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Flashcards in Acoustic immittance Deck (29):
1

What are acoustic immittance measures? Define acoustic immittance. How is energy transferred? How are the middle and inner ear systems set into motion? Where can energy be measured? Is the middle ear a good transducer? What is the opposition to energy transfer called? What is the ease of energy flow called? What does acoustic immittance refer to? 

  • Series of measures testing integriy of middle ear system and the peripheral afferent portion of auditory system. Started in the 1960's.
  • Acoustic Immittance – general term referring to an acoustic energy transfer
    • Energy is transferred when sound waves reach the external auditory canal (EAC) and sound pressure is applied to the tympanic membrane (TM)
    • With sufficient sound pressure, TM and entire middle ear system and inner ear system are set into motion
    • Can measure this energy flow at lateral surface of TM
    • Middle ear isn’t a perfect energy transducer – all of the energy at the TM doesn’t flow through the system
      • Opposition to energy transfer is acoustic impedance, Za
      • Resulting ease of energy flow is acoustic admittance, Ya
    • Acoustic immittance refers to either Za  or  Ya
      • Now only do admittance

2

What is the basis of measurement? What is the probe signal? What is measured? Describe stiff and mobile systems.

  • Basis of Measurement
    • Introduce an acoustic signal to ear and measure resulting energy transfer
      • Probe signal: 226 Hz (or 678 Hz)
      • Measure SPL of reflected probe signal – an indirect index of acoustic immittance
      • Ex: stiff ME system has hi Z and low Y – restricts flow of energy, and most of energy in probe tone is reflected back
      • Ex: mobile ME system has low Z and hi Y – hi energy transfer into the system; little energy reflected back
         

3

Where are actual measures made? Where is the oprimal point of measurement? What needs to be corrected? HOw is this done?

  • Plane of Measurement
    • Actual measures are made at entrance to EAM (remote from TM)
    • Optimal point of measurement – TM (because acoustic immittance at TM describes energy transfer reliably)
    • Need to correct measurements at probe tip by contribution of the ear canal
      • 1. Measure total acoustic immittance at probe tip at 0 daPa (includes contribution of EAC, TM, and ME system)
      • 2. Measure acoustic immittance at – 400 daPa – stiffens ME system (includes contribution of EAC only)
      • 3. Subtract #2 from #1 above -> immittance at TM

4

What are teh compnents of impedance? What are each due to? WHat is the formula for Z? What is it measured in?

  • Components of Impedance:
    • resistance (R) (friction) – component that converts energy into heat by friction
      • Due to rubbing of joints between ossicles
    • reactance (X) – opposition of energy flow through storage of energy before producing movement
      • partially due to stiffness – tendency of system to retain its shape
      • partially due to mass – weight and density of ME system
    • FORMULA FOR Z: Za = √R² + (2πfM - s/(2πf))²
  • R = Resistance   M = Mass   s = stiffness
  • (measured in Ώ)
  • * note: mass and stiffness vary with frequency 

5

What is admittance? What are its components? What is the relationship among stiffness, susceptance, mass susceptance and frequency?

  • Admittance–ease with which acoustic energy passes through a system; Components are the opposite of components of impedance
    • Impedance (Z)
      • resistance (R)
      • reactance (X)
        • mass (+Xm)
        • stiffness (-Xs)
    • Admittance (Y)
      • conductance (G)
      • Susceptance (B)
        • mass susceptance (-Bm)
        • stiffness susceptance (+Bs)
  • Relations among stiffness susceptance, mass susceptance, & frequency:
  • +Bs = (2fπ/s)
  • -Bm = 1/(2fπM)
  • Y² = B²+G² OR Y = 1/Z

6

What happens to SPL as cavity size varies? How is ear canal volume measured?

