The term nominal impedance is used primarily in acoustics for the inherent impedance of microphones, loudspeakers, speaker cabinets and headphones. The nominal impedance is an impedance averaged over the frequency range that represents the corresponding component to other electronic circuits.
To achieve the best possible power transfer from microphones to microphone preamplifiers or from power amplifiers to loudspeakers, the impedance of the sensors must be matched to the input impedance of the preamplifier and the impedance of the actuators must be matched to the output imp edance of the power amplifiers. To standardize the matching values, nominal impedances have been established that allow for a slight deviation from the values. This applies not only to amplifiers, but also to transducers that operate internally with impedances quite different from those of the ribbon microphones, electret micro phones, or electrostats connected to them.
In order to protect power amplifiers against overloading, DIN and IEC defined the term nominal impedance for loudspeaker impedance as early as the 1980s. According to this, the nominal impedance is the loudspeaker impedance, which may not be undercut by more than 20% at any frequency. The nominal impedance in ohms refers to a frequency of 1 kHz. Loudspeakers or speaker cabinets are typically low impedance and have nominal impedances of 4 ohms and 8 ohms, rarely higher; headphones have nominal impedances between 50 ohms and 600 ohms. Microphones usually have a nominal impedance of 200 ohms, but there are also low-impedance microphones where this value is significantly lower.