Technology / Home Audio / Impedance: The totality measured in Ohms of all electrical opposition to current flow: resistance, reactance, capacitance, as well as all mechanical factors inhibiting the completion of energy transfer in a contained system. In practical terms, this means that most Drivers are assigned a certain nominal impedance based on their DC voice coil resistance and mechanical stiffness. For car audio this is usually 4 ohms; for home stereo, 8 ohms is the standard.
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Technology / Home Audio / Source Impedance: A parameter of an audio component referring to the output impedance. A low output impedence .5 to 4 ohms, is preferred for automotive applications. MORE
Technology / Home Audio / Nominal Impedance: The minimum impedance a loudspeaker presents to an amplifier, directly related to the power the speaker can extract from the amplifier. MORE