Audio Engineering

700 Chapter 23

Figure 23.17 shows that the inductance of the voice coil is refl ected into the mechanical
circuit as a compliance [very much smaller than Cms , which will resonate with the total
mass at a midfrequency (usually 150–700 Hz); see Figure 23.18 ]. At this frequency, the
total electrical impedance is resistive, has the lowest absolute value, is the value that is
(or should be) quoted as the rated impedance in the manufacturer’s specifi cation, and
corresponds to (d) in Figure 23.18.

Instead of referring all the parameters to mechanical mesh, it is sometimes more
convenient to refer to the electrical input (see Figure 23.19 ) in which

R

C

m

B

c

c

em

md

voice coil resistance

voice coil inductance

C





 2

ll^2

Farads (23.40)

Figure 23.18 : Simplifi ed circuit, valid over restricted frequency ranges: (a) very low

frequencies, (b) at principal resonance frequencyω 0 , (c) above principal resonance

frequency, (d) at second resonance frequency, and (e) at high frequencies.

uc

Cmd

EBI

Rc

(a)

EBI Mmd Rmd Zma Rma j Ma

Rc

B^2 I^2

Rc

(c)

EBI Rmd Rma

Rc j Lc

B^2 I^2

ZRcj Lc

(d)

EBI Rmd R

Rc md

B^2 I^2

Rc

(b)

EBI Mmd ρocπa^2

Lc

(e)