copyright catherine m. burns 1 sound and noise chapter 5

Copyright Catherine M. Burns

1

SOUND and NOISE

Chapter 5

Copyright Catherine M. Burns 2

Anatomy of the Ear

Source: internet “earspin”

White: ear drum (tympanic membrane)

Beige: hammer and anvil

Yellow: cochlea

Blue/Yellow/white: semi-circular canals

Teal: auditory nerve


Tech Details!

Ear: pressure/force to electrical wave transducer interpretation in the brain

Sound basically air pressure waves frequency determines tone, vibrations per second

(Hz), "pitch" amplitude of pressure variation is intensity

tone

loudness


Intensity/Volume

really measured in pressure units (Pa) human range is from 20mPa to about 20, 000

Pa (one million times more) max idea is jet engine large range so use a log scale, decibel scale


The Decibel Scale

20mPa is reference. every increase of x10, is addition of 20dB Sound pressure level (dB) = 20 log (P/Pref) for

absolute measures where Pref = 20mPa So absolute sound intensity (dB) of P = 20

log (P/20mPa) See Table 5.1 for example sounds and their

volume


Pitch or Tone

Young people typically 16Hz to 20, 000 Hz about 9 octaves below 16Hz you feel as vibrations above 20 000 Hz is "ultrasonic", we can't hear


Loudness (versus intensity)

Loudness is the psychological experience of sound volume

Differs from intensity

Loudness

Intensity (of 1000HZ tone)

Figure 5.3

Key idea: Very loud sounds seem even louder


Loudness and Pitch

generally high pitched sounds sound louder most sensitive range is about 4000Hz dB(A) weights sounds by pitch to reflect

psychological loudness Human speech: vowels below 1000Hz,

consonants higher frequency


Equal Loudness Curves

Fletcher.H. and Munson.W., ``Loudness, its definition, measurement and calculation,'' J. Acoust. Soc. Am., vol. 5, pp. 82-108, 1933.


Masking

Sounds can be masked by other sounds Minimum intensity difference to ensure a sound is

heard is 15dB above the masking sound Sounds in the same frequency band are masked Low pitch sounds mask high pitched more than the

reverse.


Alarm Design

Auditory signals are used for alarms because they don’t require orientation to be heard

People can’t “close their ears” Auditory alarms should be reserved for

highly critical events, affecting multiple personnel (fire alarms)


Criteria for Alarm Design

must be heard over the background (15dB more minimum, usually 30dB is suggested)

cover different frequencies to avoid masking (chord alarms)

shouldn’t exceed 85-90dB (dangerous levels) avoid startling people not interfere with communications, other alarms be informative


Other Alarms

Voice Alarms can be confused with speech can be clearer in meaning

False Alarms people will ignore and distrust the alarm maybe even turn it off


Sound localization

Demo


Noise

unwanted sound generally “too loud”


Noise Induced Hearing Loss

slow progressive degeneration of cells in the inner ear

increases with intensity and repetition high frequency and intermittent is worse usually starts at 4000 Hz and moves to lower

frequencies How measured - "pure tone audiometry" -

progressively trying tones and adjust volume level


Basic Pure Tone Audiometry

500Hz 1000Hz 2000Hz 3000Hz

1. Calibrate to a test tone (about 1000) at lowest level person can here

2. Can you hear the tone


Temporary Hearing Loss

hearing returns to normal, temporary threshold shift

begins at 80-90dB, causes 8-10dB shift also affected by duration 100dB sound for 10min shifts 16dB, 100

minutes, 60 dB


Permanent Hearing Loss

Permanent Threshold Shift Extensive exposure to noise Often high frequencies (e.g. 4000 Hz)


Age Related Hearing Loss

worse men than women 50 years 10dB 60 years 25dB 70 years 35dB

High frequency losses


What life is like if you have hearing loss

Loss at 2000 Hz (2000Hz_9)

Normal Speech

Loss at 4000 Hz (4000Hz_9)

General loss due to middle ear infection (mild_hl)

Typical of noise exposure

Masking effects of noise


Noise Level Guidelines

ISO standards - considers anything above 90db(A) to be damaging

90dB must reduce noise, 85dB must provide ear protection Equivalence over duration Hours dB(A)

8 90 6 92 3 97 1.5 102 0.5 110


Physiological and Psychological Effects of Noise

impaired alertness disturbed sleep annoyance loss of communication


Physiological Effects

increased blood pressure accelerated heart rate contracted blood vessels on the skin slowed digestion increased muscular tension waking people from sleep - connect with circadian rhythms research is unsure whether people adapt to noise or become

increasingly sensitive about it


Designing to reduce noise

recommending ear protection designing quieter equipment designing buildings and surfaces that don't

propagate noise sound absorption enclosing the noise source acoustic tiles Designing to avoid masking


Ear Protection Solutions

Ear plugs - can reduce about 30dB Ear muffs - about 40dB Problems though - workers can't hear other

workers don't like wearing them sound reduction is somewhat isolating


Touch/Haptic

Alternative form of information (force feedback mouse)

Identification of shape, texture Alerting when sounds can’t be used (cell

phones that vibrate) Braille Could be used more powerfully


Kinesthetic Senses

Knowledge of where your limbs are Critical when doing tasks without looking

(e.g. touch typing, driving)


Vestibular Senses

Sense of acceleration (in the ear) Sense of turning and motion Key role in motion sickness, vertigo,

simulator sickness

copyright catherine m. burns 1 sound and noise chapter 5

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