Figure 30-30 Woofer and tweeter. Stereo speakers use resistors to act as impedance to the high-frequency components of sound, and only the bass frequencies are allowed to pass to the woofer speaker. Capacitors are used as a high-pass filter to allow only the high frequencies to get to the tweeter. The 60-Hz interference from electrical appliances can be decreased by a similar process. (From Hecht E: Physics: Algebra/Trig. Pacific Grove, CA, Brooks/Cole, 1994.)