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