Physics Of Sound Final

Front 1

Inverse-square law

Back 1

Intensity= Power/area= P/4πr2

Front 2

When does intensity fall off for sound

Back 2

as 1/r2

Front 3

when will the inverse-square law fail?

Back 3

when you have reflections, absorption, or background noise

Front 4

span of practical intensities

Back 4

1 pW/m2 to 1 W/m2

Front 5

Softest sound you can hear:
“Threshold of hearing”

Back 5

I0 (0 dB) = 0.000000000001 W/m2
(10−12 W/m2 or 1 picowatt per sq. meter)

Front 6

Sound loud enough to cause pain:
“Threshold of pain”

Back 6

1 W/m2 (120 dB)

Front 7

Response of most senses, including hearing, is....

Back 7

logarithmic. The incremental sensation is proportional to the ratio. Lighting 100 candles after 10 feels like the same change as lighting 10 candles after 1.

Front 8

intensity level=

Back 8

L=10 log (I/Isub0)

unit: dB

Front 9

range of sound

Back 9

0-120 dB

Front 10

why should you not use the full range of in music?

Back 10

Below some level will be inaudible for some, above some level will cause pain and permanent damage

Front 11

useable working range

Back 11

~60dB (from 40-100dB)

Front 12

standard dynamic levels
forte, piano, ff, pp

Back 12

ff (fortissimo) = 10−3 W/m2 == 90 dB
f (forte) = 10−4 W/m2 == 80 dB
p (piano) = 10−6 W/m2 == 60 dB
pp (pianissimo) = 10−7 W/m2 == 50 dB

Front 13

is intensity or pressure easier to measure?

Back 13

pressure

Front 14

sound pressure level (SPL) =

Back 14

Lp = 20 log (P/Psub0)

Front 15

how will pressure variation drop?

Back 15

1/r

Front 16

typical levels of sound sources: rock music, city traffic, whisper, shout, urban street, living room, rustling leaves

Back 16

110dB, 70, 20, 100, 80, 40, 10

Front 17

purposes of the pinna

Back 17

1. funnel and concentrate feeble signals
2.resonate at speech frequencies
3. multiple reflections cause interference which aid localization

Front 18

what do multiple reflections from the pinna produce?

Back 18

direction dependent fine structure in the spectrum reaching the eardrum

Front 19

What then is the reason for the barn owl’s excellent hearing acuity?

Back 19

An owl’s whole head acts like a pinna (it has a concave shape)

Front 20

purposes of the ear canal

Back 20

(1) channels sound to the ear drum (tympanic membrane)
(2) resonates and amplifies speech frequencies.

Front 21

what does the eardrum do?

Back 21

It converts air vibrations to solid vibrations. It is a resonator in reverse. Provides (some) impedance matching from air to a solid.

Front 22

what are the ossicles?
What do they do?

Back 22

three smallest bones in body. they do not grow

Transmit these vibrations to the oval window of the liquid filled cochlea. and once again resonate at speech frequencies.

Front 23

how does bone size reflect animal lifestyle?

Back 23

Small bones for bats that need to detect high frequencies; large bones for rats that need to hear low-freq sounds (slithering snakes, wings, etc.)

Front 24

Why transmit vibrations from one membrane to another through a series of bones? Why not simply expose the oval window to incident sound?

Back 24

Because of vastly different densities of air and water, there is an impedance mismatch and the transmission coefficient is only ~ 1%.
• Ossicles provide lever action (crowbar) giving a 1:1.3 mech. advantage. • Hydraulic amplification provides another 20-fold boost in pressure.
• Total pressure amplification = 26

Front 25

what is the acoustic reflex

Back 25

Muscle restraint of ossicle movements for protection against loud sounds.

Front 26

how does the acoustic reflex work

Back 26

1) Stirrup is pulled away from the oval window and
(2) the eardrum is tightened.

Front 27

what is the reaction time for acoustic reflex

Back 27

Reaction time ~ 1/100 s (too slow for gunshots). T

Front 28

what is one of the loudest sounds you are often exposed to?

