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Objectives 2  List the four main parts of the auditory system and the two main parts of the outer ear  Label a diagram of the pinna with the parts discussed in class  List the main functions of the outer ear  Describe how standing waves in the ear canal create amplification at certain frequencies  Explain how pinna cues occur and the frequency ranges over which they occur  Define head-related transfer function and describe what it shows  Describe and differentiate between ITDs and ILDs  Label a diagram of the tympanic membrane with the parts discussed in class  Name the three ossicles in order from lateral to medial

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The Outer Ear & Middle Ear 4

The outer ear & interaction of sound with the body I. Anatomy A. Pinna B. External Auditory Canal II. Physiology A. Sound amplification B. Directional function of the ear 1. Pinna cues (elevation & front/rear) 2. ITD and ILD C. Protection 5

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THE PINNA Anatomy 7

Anatomy of the pinna Helix Anti-helix Tragus Anti-Tragus Scaphoid Fossa Triangular Fossa Concha Lobule 8

THE EXTERNAL AUDITORY CANAL Anatomy 9

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The ear canal The ear canal is more technically called the external auditory canal. In adults, this canal is about 2.5 cm long. The canal forms an S-shaped curve ending at the tympanic membrane. 12

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Cartilagenous portion has hair and glands that make ear wax (cerumen). 14

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Memorize your anatomy Know the pinna diagram used in class Know the two sections of the ear canal, its shape, and which part produces cerumen 16

PROTECTION Physiology 17

Ear protection function The outer ear protects the tympanic membrane and the middle ear from physical damage and abuse because it is narrow and curvy. Dust and other very small objects that enter the ear are trapped by the small hairs in cartilagenous part of the ear canal. 18

Ear protection function Ear wax (cerumen) provides a lubricating protective layer for the skin. Why have cerumen? prevents water loss repels bugs antibacterial properties 19

SOUND AMPLIFICATION Physiology 20

The pinna collects sound Sounds from the front are collected more than sounds from the back. You can increase how much sounds gets into your ear by about 8 dB simply by cupping your ear 21

Resonance cavities of outer ear A resonance cavity is an enclosed space that amplifies sound. The main resonance cavity of the ear is the ear canal. The pinna also has some resonance cavities in it that amplify sounds depending on frequency and location. (More on this in a few minutes). 22

Tympanic membrane EAM is like a tube open on one end and closed on the other Pressure Node (air is at rest)

Sound reflects off the tympanic membrane (closed end) Reflects because of encountering higher acoustic impedance

Pressure waves in the EAM Keep in mind that we are dealing with sound waves here—alternating sequences of relatively lower and higher air pressure. So it really looks something more like THIS: compression (higher pressure)

Pressure waves in the EAM And the sound wave is moving over time, so it might look like this one moment….

Pressure waves in the EAM And this the next.

Interference Incident and reflected waves could interfere constructively or destructively. (Changes moment by moment) This would be an example of destructive interference because the sound pressure is no longer changing very much over time.

Interference Incident and reflected waves could interfere constructively or destructively. (Changes moment by moment) This would be an example of constructive interference because the sound pressure changes even MORE over time. Note that both the compressions and the rarefactions line up.

Because the EAM is like a tube open at one end and closed that the other, certain frequencies create standing waves Resonances of the EAM

Standing Wave

For the EAM, these frequencies are any with a pressure anti-node at the tympanic membrane Resonances of the EAM

The smallest possible distance between a pressure node and anti- node is ¼ of the wavelength of the sound. pressure

Resonances of the EAM ¼ wavelength The smallest possible distance between a pressure node and anti- node is ¼ of the wavelength of the sound. pressure

Ear canal as a resonator The ear canal is like a tube that is open on one end (concha) and closed on the other (tympanic membrane). Therefore it has resonances based on the equation: where f n is a resonance (first, second, etc.). c is the speed of sound. l is the length of the tube. The effective length of the ear canal is 3 cm or 0.03 m (with end correction) 35

Ear canal as a resonator 36 The lowest resonance frequency can be estimated: Because the lowest resonance frequency has a wavelength 4 times the length of the tube (see denominator of equation), we call this a ¼ wave resonator.

