Download How We Hear

 Pathways
of Sound
• Transmission through bony structures
• Through the ear canal
3
divisions of the ear
 Outer Ear
• Auricle and external auditory meatus
• Sound waves travel in
• Resonance effects amplify the intensity of the sound
by 10-15 dB by the time it reaches the eardrum
 Middle ear
• Separated from the external ear by the tympanic
membrane
• 3 ossicles that transmit sound: malleus, incus, stapes
• Oval window and round window
• Eustachian tube – connects middle ear to the
pharynx and allows for pressure equalization
 Inner Ear
• Vestibule, cochlea and semicircular canals
• Sound vibrations create shifts within the perilymph and
endolymph
• Fluid motion deforms the basilar membrane (cochlea)
• Stimulates the organ of corti
• Nerve impulses are generated and transmitted to the brain
via the auditory nerve
 Sound
• Vibration that stimulates the auditory sensation
• Contains a mixture of frequencies (Hz)
• Tone – single frequency oscillation
• Pitch – personal perception of tone frequencies
 Frequencies we hear
• 16Hz – 20 kHz
• Infrasonic vs ultrasonic
• With aging, the max frequency diminishes to 10kHz
• Hearing is most sensitive between 2-5kHz
• Most speech occurs between 300—700 Hz
 Psychophysics
of Hearing
• People react and interpret sounds differently
• Sensation of tone depends on intensity,
frequency and subjective feelings
 Loudness
• Affected by both frequency and intensity
• At lower frequencies, the sound pressure must
be increased to achieve equal loudness
 Ex. 50 Hz tone must have 75 db to sound as loud as
1000 Hz with 50 db
• At high frequencies, tone intensity can be
lowered to achieve equal loudness
• Figure 6.4 Phon Curves
 Only for pure tones, not if we hear different
frequencies at different times
 Responses
to music
• Little is known about psychosocial responses of
music on well being and productivity
• Music while you work is meant to break up the
monotony and generate excitement towards an
activity
 Consider timing, varying rhythms and vocals, music
popularity
 Improved morale and activity; no clear scientific
connection
 Muzak
• Background music
• Creates a welcoming atmosphere, relaxes
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customers, reduces boredom, masks disturbing
sounds
Subdued, intermediate tempo, vocals are
avoided
Workers – monotonous; customer – pleasant
Choosing music for specific activities,
environments and populations is an art
Market ploy
 Acoustic Events
1. Directional hearing

Difference in arrival times (phase difference) and
intensities
2. Distance hearing

Sound energy diminishes with the square of the distance
travelled

Human perception depends on Frequency

More distant with low intensity and low frequency
3. Doppler effect

As the distance between the source of the sound and the
ear decreases, one hears an increasing higher frequency

Larger the velocity, the more pronounced the shift in
frequency
Common difference in tone
4.
•
With a frequency interval of 100Hz or more
separates several tones, one hears an additional
frequency
Concurrent tones
5.
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When 2 tones of the same frequency are played at
the same time, they are heard as a single tone
Loudness equals the sum of the 2 tones
Destructive interference -2 tones played in
opposite phases cancel each other out; cannot be
heard
 Noise
• Unwanted or objectionable sound
• Psychological and subjective
• Many sources
 What noise can do
• Create negative emotions, surprise, frustration, fear,
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etc.
Delay, disturb or awaken a person from sleep
Drown out desirable sounds
Produce alterations in body chemistry
Interfere with human sensory and perceptual
capabilities
Change hearing capabilities
 Permanent Threshold Shift (PTS)
• Exposure to intense sound resulting in permanent
hearing loss
• Damage to the middle ear ossicles, organs of Corti,
or acoustic nerve (frequency and intensity)
 Temporary Threshold Shift (TTS)
• Exposure to a less acute sound resulting in a
temporary loss of hearing
 Severity
depends on duration,
characteristics of the sound, nature of
exposure
• Victim may not be aware of incurring injury
 Task performance
• Depends on job
 Simple, repetitive tasks – little impairment
 Difficult tasks – degrades execution
• Unexpected and irregular noise has a more
negative effect
 Signal to noise ratio
• Noise interference with spoken communication
• Workers in loud environments
• There must be a difference in speech intensity
(signal) and noise (S/N)
 Shouting
in Noise
• Lombard reflex – tendency to raise one’s voice to
speak over noise
• Males vs females
 Quiet environment – men 58 dBA, women 55-56 dBA
 Loud environment – men76 dBA, women 68-71 dBA
 Shouting – men 89 dBA, women 82-84 dBA
• S/N ratio is hard to adjust over 70 dB
• At extreme outputs, articulation becomes
distorted