Back 28

your own voice esp vowels. Hence muscles tug following intonation of syllables. Strangely this protection occurs even during subvocalization (“mind’s voice”).

Front 29

what causes hearing loss in the external ear regions?

Back 29

1. Water retention:
2. Wax buildup:
3. Tympanum rupture (caused by insertion of sharp things, pressure changes, disease, etc):

Front 30

what cause hearing loss in the middle ear?

Back 30

1. Blocking of the eustachian tube causes unequal inner and outer pressures. Pressure differences get trapped after sudden changes in elevation (flying) or depth (diving).
2. Fluid/infections in the middle ear: “tubes” can drain fluid. Prolonged fluid accumulation can dissolve the ossicles leading to permanent loss.
3. Arthritis of the ossicles can keep bones from moving freely.

Front 31

what are the two main subsystems of the inner ear

Back 31

1. vestibular system (govern balance)
2. cochlea (organ of hearing)

Front 32

evolution of vestibular system:

Back 32

provided balance and orientation in fish; hearing organ; 3 chambered cochlea found in mammals

Front 33

why are problems often shared between the cochlea and vestibular system?

Back 33

because fluids are shared between the two

Front 34

Threshold Basilar Membrane (BM) displacement?
and who discovered it

Back 34

Smallest detectable displacement
~ 100 fm=0.1 pm = 1/10,000 nm! Determined by Békésy in 1960
He received the Nobel prize in 1961

Front 35

Tonotopy

Back 35

Each frequency resonantly excites a certain point on the BM maximally

Front 36

tonotopy vibration max and mins

it is the reason for what?

Back 36

The vibration maximum is near the cochlear entrance (oval window) for highs and at the farthest end (helicotrema) for lowest frequencies. Maximum moves by ~4mm per octave. This motion is the reason for the logarithmic sensation scale.

Front 37

critical range

Back 37

tonotopy excites hair cells within a certain vicinity. It occupies 1.2mm spatially and 1/3 octave tonally.

Front 38

what causes dissonance of close notes?

Back 38

Frequencies within this critical range aren’t clearly separated

Front 39

which hair cells and thus frequencies are the first to go?

Back 39

hair cells near the entrance; high frequencies

Front 40

cochlear amplifier

Back 40

This active feedback system stimulates the OHCs to vibrate and resonantly amplify weak sounds while acting as an active cancellation system to reduce loud sounds through destructive interference so as to protect the IHCs.

Front 41

what provides basis for hearing test in infants?

Back 41

cochlear amplifier

Front 42

what can damage the cochlear amplifier

Back 42

high androgen levels during gestation

Front 43

official frequency range for a young healthy listener

Back 43

16 hz - 18kHz

Front 44

how are hearing tests conducted

Back 44

1. behavioral response to tones
2. auditory brainstem response
3. otoacoustic emmissions

Front 45

otoacoustic emissions

Back 45

the sounds outer hair cells emit

Front 46

what kind of otoacoustic emissions are there?

Back 46

both spontaneous and evoked

Front 47

the stronger the OAE, the...

Back 47

better the sensitivity and the better the protection of an individual’s hearing

Front 48

types of hearing loss in the inner ear

Back 48

conductive loss and sensorineural loss

Front 49

conductive loss

Back 49

problems in the external/middle ear that prevent sound from reaching the cochlea.

Front 50

sensorineural loss

Back 50

hearing loss from inner ear damage, i.e., hair cell loss. (Similar to retinal damage in the eyes.)

Front 51

loudness recruitment

Back 51

loss of outer hair cells reduces the dynamic range (afflicted Beethoven)

Front 52

how can excessive amplification through hearing aids accelerate hearing loss

Back 52

through further hair cell destruction

Front 53

presbyacusis

Back 53

gradual loss of high-frequency sensitivity with age

Front 54

tinnitus: what is it, how is it caused, what can aggregate it, what can alleviate it

Back 54

ringing in the ears; results from irritation of the nerves producing “false alarms”. Caffeine and NSAIDS (ibuprofen, aspirin, etc.) aggravate tinnitus. Nicotinic acid (e.g., Slow Niacin), Gingko Biloba, etc. can alleviate it

Front 55

how can caffeine damage the inner ear

Back 55

it builds up pressure in the inner ear

Front 56

what can alleviate nerve inframmation in inner ear

Back 56

certain steroidal preparations such as prednisone

Front 57

what can mitigate hearing loss after exposure of hair cells to free radicals and other toxic chemicals?

