Acoustic impedance

Acoustic impedance is the ratio of sound pressure to volume velocity at a point in an acoustic system. It describes how strongly that system opposes oscillatory airflow.

Acoustic pressure: The local deviation of pressure from its ambient value as a sound wave passes. It is the pressure component in the pressure-to-volume-velocity ratio.
Specific acoustic impedance: The ratio of sound pressure to particle velocity at a point in a sound field. Unlike volume-velocity impedance, it uses particle velocity and has different units.
Sound reflection: The return of sound energy from a boundary into the medium from which it arrived. An impedance mismatch at a boundary determines how much sound is reflected.
Acoustic wave: A disturbance that propagates through a material medium as variations in pressure, density, or particle motion. Impedance characterizes the pressure-flow relation within acoustic waves and systems.
Volume velocity: The volume of fluid passing a surface per unit time, often measured in cubic metres per second. It is the airflow component paired with pressure in acoustic impedance.
Characteristic acoustic impedance: The pressure-to-particle-velocity ratio for a traveling plane wave in a medium. It provides a reference impedance for waves propagating through an unbounded medium.
Ultrasound transducer: A device that converts electrical energy into ultrasound and ultrasound back into electrical signals. Its front layers are designed to couple vibrations into tissue with less impedance mismatch.
Acoustic energy: Energy carried by sound through a medium or stored in an acoustic system. Pressure and volume velocity together determine the rate of acoustic energy transfer.
Acoustic reactance: The imaginary component of acoustic impedance, associated with stored energy and phase differences. It captures the reactive part of impedance when pressure and flow are out of phase.
Acoustic impedance matching: The design of adjoining acoustic systems to reduce reflection and improve energy transfer. Matching depends on the impedances on both sides of an interface.

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