Audio Engineering

Loudspeaker Enclosures 725

1 kHz. The traditional, simplistic “ damping factor ” takes this ideal impedance at a nominal
point (say 100 Hz) and then describes attenuation against an 8-ohm resistor. This gives a
damping factor of about three orders, that is, 1000, but up to 10,000 at 30 Hz. Now look at
the middle curve of Figure 24.10. This is what the amplifi er’s damping ability is degraded

BDPFAZ3.CIR Temperature  27 Swr.dc.value  0.1

100

10

1000m

100m

10m

1m 10 100

v(Zo1) v(Zo3) v(Zo4)

1 K 10 K

A view of the damping surface

What the 5" driverZobel look like

at the cabinet terminals

What the amplifier looks like at the speaker’s end (inc. cable & DCP cap)

Series capacitor

Cable DC res

Skin effects

NFB reduction

What the amplifier looks like – at its output terminals

Frequency

Figure 24.10 : Views of the damping surface in 2D. The lower plot shows the very low steady-

state output source impedance of a typical transistor amplifi er with high NFB. The middle

plot shows how this degrades after passing down a few meters of reasonably rated cable

and a series capacitor (which might be the simplest crossover, or for fault protection). The

upper plot repeats the impedance versus frequency behavior of the 15 ” bass driver. The

effective damping factor is the smaller and highly variable difference between the upper and

the middle plots, not the difference between the highest impedance on the upper plot and

the lowest on the lower plot used by amplifi er makers!