The ear canal varies somewhat in shape and size, has several bends, and is covered with soft tissue, so it does not always resonate at exactly this frequency, but it is a pretty good estimate for most people. We usually give a range of Hz for the frequencies most amplified by this resonance in humans. Ear canal as a resonator 37

What is the main resonance cavity of the ear? A.Pinna B.Ear canal C.Middle ear 38

ACOUSTICS REVIEW: REFLECTION OFF AND DIFFRACTION AROUND OBJECTS 39

Reflection Any sound energy that bounces off the next medium is reflected. θ θ 40

Reflection Any sound energy that bounces off the next medium is reflected. θ θ 41

Diffraction AROUND an object occurs when the wavelength of the sound (distance a sound travels in one cycle or distance a sound travels during one period) is longer than the object 42

Diffraction AROUND an object occurs when the wavelength of the sound (distance a sound travels in one cycle or distance a sound travels during one period) is longer than the object 43

A sound shadow forms when the wavelength of the sound is smaller than the dimensions of the object. 44

A sound shadow forms when the wavelength of the sound is smaller than the dimensions of the object. 45

DIRECTIONAL FUNCTION OF THE EAR Physiology 46

Pinna cues Sounds in front are collected by the pinna more than sounds in back—this helps us localize a sound in the front/rear direction. Pinna has resonance cavities that amplify sounds depending on frequency and elevation. These are called pinna cues and are MONAURAL CUES because you only need one ear (pinna + rest of the system) to do this. 47

Reflections off pinna help with localization in elevation 48

Direct and reflected waves How can the direct and reflected waves differ? 1. Amplitude: The direct wave should be slightly more intense. 2. Phase: The direct wave should arrive at an earlier phase because it has to travel a shorter distance. 3. Timing: The direct wave should arrive at a slightly earlier time. EAM direct path: earlier phase reflected path: later phase EAM 49

Pinna cues Direct paths are shorter, leading to earlier phase. Reflected paths are longer, leading to later phase. Both direct and reflected sounds will be in the ear canal at the same time. Therefore, they will interfere with one another. Some frequencies will be amplified and others attenuated depending on the phase difference between the two waves (constructive vs. destructive interference). 50

Pinna cues Path length will change depending on the elevation of the sound, so different frequencies will be amplified or attenuated when sound comes from different locations. 51

Pinna cues General trends based on the wavelength (and therefore frequency) of the sound. Few pinna cues below 1 kHz due to diffraction Primarily constructive interference between 1 and 6 kHz—serves amplification function, but not really a localization function because the result is the same at all elevations. Either constructive or destructive interference can occur above 6 kHz depending on frequency, elevation and individual’s ear shape. 52

Pinna notch Pinna notch: frequency at which sound level is significantly reduced due to destructive interference Theoretically, interference is completely destructive (can be the case) For some frequencies and elevations, other aspects of external ear resonance will cause there to be an overall dip in level but not a complete obliteration of the sound at the pinna notch. 53

Pinna cues Example of constructive and destructive interference for this individual with sound presented directly in front. Note the pinna notch at 10 kHz. 54

Pinna cues Example of various amounts of constructive interference for this individual with sound presented directly above. Note the pinna notch at 9 kHz. 55

Head-related transfer function (for one “average” individual) elevation Pinna cues ¼ wave resonance pinna notch amplifications due to pinna reflections 56

What type of localization does the pinna help with? A.Front/rear localization B.Left/right (horizontal) localization C.Elevation (vertical) localization D.A and C E.B and C F.All of the above 57

Horizontal localization Not exactly an “outer ear” function, but involves the interaction of sound with the body. Two mechanisms (both involving comparisons between ears) Interaural level or intensity differences Interaural time differences Onset interaural time difference (onset ITD) Interaural phase difference (IPD) These are called BINAURAL CUES because you need two working ears to do this. 58

