants?

Continuous interleaved sampling (CIS)

• Division to frequency bands

• Amplitude envelope extraction

• Compression and low-pass filtering

• Each channel is used to modulate a pulse train (One pulse train signal per electrode contact)
  

Pulses are interleaved in time

• Minimum interference between channels

• Unfortunately: reduction of temporal information (time-related aspects of a sound, such as its timing, rhythm, and changes in amplitude and frequency over time)

how does a cochlear implant look like?

Microphone	Picks up environmental sound
Sound Processor	Analyzes and codes sound into a digital signal
Transmitter Coil	Sends signal wirelessly across the skin
Receiver/Stimulator	Implanted, converts digital signal to electric pulses
Electrode Array	Delivers pulses directly to cochlear nerve