Page 130 - Master Handbook of Acoustics

FIGURE 5-22 A comparison of (A) parallel resonance and (B) series resonance. For a constant
alternating current flowing, the voltage across the parallel resonant circuit peaks at the resonance
frequency while that of the series resonant circuit is a minimum.

A series resonant circuit (see Fig. 5-22B) also uses an inductor L and a capacitor C. As the

alternating current of constant magnitude and varying frequency flows in the circuit, the voltage at the
terminals describes an inverted resonance curve in which the voltage is minimum at the natural
frequency and rising at both lower and higher frequencies. It can also be said that the series resonant
circuit presents minimum impedance at the frequency of resonance.

Audio Filters

Filters are used in equalizers, as well as loudspeaker crossovers. By adjusting the values of the
resistors, inductors, and capacitors, any type of analog filter can be constructed to achieve almost any
frequency and impedance matching characteristic desired. The common forms of filters include the
low-pass, high-pass, band-pass, and band-reject. The frequency responses of these filters are shown

in Fig. 5-23. Figure 5-24 shows how inductors and capacitors may be arranged in various passive
circuits to form simple high- and low-pass filters. Filters of Fig. 5-24C will have much sharper
cutoffs than the simpler ones in Fig. 5-24A and B. There are many other highly specialized filters
with specific features. With such filters, a wideband signal such as speech or music can be altered at
will.