Figure shows how the resistance of a resistor and the reactances of an inductor and a capacitor vary with angular frequency $\omega$ . Resistance R is independent of frequency, while the reactances  $X_L$ and  $X_C$ are not. If  $\omega =0$, corresponding to a dc circuit, there is no current through a capacitor because $X_C\to \infty$ , and there is no inductive effect because $X_L=0$ . In the limit $\omega \to \infty$ , $X_L$ also approaches infinity, and the current through an inductor becomes vanishingly small; recall that the self-induced emf opposes rapid changes in current. In this same limit,$X_C$  and the voltage across a capacitor both approach zero.
Figure shows an application of the above discussion to a loudspeaker system. Low-frequency sounds are produced by the woofer, which is a speaker with large diameter; the tweeter, a speaker with smaller diameter, produces high frequency sounds. In order to route signals of different frequency to the appropriate speaker, the woofer and tweeter are connected in parallel across the amplifier output. The capacitor in the tweeter branch blocks the low-frequency components of sound but passes the higher frequencies; the inductor in the woofer branch does the opposite.