Immittance

Immittance
Combined measure of impedance, admittance, or both. Immittance is crucial to several practices, such as tympanometry.
Common symbols	W, Za, Ya,[1] C, F, t, ∈, ℝ, jt, ϵ, χ, M[2]
SI unit	N/A

Immittance is a term used within electrical engineering and acoustics, specifically bioacoustics and the inner ear, to describe the combined measure of electrical or acoustic admittance and electrical or acoustic impedance. Immittance was initially coined by H. W. Bode in 1945, and was first used to describe the electrical admittance or impedance of either a nodal or a mesh network. Bode also suggested the name "adpedence", however the current name was more widely adopted. In bioacoustics, immittance is typically used to help define the characteristics of noise reverberation within the middle ear and assist with differential diagnosis of middle-ear disease.^[1] Immittance is typically a complex number which can represent either or both the impedance and the admittance (ratio of voltage to current or vice versa in electrical circuits, or volume velocity to sound pressure or vice versa in acoustical systems) of a system.^[3]

Immittance does not have an associated unit because it applies to both impedance, which is measured in ohms (Ω) or acoustic ohms, and admittance, which is commonly measured in siemens (S) and historically has also been measured in mhos (℧), the reciprocal of ohms.