Back 57

magnesium and vitamins a,c,e

Front 58

foam and silicone earplugs

Back 58

18-31dB

disadvantage: too much high frequency absorption and get worn quickly

Front 59

musicians earplugs

Back 59

15 or 25 dB with flat response

Front 60

earmuffs

Back 60

22-33 dB; easy to put on, take off; can be worn with earplugs;

disadvantage: too much high frequency absorption, eventually cushions get worn

Front 61

active cancellation sets

Back 61

A microphone senses the incoming (low-frequency) sound and feeds back an inverted replica of this through built in speakers. Interference then cancels the incoming sound. This works well only at low frequencies. Excellent for flights. More affordable now.

Front 62

how to prevent hearing loss on motorcycles

Back 62

(1) Wear foam earplugs,
(2) insert balloons inside the helmet,
(3) use a helmet with a chin guard to break the wind to prevent helmet resonance from “wind on the chin”

Front 63

what is the job of the brain in hearing?

Back 63

The ear provides (continuous and messy) streams of signals broken down by frequency.
All kinds of information (freqs. present, locations, etc.) needs to be extracted from incoming sound.
Perceptual grouping of information needs to take place.

Front 64

what are the two very broad characteristics of sound that need to be determined?

Back 64

What (pattern recognition) and where (localization).

Front 65

What 4 characteristics are important for music?

Back 65

Frequency (pitch), intensity (loudness), timing (rhythm), timbre

Front 66

sensory inputs for computer vs body

Back 66

camera, microphone, mouse

retina, organ of corti, tactile touch sensors

Front 67

analog sensory signal

Back 67

Bent hairs allow positive potassium ions to enter raising the voltage

Front 68

what kind of cell is a haircell

Back 68

receptor cell

A receptor cell has a continuously variable voltage.
A neuron either fires or doesn’t.

Front 69

how many frequency channels of raw info do the inner hair cells provide

Back 69

4000

Front 70

how many auditory nerves are contacted by IHCs

Back 70

~8

Front 71

why are there multiple copies of each signal?

Back 71

The fast nerves are like using high ISO-sensitivity in analog and digital photography, which allows a greater (shutter) response speed.

Front 72

Why then would you want to use a lower ISO setting at all?

Back 72

There is a trade off between speed and signal-to-noise ratio.
Faster representations tend to be noisier and less clean.

Front 73

signals coming from the auditory nerves fibers are crude and weak. Where is the preconditioning and cleanup done?

Back 73

the cochlear nucleus

Front 74

how does the brain judge musical sounds?

Back 74

the brain needs to determine the frequency (to decide the pitch) and relative intensities between different frequencies (to get the timbre from the spectrum and harmonic amplitudes).

Front 75

why can't you compare frequencies with raw auditory nerve signals?

Back 75

you need a moving average which is the job of the bushy neuron in the cochlear nucleus

Front 76

what does the bushy neuron help decipher?

Back 76

pitch and spectrum

Front 77

one factor that governs timber (think of a piano)

Back 77

attack; the sharper the attack, the closer in time will all freq. components begin together

Front 78

how do octopus neurons measure attacks?

Back 78

through synchronicity
Their leaky cell membranes cause them to fire only when multiple frequencies arrive simultaneously as in a sound with a rapid “attack”.

Front 79

How can you encode intensity (loudness) if nerves fire with a fixed amplitude?

Back 79

Stellate cells in the cochlear nucleus encode intensity through their firing rates.

Front 80

how to determine localization

Back 80

elevation, azimuth, and distance cues

Front 81

elevation cues

Back 81

Multiple reflections from the pinna produce direction dependent structure (minima/maxima) in the spectrum, which is decoded in the DCN (dorsal cochlear nucleus) yielding elevation info.

