11 - ME and Acoustic Immitance Components

Question 1

Acoustic Immittance is a general term that refers to acoustic energy flow through a system, and is a combination of what two words?

Answer 1

Admittance (Ya) - ease of sound flow

Impedance (Za) - opposition to sound flow

Question 2

What are the roles of the Middle Ear Transformer?

Answer 2

• to transform acoustical energy into a more efficient mechanical system for driving the inner ear fluids
• to increase the effective pressure at the stapes/oval window interface
• impedance matching
• amplification

Question 3

Name 2 of the ME structures and their contribution to the vibratory ME system

Answer 3

• ME cavity - filled by a “compliant” volume of air
• TM and ossicles - contribute mass
• stapes footplate, oval window, perilymph - contribute stiffness and resistance
• mucosa, ligaments, tendons - reduce friction as TM, muscles and ossicles move

Question 4

Acoustic Admittance (Ya) is the ease at which acoustic energy is transmitted. What is the equation that describes this?

Answer 4

Ya = Ga + Ba
where Ga= acoustic conductance
and Ba= acoustic susceptance

Question 5

What is Ga?

Answer 5

Conductance

• ME system designed to overcome resistance (facilitate sound conductance) to sound-induced vibration
• tendons, ossicular chain, etc facilitate vibration and limit energy loss due to friction

Question 6

What is Ba?

Answer 6

Susceptance

• Energy stored in the ME mechanical system
• 2 elements of mass and compliance form the acoustic susceptance Ba

Question 7

What 3 key elements does acoustic admittance (Ya) rely on?

Answer 7

Ga (acoustic conductance)
+Ba (positive compliant acoustic susceptance; springiness)
-Ba (negative Mass Acoustic Susceptance)

Question 8

Which components of the ME system enhance the Ga acoustic conductance by reducing the “frictional resistance”?

Answer 8

• ligaments and muscles that suspend the ossicles

- mucosal lining

Question 9

The passage of sound, especially low frequencies, through the ME system is dominated by which factor?

Answer 9

ME “springiness” known as +Ba Compliance Component
- vibration of ME system at low frequencies is most affected by springiness of air molecules, TM, tendons and ligaments, and “stiffness” at air-perilymph interface at the oval window

Question 10

What is the -Ba component of the ME system comprised of?

Answer 10

The combined mass of ossicles and TM

-mass has little effect on passage of low frequency sound, but influences high frequencies

Question 11

Which frequencies does the Ga component of the ME affect?

Answer 11

All sound frequencies through the normal ME, but minimal effect because frictional resistance is negligible

Question 12

Which frequencies does the mass element (-Ba) affect in the ME system?

Answer 12

High frequencies, but is a relatively minor component of normal ME vibration

Question 13

When might the TM become more stiff?

Answer 13

• when air pressure in the ME is very different than the ambient pressure
• reduced compliance of ossicular chain, or related tendons and ligaments

Question 14

The “best” (resonant) frequency of a vibrating object depends on what two things?

Answer 14

Mass (high frequency sound transmission) and Stiffness (low frequency sound transmission)

Question 15

What is the primary advantage and disadvantage of driving the ME with a 226 probe tone?

Answer 15

• sensitive to stiffness-related changes affecting ME vibration in pathological conditions
• pathologies affecting ME mass are undetected