Audio Engineering

Loudspeaker Enclosures 721

At and about the resonant frequency, the impedance variation at the loudspeaker’s
terminals is due to the refl ection of mechanical energy storage, and damping, back to
the electrical domain. Figure 24.9 shows that the loading is capacitative on the right side
of the resonant peak, where impedance is falling with increasing frequency, while the
impedance that slopes upward with increasing frequency, on the left side of the resonant
peak, is inductive. Figure 24.9 shows this in another domain. When the phase (lower
graph, left scale) is positive, the impedance is capacitative; when negative, it is inductive.
When toward the center, it is resistive. Dead center is pure resistance.

The resonant frequency area(s) of any bass speaker system is are commonly highly
stressful to amplifi ers, and with many BJT amplifi ers, when contact is prolonged by a low
enough frequency and perhaps an insistently enough pounded note, it has frequently been
fatal. Other amplifi ers have been known to simply burst into uncontrollable oscillation.

50.00

PA–LSZ–1.CIR Temperature  25

Inductive Capacitative

Inductive

Resistive

40.00

30.00

20.00

10.00

0.00 10 100 1 K

v(2) Frequency (Hz)

10 K

Ohms

Resonance

Figure 24.8 : The impedance of a 15  drive unit mounted on a nominal baffl e. In some

cabinet designs, there could be two or more resonant peaks. Note the labeling of the

resistive, capacitative, and inductive impedance zones.