  • Equivalent Volume
  • What happens to SPL as we vary cavity size? 
    • Small cavity --> SPL is high
    • Large cavity --> SPL is low
  • What we do in acoustic immittance to measure individual’s ear canal volume (cavity size)
    • Present tone of known intensity to a hard-walled cavity of known volume and measure SPL (.5, 2.0, 5.0 cm3)
    • Take calibrated signal to individual’s EAC – measure SPL developed
      • Compare to actual ear SPL
    • Infer volume of EAC re: equivalent volume in cavity
    • Also based on principle that with a 226-Hz probe, a 1-cm3 volume of air has Y = 1 acoustic mmho
      • mmho and cm3 used interchangeably 

7

Describe acoustic immittance instrumentation. What do impedance bridges measure?. What do otoadmittance units measure? How do otoadmittance meters derive admittance? What can be measured? 

  • Impedance Bridges - measure Z (CURRENTLY OBSOLETE)
  • Otoadmittance Units –measure Y in two components:
    • Susceptance (B – stiffness and mass); vary with frequency
    • Conductance (G – resistive component); doesn’t vary with frequency
  • Otoadmittance meters derive admittance from two aspects:
    • 1. volume velocity of reflected signal (motion of air molecules in EAC) (U) sound pressure is constant, and Y is proportional to U Y ≈ U/P
    • 2. phase angle of reflected signal (lag between pressure presented and flow of energy)
      • a. in-phase component: conductance, G
      • b. out-of-phase component: susceptance (B)
      • Y is calculated from these values 
    • Can measure tympanometry, acoustic reflex thresholds, and can calculate static (peak) admittance
       

8

Describe the 3 lines in the acoustic immittance instrumentation. What is line 1? What are the reasons for a low frequency probe? Multi-frequency? What allow presentation of probe tone to ear? What is line 2? What are the typical manometer values? What is line 3? What are its components and what does each do?

  • Line 1: signal presentation system
    • oscillator (probe freq’y): 226 (85 dB SPL) or 678 Hz
      • Reasons for low-frequency probe 
        • Less worried about standing waves
        • To avoid ear canal resonances
          • Will show artificial measures
        • Higher frequency - more sensitive - more false positives 
      • Multi-frequency tympanometry
        • High for neonates
    • potentiometer and loudspeaker – allow presentation of probe tone to ear
  • Line 2: air pressure system
    • air pump (change pressure to ear canal)
    • manometer (measure pressure in ear canal)
      • Range: -400 daPa -> +200 daPa (normal)
      • daPa – units in System International; 0 mm H20 = 0 daPa
      • Pump speed – varies: 12.5, 50, 200, 600/200 daPa/sec
  • Line 3: measuring and analyzing system
    • Microphone: monitors reflected probe tone
    • Amplifier: amplifies reflected tone for analysis
    • Analyzing system: compares reflected wave to incident wave and determines U, and phase angle -> derives G and B, calculates Y
       

9

How is calibration conducted? How is an air-tight seal verified?

  • Conduct calibration checks (ANSI S3.39; see equipment manual):
    • use the test cavity provided with the instrument to check the calibration of the ml/mmho meter and graphic display
    • Can check pressure range used for tympanometry
    • Also check reflex threshold mode and intensity limits
    • Check intensity of probe tone and signal activators for acoustic reflex
  • Verify an air-tight (hermetic) seal
    • Place probe assembly in ear canal with rubber cuff/eartip over probe
    • Introduce +200 or +400 daPa pressure – is pressure stable?
    • “occlusion” message – examine  probe tip for cerumen
    • “leak” message - reposition the probe, or select an alternate size eartip

10

What is the definition of tympanometry? What is the procedure of measurement? 

  • Definition: measure of ME admittance as pressure to TM is varied
    • Y is measured indirectly as we monitor U and phase of reflected probe tone
    • More admittance as you approach 0 dapa
  • Procedure for measurement
    • Present a range of pressures to EAC
    • These change stiffness of system
    • Measure Y at each pressure value

11

What are tympanometric findings in a normal system?

  • findings in normal auditory system
    • +200 daPa -> stiff system -> high SPL reflected (low U) -> low Y
    • 0 daPa -> mobile system -> low SPL reflected (high U) -> high Y
    • -200 daPa -> stiff system -> high SPL reflected (low U) -> low Y
    • Plot of results is called “tympanogram” – a pressureadmittance function
  • findings in abnormal auditory system
    • Fluid - high impedance - not much motion
       

12

What are the four components of the tympanometry quantification system?