Intensity differences between the ears result in interaural level or intensity difference (ILD) cues. For example: Sound arriving from the left side will be louder in the left ear. Horizontal localization 59

Interaural Level or intensity Difference (ILD) BINAURAL CUE 60

Head shadow (ILD) for sounds above 1600 Hz (when the wavelength is much smaller than the head) Partial head shadow (ILD) for sounds Hz (when the wavelength is about the same as the size of the head) No head shadow when sounds are less than 800 Hz (wavelength much bigger than head) 61

Horizontal localization Timing differences between ears result in interaural time difference (ITD) cues. Onset ITD is a difference in STARTING time between two ears and only works if you heard the sound start. Interaural phase differences (IPD)—the sound will arrive at a later phase at the farther ear 62

Onset interaural time difference (onset ITD) and interaural phase difference (IPD) arrives at a later time and phase BINAURAL CUE 63

ambiguous cue if wavelength smaller than distance between ears Meaningful cue if λ > size of head (1.6 kHz or lower) Only cue for if λ > twice the size of the head (0.8 kHz or lower) 64 Interaural phase difference (IPD)

What do the binaural cues (ITD and ILD) help with? A.Front/rear localization B.Left/right (horizontal) localization C.Elevation (vertical) localization D.A and C E.B and C F.All of the above 65

There is no sound shadow for frequencies below 800 Hz because... A.The sound reflects off the head B.The sound diffracts around the head C.The sound is too slow D.The phase difference is uninterpretable 66

Why are interaural phase differences uninterpretable above 1600 Hz? A.The phase difference between ears becomes too small. B.There is no sound shadow. C.There’s no way for our brain to know how many cycles of the sound passed as it travelled from one ear to the other 67

The middle ear I. Anatomy A. Tympanic membrane B. Ossicles and middle ear space C. Other key structures II. Physiology A. Sound transmission/ Impedance transformation B. Pressure Equalization C. Protection 68

THE TYMPANIC MEMBRANE Anatomy 69

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tympanic ring *The pars tensa is everything that isn’t the pars flaccida. The tympanic membrane 71

The tympanic membrane Attaches to malleus, which is first middle ear ossicle. Two landmarks: Manubrium (arm) of malleus can be seen from center to top of tympanic membrane Umbo is end of manubrium of malleus and tip of cone Tympanic annulus attaches tympanic membrane to bony wall of ear canal. 72

tympanic annulus *The pars tensa is everything that isn’t the pars flaccida. The tympanic membrane 73

The tympanic membrane Cone of light is the reflection of an otoscope light. You can tell which ear it is based on the position of the cone of light. Right ear: cone of light on your right side (3 o’clock with manubrium) Left ear: cone of light on your left side (9 o’clock with manubrium) 74

Which ear is this? A.Left B.Right 75

OSSICLES AND MIDDLE EAR SPACE Anatomy 76

Middle ear space and ossicles The middle ear is an air-filled cavity called the tympanic cavity. The tympanic cavity contains three bones called the ossicles: Malleus Incus Stapes 77

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What is the smallest bone in the human body? A.Malleus B.Incus C.Stapes 82

OTHER KEY STRUCTURES Anatomy 83

Other key structures Eustachian tube on front/anterior wall Middle ear muscles Tensor tympani enters middle ear space from anterior wall and attaches to the malleus Stapedius muscle enters middle ear space from posterior wall and attaches to the stapes 84

IMPEDANCE MATCHING Physiology 85

Impedance mismatch between TM and inner ear Low impedance tympanic membrane High impedance oval window 86

Impedance mismatch Most energy will be reflected if sound encounters a boundary with higher impedance— 99.9% of sound energy in this case, dB loss Higher impedance means you need higher energy to allow sound to continue propagating Impedance matching is the idea that you can increase energy (pressure) in order to allow sound transmission to continue. 87