Noise induced hearing loss
• Occurs around 4000 Hz
• Also reduced with aging
 10 dB at 50 years
 25 dB at 60 years
 35 dB at 70 years

Sounds that damage
• Sounds above 85 dBA are hazardous
• Magnitude of loss relates directly to the sound level
• US regulations
 16 hours of 85 dBA
 8 hours of 90 dBA
 4 hours 95 dBA
• Indicators of dangerous sound environments
 Louder than conversational level, difficult to communicate,
tinnitus, muffled sounds after leaving noisy area
3
strategies to prevent NIHL
1. Avoid generation
 Properly design machine parts, reduce rotational
velocities, change the flow of air, replacing a noisy
apparatus
2. Leave the Area
3. Impede transmission
 Mufflers, encapsulate source, increase distance,
sound absorbing medium
 Planning
for no noise
• Select technologies and sounds that produce
acceptable sound levels
• Certain machines and jobs are inherently noisy;
prevent noise propagation
• Architect – locates offices away from noise
• Factory – intervening spaces between
machinery and workers if possible
 Noise
barriers
• Best way to reduce propagation is to enclose the
source
 Trees and bushes
 Buildings reduce sound by 20 – 30 dB (Table 6.1)
 Hearing
protection devices (HPD)
• Helmets, earmuffs, earplugs
• Varying effectiveness
 Passive
HPDs
• Sound passes through material that absorbs,
dissipates and impedes energy flow
• Highly protective if worn properly
• Attenuate high frequency more than low
frequency, speech is distorted
 Plugs and Muffs
• 500 – 2000 Hz earmuffs are more effective
• Proper fitting and use influence effectiveness
• Muffs are easier to fit but more uncomfortable
in hot environments
• Tendency to lower one’s voice due to bone
conduction amplification
 Active HPDs
• Attenuation qualities
can be tailored to
the prevailing noise
levels, job demands
and users’ hearing
abilities
• Use destructive
interference
• Works well below
1000Hz
 Voice communications
• Intelligibility - Ability to understand the
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meanings of words, phrases, sentences and
speech
75% intelligibility is required for satisfactory
communication
Direct communication – visual cues
Indirect – distance, background noise level,
voice level
Air pressure and composition affect efficiency
and frequency of voice transmission
 Intelligibility
• Intensity of speech relative to noise is a basic
determinant
• S/N ratio (difference)
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+ 10 dB or greater, 80%
5 dB, 70%
0dB, 50%
-5 dB, 25%
• Frequencies 200 -8000Hz are important in voice
communication
• Consonants are more critical for understanding than
vowels
 Have higher frequencies and less energy and more
masked by noise
 Components
of speech communication
1. The message – clearest if in context and clear
2.
3.
4.
5.
wording is used
The speaker – speak slowly, using common
vocabulary
Message transmission – system that causes
little distortion of frequency, amplitude or time
The environment – noise affects listener’s
ability to receive the message
The listener
 Design
of warning signals
• Must penetrate sound; use frequencies below
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500 Hz
Low frequencies diffract easily around barriers
Within the range 1000-4000 Hz
Intensity should be15 dB above masking noise
Auditory signals can be combined with
indicators appealing to different senses
 Improving
defective hearing
• Modern digital hearing aids
 Amplify sound, filter out background noise and make
the sounds clearer
 Behind the ear vs Ear canal
 Adjustments
• Microphone adjustments for different
environments
• Settings for the left and right ear
 Surgical implants
• Bone anchored hearing aids
 Single sided deafness
 Transmitter picks up sound and conducts it to the good ear
• Middle ear implants
 Mild to moderate hearing loss
 Attach to the ossicles and amplify sounds
 Part behind the ear houses a microphone
• Cochlear implants
 Severe hearing loss
 Convert sound into nerve impulses to be transmitted to the
brain
 Transmitter under the skin and behind the ear with
electrodes implanted inside the cochlea
 Ears
provide necessary information for
everyday life
 Sound is relayed as a combination of
different frequencies and intensities
changing over time
 Information is interpreted based on
individual experiences and hearing
capabilities
 Noise
influences us in many different
ways
 Hearing protection devices and hearing
aids help prolong and restore our
hearing capabilities