Impedance (Ω)
Definition
Impedance is the opposition a circuit presents to alternating current, expressed in ohms (Ω). Unlike pure resistance, it varies with frequency. Shure classes microphones below 600 Ω as low impedance, and professional models are commonly rated at 150 Ω, with actual values often around 150 to 300 Ω. Audio interfaces follow the bridging principle, in which the input impedance is at least ten times the source impedance, so that almost all of the source voltage reaches the input. A 150 Ω microphone therefore calls for an input of at least 1.5 kΩ. Loudspeakers have a rated (nominal) impedance, commonly 4, 8 or 16 Ω, and IEC 60268-5 requires the lowest impedance within the rated frequency range to be no less than 80% of the rated value.
In the field
A passive guitar pickup has a high output impedance, of the order of tens of kilohms, so a low-impedance microphone input loads it and dulls the high frequencies. A DI box or a dedicated high-impedance instrument input avoids this loss. On the amplifier side, a load below the minimum impedance specified by the manufacturer, such as 4 Ω on an amplifier rated for 8 Ω minimum, draws more current than the design allows and can cause overheating or trigger the protection circuits.
Related lessons
Sources
- Shure, « Microphone Volume - High and Low Impedance » : basse impédance = moins de 600 Ω · service.shure.com
- Shure, « Should I match impedances of my microphone to my mixer? », pontage : impédance d'entrée d'au moins 10 fois celle du micro · service.shure.com
- IEC 60268-5 (haut-parleurs) : impédance minimale ≥ 80 % de l'impédance assignée dans la bande assignée · webstore.iec.ch
- Dynaudio, « What is impedance? » : application de la règle des 80 % de l'IEC 60268-5 · dynaudio.com
- Radial Engineering, DI Questions : les micros de guitare passifs demandent une entrée à haute impédance, une entrée basse impédance les charge · radialeng.com
- Shure SM57 (Wikipedia) : impédance assignée de 150 Ω pour une valeur réelle de 310 Ω · en.wikipedia.org