Impedance matching mechanisms of the middle ear 1. Area transformer *most important* 2. Ossicular lever transformer 3. Catenary lever transformer 88

Area transformer 89

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Larger Area Same Force Lower Pressure Larger Area Same Force Lower Pressure Smaller Area Same Force Higher Pressure tympanic membrane stapes footplate AREA TRANSFORMER Increases pressure by decreasing area at the stapes footplate. 91

Larger Area Same Force Lower Pressure Larger Area Same Force Lower Pressure pressure = force ÷ area Smaller Area Same Force Higher Pressure 92

Larger Area Same Force Lower Pressure Larger Area Same Force Lower Pressure pressure1 = force ÷ 55mm 2 pressure2 = force ÷ 3.2mm 2 Smaller Area Same Force Higher Pressure 93

Larger Area Same Force Lower Pressure Larger Area Same Force Lower Pressure pressure1 x 55mm 2 = force pressure2 x 3.2mm 2 = force Smaller Area Same Force Higher Pressure 94

Larger Area Same Force Lower Pressure Larger Area Same Force Lower Pressure pressure1 x 55mm 2 = pressure2 x 3.2mm 2 Smaller Area Same Force Higher Pressure 95

Larger Area Same Force Lower Pressure Larger Area Same Force Lower Pressure pressure1 x 55mm 2 ÷ 3.2mm 2 = pressure2 Smaller Area Same Force Higher Pressure 96

Larger Area Same Force Lower Pressure Larger Area Same Force Lower Pressure pressure1 x 17 = pressure2 Smaller Area Same Force Higher Pressure 97

True or false: The tympanic membrane has a larger area and vibrates with greater force than the oval window/stapes footplate. A.True B.False 98

Impedance matching mechanisms of the middle ear 1. Area transformer *most important* 2. Ossicular lever transformer 3. Catenary lever transformer 99

The ossicular lever Increases pressure by increasing force at the stapes footplate. 100

Two sides of a lever are like two sides of an equation Force X Displacement = Equal amount of work done on the two sides Work = Force x Displacement 101

Force X Displacement = displacement 2 Force can be imagined as what is needed to press one side down or the force with which the opposite side moves up. force 1 displacement 1 force 2 102

Force X Displacement = force 1 displacement 1 displacement 2 force 2 103

Force X Displacement = force 1 displacement 1 force 2 displacement 2 104

Force X Displacement = 105

Force X Displacement = displacement 2 force 1 displacement 1 force 2 106

Force X Displacement = displacement 1 force 2 displacement 2 force 1 107

Force X Displacement = force 1 displacement 1force 2 displacement 2 108

manubrium of malleus long process of incus force 1 displacement 1force 2 displacement 2 Ossicular lever 109

manubrium of malleus long process of incus force 1 force 2 displacement 2 displacement 1 110

force 1 displacement 1 manubrium of malleus long process of incus force 2 displacement 2 111

force 1 displacement 1 manubrium of malleus long process of incus force 2 displacement 2 112

manubrium of malleus long process of incus stapes tympanic membrane 113

Because the manubrium of the malleus is longer than the long process of the incus… A.The manubrium of the malleus moves more/faster, but the long process of the incus moves with more force. B.The manubrium of the malleus moves with more force, but the long process of the incus moves more/faster. 114

Impedance matching mechanisms of the middle ear 1. Area transformer *most important* 2. Ossicular lever transformer 3. Catenary lever transformer 115

Catenary lever transformer The curvature of the TM causes internal tensions that increase the displacement of curved membranes and decrease the displacement of the manubrium. Increases pressure by decreasing displacement and thus increasing force at the manubrium of the malleus. 116

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Let’s simplify our lever for a minute Force X Displacement = Equal amount of work done on the two sides Work = Force x Displacement Keeping work constant, if displacement decreases, force increases 118