Front 82

bipolar neurons

Back 82

a type of cell that fire when both inputs arrive simultaneously

Front 83

How will you design a circuit that can encode angle of arrival of sound (the azimuth) using a bank of bipolar neurons and nerve fibers of different lengths?

Back 83

having an array of bipoloar neurons with different nerve lengths going to the right and left cochleas will cause different ones to fire depending on which one receives the two signals simultaneously. Since the nerve lengths are different, the sound would have to reach the two ears at different times to compensate for the internal delay.

Front 84

what do these neurons encode?
bushy
stellate
octopus
pyramidal cells
bipolar cells

Back 84

frequency info
intensity info
synchronicity info
elevation
left-right time level differences

Front 85

• Reflection delays of ~5 to 35 ms:

Back 85

associated with the same location as the earliest (direct) sound (brain selectively ignores location of reflection): broadening & definition.

Front 86

• Delays longer than 80 ms:

Back 86

usually heard as separately located echoes.

Front 87

• Very early reflections (within a few ms):

Back 87

produce summative localization. Stereo sound relies on this. This causes smearing and spaciousness (intimacy for live sound, bad news for reproduced sound).

Front 88

chain of decaying reflections

Back 88

reverberation and ambiance

Front 89

why does the brain hide info

Back 89

so that you are not overwhelmed with info and can make timely decisions

Front 90

precedence effect

Back 90

hides irrelevant locations of reflections

Front 91

stream segmentation

Back 91

fuses associated harmonics together so that you hear one single tone.

Front 92

Franssen Effect

Back 92

Direction of sound is associated with the high-frequency components related to the onset rather than the low- frequency components even when the latter carry most of the energy.

Front 93

what does the franssen effect allow for?

Back 93

low-frequency integration and the use of monophonic (vs stereophonic) subwoofers in less critical applications (but not Hi Fi!)

Front 94

why is it important to sit on the midline between stereo speakers

Back 94

– to allow proper summative localization
Need sufficient distance from reflecting walls to provide ambience without smearing and blurring (3’ is minimum, >5’ is better).

Front 95

what will improve the overall imaging

Back 95

Stereo subwoofers improve overall imaging
(time alignment between subs and main speakers is critical to 1⁄2”).

Front 96

good qualities of a concert hall

Back 96

good mixture of direct and early reflected sound

• Avoid echoes, especially flutter echoes (mainly sides not floor-ceiling)
– use splaying and treatment.

Front 97

Why do we prefer early reflections/reverb from the side rather than the ceiling?

Front 98

what does reverb do?
reverb time for chamber music, mozart concerto, mahler symphony, church music

Back 98

extends sonic memory for complex pieces – needed to build up relationships. Desirable reverberation times: 1 sec. for chamber music, 1.5 s for a Mozart concerto; 2.25 for a Mahler symphony; much longer for church music for choir and organ (some cathedrals have ~7 sec reverb. time)

Front 99

direct sound for
very early reflection for
early sound for
strong reverb for
long reverb for

Back 99

localization
intimacy
definition
big sound
warmth

Front 100

the science of reverberation

Back 100

A sharp initial sound will ricochet off walls infinite times and produce and uniform isotropic reverberant sound field like smoke filling a room.

Front 101

outer hair cell provides
inner hair cell is like a
number of each

Back 101

active cancellation, amplification, and sharpening
microphone
4000 IHCs vs 12000 OHCs

Front 102

why is hearing so effective and impactive

Back 102

massive neural processing

Front 103

overall size of the organ of corti

Back 103

1/250th of an inch

Front 104

how does the organ of corti work?

Back 104

The basilar membrane vibrates the hair cells against the tectorial membrane, or "roof"

Front 105

difference between analog and digital binary signals?

Back 105

initial signals are analog voltages
final signals are binary digital signals

Front 106

describe the first step of the analog sensory signal

Back 106

bent hairs allow positive potassium ions to enter raising the voltage