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How We Hear. Our Ears. Pathways of Sound Transmission through bony structures Through the ear canal 3 divisions of the ear. Our Ears. Outer Ear Auricle and external auditory meatus Sound waves travel in - PowerPoint PPT Presentation
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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
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 Events1. Directional hearing Difference in arrival times (phase difference) and
intensities2. 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
4. Common difference in tone• With a frequency interval of 100Hz or more
separates several tones, one hears an additional frequency
5. Concurrent tones• 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, 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 NIHL1. Avoid generation Properly design machine parts, reduce
rotational velocities, change the flow of air, replacing a noisy apparatus
2. Leave the Area3. 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
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)
+ 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 communication1. The message – clearest if in context and clear
wording is used2. The speaker – speak slowly, using common
vocabulary3. Message transmission – system that causes
little distortion of frequency, amplitude or time
4. The environment – noise affects listener’s ability to receive the message
5. The listener
Design of warning signals• Must penetrate sound; use frequencies
below 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