In order for speakers to produce sound, they need power in the form of electric current. A speaker’s impedance is a measure of its resistance to that current, determining the amount of energy it draws from the amplifier. The lower the impedance of a speaker, the more power it will draw. Measured in ohms (Ω), the impedance of car speakers usually ranges from 2 to 4 ohms. Amplifiers send twice as much power to speakers with an impedance of 2 ohms than to those with an impedance of 4 ohms. The impedance of a speaker constantly changes based on the frequency of the sound it is producing at any given time. Instead of publishing a speaker’s impedance curve, which details its impedance at any given frequency, speaker manufacturers offer “nominal” impedance, a constant that is typically slightly higher than the speaker’s minimum impedance. This simplification makes it easy to choose the right combination of speakers and amplifiers.
Paying attention to the ways a speaker interacts with power is extremely important when considering which speakers to install in your vehicle. A low-power automotive speaker system, with 15 watts RMS or less, should be combined with high-sensitivity speakers (90 dB or more) and vice versa. It is also important to match the speaker’s maximum RMS to the power provided by the stereo or amplifier. A speaker’s impedance measures a type of resistance to the power coming to it, with lower levels of impedance offering lower resistance.