The impedance is The voltage required to produce one amp of current. [ohm] Impedance is the reciprocal of admittance. A high impedance (low admittance) is one indicator of a properly designed stack at parallel resonance, whereas a low impedance indicates a lossy accumulation.^{1}

Impedance is the complex-valued generalisation of resistance. It may refer to:

**Acoustic impedance**and**specific acoustic impedance**are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system.- In electrical engineering,
**electrical impedance**measures the opposition that a circuit presents to a current when a voltage is applied. **Mechanical impedance**is a measure of how much a structure resists motion when subjected to a harmonic force.

The lower the ultrasonic equipment's impedance, the better, the impedance is low, the energy loss is small, the heat is less, and the equipment service life is relatively longer.

### Units and symbols

Name |
Symbol |
Unit | Admittance |

Impedance | Z | Ω (ohm) | reciprocal of admittance |

### Impedance plot

A graph of impedance (vertical axis) versus frequency (horizontal axis). For a stack that is driven by a piezoelectric ceramic, the impedance for each "resonance" will consist of an impedance dip (corresponding to series resonance) followed by an impedance peak (corresponding to parallel resonance) at a somewhat higher frequency.

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