Ever wondered why your woofers have that one extra-loud note? You may have known this is the resonant frequency, but do you really know what makes the frequency resonant, and why it's louder than the others are? Normally, Impedance is the product of 3 combined opposition forces. These are: 1. Resistance - Opposition to Current 2. Inductance Reactance - Inductor's Opposition to AC Current 3. Capacitive Reactance - Capacitor's Opposition to AC current. The last 2 forces are opposite reactances, and create phase differences that are also opposite, anywhere from 0-180 degrees, depending of the applied frequency. When they both become exactly 180 out of phase with each other, they cancel each other out. Whatever frequency this cancellation occurs at, is called the Resonant Frequency. Resonance is a special condition of Impedance. By definition, Resonance is the condition where Inductance Reactance equals Capacitive Reactance. Being opposite forces, when they equal each other, they become ZERO. All that is left of the 3 opposition forces is Resistance, and another rule of a Resonant Frequency is that Impedance = Resistance (Z = R) Okay, now that I've beaten that horse to death, here is where the over-simplification starts. Where you normally had 3 forces, you now only have 1. Your opposition is reducing by 2/3, so now power is increasing by 2/3, BUT ONLY AT THAT ONE FREQUENCY. The sudden drop in Impedance allows your receiver to suddenly increase Wattage by an equal amount. This is why your resonant frequency sings so loudly.