Measurement of Hearing - Quiz 7 Tymps and Acoustic Reflexes

Objective measurements

-”objective” tests don’t require a behavioral repsonse

• provide information about the function of various portions of the auditory system (but not perception!)

-Acoustic immittance test battery

• tympanometry

• acoustic reflex threshold

• acoustic reflex decay

Acoustic impedance and admittance

-immitance = a hybrid term that refers to both acoustic impedance and acoustic admittance

• acoustic impedance (Z_a)- the total opposition to the flow of sound energy (measured in ohms)

  • ratio of sound pressure (P) to sound flow (U) Z_a = P/U

• acoustic admittance (Y_a)- the total ease with which sound energy flows through a system (measured in mhos)

-These values have a reciprocal relationship

• Y_a = U/P or Y_a = 1/Z_a

Acoustic immittance

-the ease (admittance) or opposition (impedance) of sound energy trasnmission through the external and middle ear can be affected by conductive pathologies-

• middle ear effusion

• eardrum perforation

• ossicular disarticulation

• otosclerosis/stapes fixation

-Sensory or neural pathologies have minimal effec

What factors affect impedance and admittance (immittance)?

-ease, or opposition, of energy transmission through a system (immittance) is affected by the:

• system mass (its inertia)

• system stiffness/elasticity/compliance (its restoring force)

• system resistance (friction)

-The mass, stiffness and resistance of the system components modulate the total impedance/admittance of the system

Terms used to describe the effects of mass, stiffness, and friction for impedance

mass reactance, stiffness reactance, and resistance

Terms used to describe the effects of mass, stiffness, and friction for admittance

mass susceptance, stiffness susceptance (or compliance) and conductance

components of impedance

imagine how energy transfer through this system would be affected by the mass of the block, the stiffness of the spring, and the friction (resistance) of the surface the block moved on

components of admittance

these terms are the reciprocal of the components of impedance and dictate the ease of energy flow through the system.

frequency, impedance, and resonance

-opposition to energy flow due to mass and stiffness reactance varies with freq

• mass reactance increases with freq

• stiffness reactance decreases with freq

• opposition due to resistance is constant with freq

-a system “resonates” at a certain freq based on its mass an stiffness

• resonant freq occurs when “net reactance” is smallest

Immittance of our system

-immittance is due to

• stiffness of

  • air in the EAC and ME

  • TM, ME ligaments and muscle contractions

• mass of

  • ossicles, TM

• system friction

  • perilymph, tendons, ligaments

• immittance changes (primarily changes in stiffness) are associated with various pathologies

Clinical measurement of acoustic immittance

-clinical instruments measure the “admittance” of the ear in millimhos (mmhos) or milliliters (mLs)

• most also provide separate measures of susceptance (B) and conductance (G)

• components:

  • probe loud speaker

  • microphone

  • air pump

  • a second loudspeaker for reflex testing

-present pure tone at 85 dB SPL

• generally LF 226

measuring middle ear admittance (vs total system admittance)

-diagnostically it is helpful to differentiate ME (Y_ME) and total admittance (Y_total)

-”stiffen” TM to measure admittance of EAC only (Y_EAC)

• stiffen TM by increasing EAC pressure to +200 daPa (2000 Pa)

-we can infer ME admittance

• Y_ME = Y_total - Y_EAC

Estimating ear canal volume via admittance measures

-In terms of admittance, a cavity (like the EAC) is essentially a tube of air- a stiffness susceptance

• the stiffness/admittance of a tube of air varies systematically with its volume (cavity size)

• For a 226 Hz tone, admittance (in mmhos) is equal to its volume (in mLs or cc)

  • this is one reason why we use a 226 Hz tone

-with a 226 Hz tone we can estimate ear canal volume while measuring middle ear admittance!

• EAC volume can be diagnostically important

  • indicative of TM perforation

  • clogged or patent PE tube or

  • occluded EAC

What measures make up the immitance test battery?

-tympanometry: admittance as a function of EAC pressure

• static acoustic admittance

  • compensated/uncompensated

• tympanometric peak pressure

• tympanometric width/gradient (we don’t use the gradient measure)

-Ear canal volume

-ipsilateral and contralateral acoustic reflex thresholds

-These are VERY informative measures that should be routinely obtained as part of the diagnostic test battery, but

-immitance measures should NOT be completed when contra-indications exist, e.g.

• recent middle ear surgery (within 6 months)

• patient reports extreme discomfort

• if the referral source specifically requests no immittance

Tympanometry

the measurement of acoustic immittance in the external auditory meatus as a function of air pressure within the external auditory meatus