A basic discussion of voltage dividers is shown here . The voltage drop across each of the fixed resistors is proportional to its relationship to the total resistance in the circuit. Likewise, the voltage drop across the speaker voice coil is proportional to its resistance relative to the total resistance including the fixed resistor. At frequencies where the impedance is higher, the voltage drop across the speaker voice coil is higher and results in a relative increase in loudness at that frequency.

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