  • Quantification System
    • TPP
    • Peak Y
    • Equivalent volume
    • Tympanometric Width/Gradient

13

What is the TPP? What does it indicate? What is it an estimate of? What is normal? What does negative value indicate? in children? What about positive values? What does the height of the tail value represent?

  • TPP: tympanometric pressure peak
    • The pressure corresponding to the peak point on the tympanogram
      • Assumes that this is where pressure in ME = presure in EAC
    • Indicates pressure at which energy transfer is greatest
    • A gross estimate of pressure in the ME
    • If near 0 daPa – indicates normal ME pressure
    • If significant (-) value in adults – indicates (-) pressure in ME (
    • (-) values in children – evaluate cautiously
    • (+) TPP values – no dx significance
    • Height at tail value - volume of ear canal
      • -400 dapa preferred
      • Now compensate for ear cana l volume.
         

14

What is peak compensated admittance? How is it measured? What are normative values?

  • Definition: the resting acoustic Y at the lateral surface of the TM
  • Method of Measurement
    • Derived by comparing Y when pressure is at 0 daPa and Y when pressure in EAC is max (volume measure; at -400 or +200  daPa)
    • Ytm = Ymax -Y min
    • compensated - height of peak
    • uncompensated - peak tail value
  • Normative Values (Y, in mmhos)

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15

What is the diagnostic significance of peak admittance?

  • Normal range (adults): .3 – 1.8 mmho
  • Abnormally low Peak Y: <.3 mmho>
  • Reasons? ME is not mobile
    • Fluid - otitis media
    • otosclerosis
  • Abnormally high Peak Y: >1.8 mmho
    • Reasons? Hypermobile ME system
      • disarticulation of ossicular chain
      • scarring
      • perforation that healed spontaneously
  • 16

    What is equivalent volume? How is it measured? What are normative values?

    • Vec – an estimate of volume of air in front of admittance probe tip
    • How volume is measured (+200 or -400 daPa)
    • normative values: (cm3) 
    • interpretation of abnormally small values
      • small ear canal
    • interpretation of abnormally large values
      • more than 2.5 - tm perforation
      • more than 2.0 in kids - pe tubes or perf
         

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    17

    What is the tympanometric width?

    • Describes the shape of the tympanogram
    • The pressure interval corresponding to a 50% reduction in Peak Y
    • (one type of gradient measure, or estimate of shape)
       

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    18

    What are the norms for tympanometric width?

    • Diagnostic significance: if TW > 90% range
    • Greatest variable to ID kids with ME effusion
    • greater than 275 dapa

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    19

    Describe the tympanograms.

    A. Chronic otitis media

    B. Chronic otitis media

    C. TM retraction

    D. TM atrophy - scarring

    E. Ossicular disconinuity - largest peak Y

     

    Pathology most lateral picked up by tymp.

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    20

    Describe the qualitative interpretation of tympanograms. What are they based on?

    • Qualitative Interpretation of Tympanograms
    • Typing System: Jerger-Liden (1969)
    • Type A, As, AD: Normal Shape
    • Type B: flat shape
    • Type C: normal shape, but peak is at (-) pressure
    • Type D: w-shape
      • only with 678 Hz probe - not diag. sig.
      • at 226, hypermobile TM
    • NOTE: Based on measures obtained with impedance bridges (measured compliance in relative units) **not appropriate to use today!!
       

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    21

    Describe the multiple frequency probe classification system. What is it based on? What does it monitor? What changes with probe frequency?

    • Multiple frequency probes - classification system (Vanhuyse, Creten, & Van Camp, 1975) (probes vary between 220 and 910 Hz)
      • Classification is based on amplitude and shape
      • monitors number of peaks in G & B tympanograms
        • Number of peaks in susceptance and conductance
      • normative changes seen with changes in probe frequency
        • Reflect changes in stiffness or mass dominance of ME system as probe frequency changes
        • Probe < 800 Hz – ME is stiffness dominated
        • Probe  > 1200 Hz – ME is mass loaded
        • Between 800-1200 - will be resonant - mass and stiffness equal.
           

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    22

    What is considered normal with multiple frequency tymps? What is abnormal?

