How We Hear

1. How We Hear

2. Our Ears • Pathways of Sound • Transmission through bony structures • Through the ear canal • 3 divisions of the ear

3. Our Ears • 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

4. Our Ears • 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

6. Hearing Sounds • 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

7. Hearing Sounds • Psychophysics of Hearing • People react and interpret sounds differently • Sensation of tone depends on intensity, frequency and subjective feelings

8. Hearing Sounds • 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

9. Hearing Sounds • 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

10. Hearing Sounds • 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

11. Hearing sounds • Acoustic Events • Directional hearing • Difference in arrival times (phase difference) and intensities • 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 • 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

12. Hearing Sounds • Common difference in tone • With a frequency interval of 100Hz or more separates several tones, one hears an additional frequency • 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

13. Noise and its Effects • 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

14. Noise and its Effects • 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

15. Noise and its Effects • 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)

16. Noise and its Effects • 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

17. Noise and its Effects • 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

18. Noise and its Effects • 3 strategies to prevent NIHL • Avoid generation • Properly design machine parts, reduce rotational velocities, change the flow of air, replacing a noisy apparatus • Leave the Area • Impede transmission • Mufflers, encapsulate source, increase distance, sound absorbing medium

19. Noise and its Effects • 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

20. Noise and its Effects • 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)

21. Noise and its Effects • 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

22. Noise and its Effects • 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

23. Noise and its Effects • 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

24. Noise and its Effects • 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

25. Noise and its Effect • 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

26. Noise and its Effects • Components of speech communication • The message – clearest if in context and clear 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

27. Noise and its Effects • 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

28. Noise and its Effects • 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

29. Noise and its Effects • 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

30. Summary 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

31. Summary Noise influences us in many different ways Hearing protection devices and hearing aids help prolong and restore our hearing capabilities