Admittance is a fundamental concept describing how easily a system allows energy, typically in the form of an alternating current, to flow through it. This measurement provides a holistic view of a material’s or circuit’s ability to transfer energy. Admittance is valuable in fields beyond pure electronics, such as biological and medical diagnostics, where the transfer of energy through tissues and organs is measured. By quantifying the ease of flow, admittance helps characterize the physical properties of various materials and biological structures.
Admittance Defined
Admittance, symbolized by \(Y\), is mathematically defined as the reciprocal of impedance (\(Z\)). While impedance measures the overall opposition to alternating current flow, admittance measures the overall allowance of that flow. An analogy is comparing a narrow, clogged pipe (high impedance, low admittance) to a wide, clear one (low impedance, high admittance).
The magnitude of admittance quantifies the total ease with which an electrical or acoustic signal can pass through a medium. In a direct current (DC) system, this concept simplifies to conductance, the reciprocal of resistance. In alternating current (AC) systems, the dynamic nature of the current requires the more comprehensive term, admittance, to account for effects beyond simple resistance.
The standard international unit for measuring admittance is the Siemens (S), named after Werner von Siemens. This unit is the reciprocal of the Ohm (\(\Omega\)), the unit for impedance and resistance. Admittance is a complex physical quantity that accounts for both the resistive and reactive properties of a system, providing a complete picture of energy transfer, including dissipation, storage, and release.
The Two Components of Admittance
Admittance is a complex quantity consisting of two distinct components that describe different physical processes. It is expressed as \(Y = G + jB\), where \(G\) is the Conductance and \(B\) is the Susceptance, and \(j\) represents the imaginary unit. These two parts separate the energy transfer into dissipated energy and stored energy.
Conductance (\(G\)) represents the real part of the admittance, signifying energy transferred through the system and converted into other forms, such as heat. This component is dissipative, reflecting the loss of energy from the signal. In an electrical context, conductance is directly related to resistance, quantifying the energy permanently used up by the system.
The imaginary part is Susceptance (\(B\)), which quantifies the energy that is temporarily stored and then released back into the system. This storage is a reactive process, often associated with components like capacitors or inductors. Susceptance measures the ease with which a system can store and release energy without permanent dissipation.
Application in Physiological Measurement
Admittance measurement is a fundamental tool in several non-invasive diagnostic techniques used in health and biology. By applying an electrical or acoustic signal and measuring the resulting admittance, clinicians gain insight into the structural and fluid properties of biological tissues. This is possible because the human body acts as a complex electrical and acoustic circuit, where different tissues possess characteristic admittance signatures.
Tympanometry
Tympanometry is a test used to assess the function of the middle ear. The procedure measures the acoustic admittance of the middle ear system, which includes the eardrum and the tiny bones behind it. A small probe introduces a low-frequency sound, typically 226 Hz, into the ear canal while air pressure is varied.
The resulting admittance measurement reveals how easily sound energy is transferred through the middle ear structures. A change in admittance, such as a reduced peak or a shift in the peak’s position, can indicate common issues like fluid buildup (otitis media) or excessive stiffness (otosclerosis). The test separates the acoustic admittance contributed by the ear canal from that of the middle ear, allowing for precise diagnosis of middle ear pathology.
Bioelectrical Impedance Analysis (BIA)
Bioelectrical Impedance Analysis (BIA) is used to estimate human body composition. In BIA, a small, safe electrical current is passed through the body, and the resulting admittance is measured. The body’s total water content is a strong conductor with high conductance, while body fat is a poor conductor with lower conductance.
By measuring both the conductance (\(G\)) and the susceptance (\(B\)), BIA devices estimate the proportion of fat-free mass (muscle and water) versus fat mass. The conductance value relates primarily to the total body water, which is concentrated in muscle tissue. The susceptance value relates to the capacitance of cell membranes, providing information about the integrity and quantity of the body’s cells. These measurements help monitor changes in hydration and muscle mass.