    • 3B1G
    • 3B1G
    • 5B3G
    • abnormal patterns observed:
      • more peaks than normal (>5 extrema in B tymp or > 3 extrema in G tymp), or
      • excessive distance between peaks (notch width > 75 daPa for 3B tymps or > 100 daPa for 5B tymps)

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    23

    Picture. Interaural multifrequency tympanometry. Explain the study of RF. How is it applied?

    • Where B crosses 0 - resonance of ME 800-1200
    • Disarticulation - mass dominated system
    • Compared resonance frequencies (RF) in the 2 ears in 12 males and 12 females w/o ME pathology and normal hearing
      • Used different sweep methods (pressure-sweep, frequency sweep) and different ways of calculating the RF
      • 90% ranges calculated with the different methods and showed interaural differences should be + 200 Hz  
    • Application:
      • find RF in each ear in cases of unilateral conductive lesion  
      • if RF in conductive ear > RF of normal ear then conductive lesion is stiffness dominated
      • If RF in conductive ear < RF of normal ear, then conductive lesion is mass dominated 

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    24

    Describe wideband acoustic immittance. What is it? What is the stimulus? What is R sq? What is 1-R sq? How can measures be made?

    • A broadband measure of ME function rather than measures at discrete probe frequencies (tympanometry).
    • Stimulus: a click or chirp (multiple frequencies presented simultaneously) - spectrum is 250-8000 Hz
    • Some of the sound is absorbed by the ME -> IE, some is reflected to EAC
    • Calibration technique permits measurement of
      • The pressure reflectance coefficient, R. R²=Energy/power reflectance
        • How much chirp is reflected back
        • Higher, less absorbtion of ME
      • Absorbance: 1-R sq. The proportion of sound power absorbed by the ME
        • Ranges from 0-1.0
        • 0 = little acoustic power is absorbed
        • 1 = all aoustic power is absorbed
    • WAI - variety of measures - mostly refers to absorbance
    • Measures can be made at ambient ear canal pressure, takes 1-2 sec.

    25

    How does wideband absorbance change with frequency, age, and gender?

    • Wideband absorbance changes as a factor of frequency, most absorbance in mid-frequencies (750-4000).
    • WB absorbance changes from 0-1.0
    • Younger adults show higher EA than older in mid-frequencies (higher in kids)
    • Differences in EA between males and females

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    26

    What would different disorders look like with WAI?

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    27

    Describe the eustachian function test. What is the method of the test? What do results indicate with functioning eustachian tube?

    • Goal: determine if Eustachian tube opens normally with changes in pressure
    • Method: measure tympanogram three times (always sweep in same direction)
      • Baseline
      • Introduce +400 daPa, patient swallows -> repeat
      • Introduce -400daPa, patient drinks water -> repeat
    • Results with functioning Eustachian tube
      • With +400 daPa -> TPP shifts in (-) direction
      • With -400 daPa -> TPP shifts in (+) direction
      • Look for shifts of 15-20 daPa
      • (if no shift – do Toynbee or Valsalva maneuver)
         

    28

    What are operational variables that influence a tympanogram?

    • Operational Variables that influence tympanogram
      • Probe frequency
      • rate of pressure change
        • Faster not nec. better - peak may not be as high
      • range of pressure change
        • tails will spread out more
        • might see a peak for kids at -400
      • direction of pressure change
      • Number of tympanograms recorded
        • admittance can increase

    29

    Describe the diagnostic value of tympanometry. How good is the accuracy for kids and adults? What is the emerging value of WAI?

    • Diagnostic Value of Tympanometry
      • accuracy in adults: high
      • accuracy in children: high, but some false positive results (esp. negative pressure)
        • children < 7 mos of age – poor correlation between tympanometry and the actual condition in the middle ear
        • In young infants – normal tympanogram doesn’t mean there is a normal middle ear
          • But: flat tympanogram strongly suggests a diseased ear
    • Emerging value of WAI
      • Better predictor of conductive hrg loss (air-bone gaps) in kids (2.6-8.2 yrs)  than 226-Hz tympanometry (Keefe et al., 2012)