Hearing

and other senses

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Sound

• Sound: sensed variations in air pressure

• Frequency: number of peaks that pass a point per second (Hz)

• Pitch

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Sound Measurement

• Range: p < 0.001 bar (normal breathing) p > 1000 bar (jet plane)

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Sound Pressure Level (SPL)

SPL = Lp = 20 log10 ( p/pr)

p = RMS sound pressure of target sound

pr = RMS sound pressure of reference sound

(e.g. 0.0002 bar)

SPL units: decibels (dB)

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Representative SPLs

Sound approximate SPL (dB)

Jet takeoff (at 200 ft) 125

Cutoff saw 105

Inside sports car (at 50 mph) 80

Near freeway (auto traffic) 65

Average residence 50

Soft whisper (at 5 ft) 30

Normal breathing 10

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The device on the left is a sound level meter and is primarily used for noise abatement activities and acoustical work such as determining noise control criteria for an occupancy or for ambient noise analysis and control. The device in the center is a sound level meter/noise dosimeter which accumulates, or logs noise exposure for an entire work shift. This instrument is primarily used for OSHA hearing conservation activities. The device on the right is a previous-generation sound level meter.

Sound Measurement Devices

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A, B, and C Scales

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Adapted from: (http://www.teleport.com/~veda/gallery.html)

The Ear

Hearing Anatomy & Physiology

PinnaAuditory

CanalTympanicMembrane

Ossicles

OvalWindow

CochleaAuditory

NerveAuditoryCortex

Malleus (hammer)Incus (anvil)Stapes (stirrup)LigamentsMuscles

Amplitude reductionPressure amplificationAttenuation reflex (protection, low frequency masking)

Cochlea

Stapes

RoundWindow

OvalWindow

Scala Vestibuli& Scala Media

Scala Tympani

Basilar MembraneOrgan of cortiHair cells

Helicotrema

High Frequency Low Frequency

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Auditory Experience

• Sound intensity/SPL Loudness

• Frequency Pitch

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Psychophysical Scaling

• loudness not directly proportional to intensity

• psychophysical perceived loudness) scales• Phons

• Equal loudness contours

• phons = dB @ 1000 Hz

• Sones• Relative subjective loudness

• 1 sone = 1000 Hz @ 40 dB

• 2 sones = sound judged twice as loud as 1 sone sound

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Sensitivity

Range of Hearing: 20 - 20,000 Hz

Highest Sensitivity: 1,000 - 3,000

Hz

Lowest Detectable Intensity: 0 dB

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Limits

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Discriminability

• Ability to distinguish between two simuli (e.g. sounds) • Frequency - Pitch• Intensity - Loudness• Spectrum• Phase (?)

• Just Noticable Difference (JND)• Least change in a stimulus or the least

difference between two stimuli that can be detected 50 % of time.

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Absolute Discrimination

Dimension (s) # of Levels

Intensity 4 - 5

Frequency 4 - 7

Duration 2 - 3

Intensity & Frequency 9

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Masking

• Sound A (masking sound) reduces sensitivity of ear to Sound B (masked sound).

• Raised threshold for B (masked threshold).

Masking by Pure Tones

Masking by Wideband Noise

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Reducing Masking

• Signal Control• Selection - distinction from noise• Intensity - above masked threshold

• Noise Control• Selection - distinct from signal• Intensity - reduce• Filter - alter spectrum to reduce masking

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Alarms

• Rationale for auditory alarms• sound omnidirectional• can’t “close” our ears

• Criteria for auditory alarms• must be above background sound• must not be above danger level• should not be overly startling (longer rise time)• should not interfere with other signals• should be informative

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Designing Auditory Alarms

• do task analysis

• stay within limits of absolute judgement

• capitalize on the dimensions• pitch• envelope• rhythm• timbre• design sound specifics

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False Alarms

• false alarms • loss of trust• disabling of alarms• missed signals

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Speech

• Example: Tenerife• bottom-up issues• top-down issues

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Masking Effects

• female voice more vulnerable

• consonant sounds (esp. s, ch) more susceptible to masking than vowels

• “fly to” vs “fly through”

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Measuring Speech Communication

• Bottom-Up: Articulation Index (AI)• signal-to-noise ratio

• speech db – noise db

• weighted across frequency bands

• Top-Down: Speech Intelligibility Level (SIL)• % items correctly heard

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Speech Distortions

• Examples• clipping (beginnings, ends of words)• reduced bandwidth• echoes• reverberations• low quality synthesized speech

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Temporary Hearing Loss

• Continuous noise leads to hearing loss

• Temporary threshold shift at 2 min (TTS2)

• 70 - 75 dBA : no TTS2

• 80 - 105 dBA: TTS2 proportional to exposure

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Permanent Hearing Loss

• Continuous noise may lead to permanent hearing loss

• Begins at 4000 Hz

• Generally restricted to 3000 - 6000 Hz

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Hearing Loss

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Noise - Induced Hearing Loss

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Noncontinuous Noise

• Impact Noise (e.g. drop forge)

• Impulse Noise (e.g. gunfire)

• Noncontinuous noise may lead to permanent hearing loss.

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OSHA Standards: Continuous Noise

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OSHA Standards: Impulse Noise

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Noise Dosage

• total (daily) dose = sum of partial doses

• Requirement: total dose < 1.00

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Example

Worker exposed to 90 dBA for 4 hours, 105 dBA for 30 minutes. Within dosage limits?

4 hr @ 90 dBA = 4 / 8 = 0.5

0.5 hr @ 105 dBA = 0.5 / 1 = 0.5

Total dosage = 0.5 + 0.5 = 1.0

Since 1.0 < 1.0, dosage is OK

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Physiological Effects

• Short Term Effects• Startle response

• Long Term Effects ( > 95 dBA, > 10 yrs)• Hypertension• Hypotension• Ulcers• Headaches• Irritability• Sleep disorders• etc.

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Performance Effects

• Increase confidence (increased misses)

• Attention funneling (missed info)

• Performance gaps

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Noise Control

Source Path Receiver Design Barriers Ear plugs

Maintenance Enclosures Ear muffs Mountings Baffles

Mufflers

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Hearing Protectors

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The Other Senses

• Touch• Tactile/Haptic Sense

• Proprioception• joint angles

• Kinesthesis• movement

• The Vestibular Senses• motion• acceleration• illusions of motion

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Tactual (Tactile) Displays

• Stimuli• mechanical• thermal• chemical• electrical

• Coding• shape• pattern• magnitude (pressure, vibration, size, displacement)

• Examples• braille• reading devices for blind• K-T display