Tympanometric Components

Simple simulation of impedance components in tympanometry (definitions of components below). Select probe tone frequency and then adjust slider to change the balance between stiffness and mass reactance. Negative reactance values imply a stiffness dominated system.

Resistance (R)

• Definition: Resistance is the component of impedance that represents the real part of the impedance, which accounts for the energy lost due to friction or dissipation. In the middle ear, it is primarily related to the frictional losses within the ear canal, middle ear structures, and the tympanic membrane.
• Behaviour: Resistance is typically a constant value that decreases slightly as the pressure changes, but it does not exhibit a phase shift. It is a function of the conductive properties of the ear, such as the ossicular chains mobility and the middle ear muscles.
• Unit: Ohms (Ω)
• Influence: Higher resistance means more energy loss, leading to lower admittance (the inverse of impedance).

Reactance (X)

• Definition: Reactance is the component of impedance that represents the imaginary part of impedance. It accounts for the energy stored and released by elastic structures (like the tympanic membrane and ossicles) and is associated with the frequency-dependent behaviour of the system.
• Types:
• Stiffness Reactance (Xs): When the ear structures act like a spring, storing and releasing energy (opposing displacement and thus opposing low-frequency movement).
• Mass Reactance (Xm): When the system exhibits properties similar to an mass (opposing acceleration and thus opposing high-frequency movement).
• Behaviour: Reactance changes with frequency. At resonant frequencies, the reactance may be zero (Xm and Xs are the same magnitude, but opposite in phase and thus cancel out), indicating that the system is in its most compliant state (maximum admittance).
• Unit: Ohms (Ω)
• Influence: Reactance affects how the ear canal’s compliance changes with pressure. At certain frequencies, it can cause resonance effects, leading to sharp changes in the systems admittance.

Susceptance (B)

• Definition: Susceptance is the reciprocal of reactance and represents the ease with which the middle ear system can store and release energy. It is a measure of the systems compliance and is inversely related to reactance.
• Behaviour: Susceptance increases as the pressure approaches the resonant frequency of the system (increased compliance). It is more relevant at frequencies where the ear is most responsive.
• Unit: Mhos (or mMhos)
• Influence: The greater the susceptance, the more compliant the system is, leading to higher admittance.

Conductance (G)

• Definition: Conductance is the reciprocal of resistance and represents how easily energy flows through the middle ear system. It is related to the real, dissipative part of the impedance and corresponds to the energy lost due to friction and resistance.
• Behaviour: Conductance remains relatively constant unless affected by pathological conditions like fluid buildup, ossicular chain dysfunction, or tympanic membrane stiffness.
• Unit: Mhos (or mMhos)
• Influence: Higher conductance indicates a more conductive (less resistant) ear, which generally correlates with better sound transmission through the middle ear.

Admittance (Y)

• Definition: Admittance is the reciprocal of impedance and represents the ability of the ear to admit sound energy. It is the combined result of resistance and susceptance in the middle ear system. Admittance is a key measure in tympanometry, as it indicates how much sound energy is transmitted through the system.
• Behaviour: Admittance is highest when the system is most compliant (e.g., at the resonant frequency), and it decreases when the system becomes stiffer (e.g., with changes in pressure).
• Unit: Mhos (or mMhos)
• Influence: Higher admittance means more energy transmission, which typically happens when the middle ear is in a compliant state (e.g., when the ear canal pressure is close to atmospheric pressure).

Impedance Magnitude (|Z|)

• Definition: Impedance magnitude represents the overall opposition to sound energy flow through the middle ear. It is the combination of both resistance and reactance and describes how much the middle ear resists sound transmission.
• Behaviour: The impedance magnitude is highest when the system is stiff and least compliant (such as at very low or high frequencies or at extreme pressures).
• Unit: Ohms (Ω)
• Influence: The magnitude of impedance will vary with the ears compliance and the frequency of the sound. In tympanometry, it shows the overall "stiffness" of the ear system.

Impedance Phase (φ)

• Definition: The phase of impedance indicates the relative timing of the force and displacement within the ear system. It describes the delay between the pressure wave and the ears response to it, with positive values indicating a stiffness (lagging) system and negative values indicating an mass (leading) system.
• Behaviour: The impedance phase changes with frequency and pressure. At the resonant frequency, the phase tends to be close to zero, indicating that the system is in a highly compliant state.
• Unit: Radians
• Influence: The phase shift affects the timing of sound transmission. A phase of 0 radians suggests no delay, while positive or negative phases indicate that the middle ear is either leading or lagging the pressure.