Force X Displacement = force 1 displacement 1force 2 displacement 2 Catenary lever transformer 119

outer edges of TM manubrium of malleus force 1 displacement 1force 2 displacement 2 Catenary lever transformer 120

outer edges of TM manubrium of malleus force 1 displacement 1 force 2 displacement 2 Catenary lever transformer 121

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The catenary lever transformer occurs because… A.The tympanic membrane vibrates differently at different frequencies B.The tympanic membrane vibrates more on the edges of the pars tensa than at the umbo C.The tympanic membrane vibrates more in the pars tensa than in the pars flaccida D. The tympanic membrane vibrates more in response to louder sounds 123

Impedance matching mechanisms of the middle ear 1. Area transformer *most important* Pressure ratio of 17:1 (24.6 dB) 2. Ossicular lever transformer Pressure ratio of 1.3 :1 (2.3 dB) 3. Catenary lever transformer Pressure ratio of 2:1 (6 dB) Total advantage ratio= approx. 44.2:1 at 1000 Hz Equivalent to about 33 dB 124

Impedance transformation Together, the middle ear impedance matching mechanisms result in an increase in sound pressure of 33 dB. This is very close to the pressure loss from the outer to inner ear (35-40 dB). In reality, somewhat lower gain: 125

Sound transmission through outer and middle ear The outer ear resonances, middle ear impedance matching, and some other factors work together to most effectively transmit sounds between about 500 and 6000 Hz. 126

Threshold of hearing 127

Threshold of hearing 128

PRESSURE EQUALIZATION Physiology 129

Pressure equalization 130

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How does the Eustachian tube open? Yawning and swallowing Tensor veli palatini and levator veli palatini muscles open it. 132

Why do we care? Open Eustachian tube forces the pressure in the tympanic cavity to stay constant. This means that “additional” sound energy flows through to the next part of the system. Closed Eustachian tube or built up pressure turns in the tympanic cavity into a closed space and sound energy can get trapped there. 133

What happens when the Eustachian tube opens? 1. Air enters the middle ear cavity 2. Any pressure difference across the TM is equalized, allowing better sound propagation into the inner ear. 3. Small amounts of fluid (such as from illness) can drain out of the ear, down the Eustachian tube. 134

True or false: The primary function of the Eustachian tube is to drain fluid from the middle ear space. A.True B.False 135

PROTECTION Physiology 136

Protection: Acoustic reflex A reflex is an involuntary muscle contraction. When muscles contract, they get shorter. This reflex is ACOUSTIC because it is triggered by sound. It is a protective response to relatively high amplitude sounds (>80 dB). 137

Protection: Acoustic reflex For the acoustic reflex, that means the muscles pull towards the place they enter the middle ear cavity. Tensor tympani enters middle ear space from anterior wall and attaches to the malleus Stapedius muscle enters middle ear space from posterior wall and attaches to the stapes 138

posterior anterior medial is where you are 139

Protection: Acoustic reflex The contracting muscles pull in opposite directions and so STIFFEN the ossicular chain Stiffer things are have high impedance to vibration at low frequencies. Therefore, the acoustic reflex primarily protects from lower frequency sounds (<2000 Hz). 140

Protection: Acoustic reflex Takes between 10 ms (for very intense sounds) and 150 ms (for sounds near 80 dB SPL) to happen Provides about dB SPL attenuation Tensor tympani not often active in humans, but reflex works anyway 141

The acoustic reflex is triggered by sounds that have an amplitude of about _________ or above and is only protective for sounds that are ________________. A.60 dB SPL, less than 2000 Hz B.80 dB SPL, less than 2000 Hz C.100 dB SPL, less than 2000 Hz D.60 dB SPL, greater than 2000 Hz E.80 dB SPL, greater than 2000 Hz F.100 dB SPL, greater than 2000 Hz 142

Which mechanism is responsible for most of the impedance matching by the middle ear? A.Ossicular lever transformer B.Catenary lever transformer C.Area transformer 143