Electrical model of a loudspeaker

AN3340

8/24

Doc ID 18453 Rev 1

Rdc can be identified immediately considering the impedance at very low frequencies, for

example, at 20Hz or even at DC.

The effect of Ls is predominant at high frequencies and so this parameter can be measured

at a frequency around 20 kHz.

The remaining three parameters can be calculated considering the property of a parallel

resonant circuit. Rr is the mathematical difference between the peak impedance measured

at the resonant frequency and Rdc, assuming that the term with Ls is negligible.

The last two parameters Cr and Lr can be calculated considering the value of the resonant

frequency and the quality factor of the parallel resonator or simply using two other

frequencies.

The impedance characteristic is important because it provides useful information:

to identify potential overload situations

to optimize the output filter design

to identify the resonant frequency.

The last point is important when the speaker compensation procedure is applied. Due to the

nature of the speaker, the resonant frequency represents the lower limit of the frequency

range that the speaker is able to reproduce efficiently, that is, with good sensitivity.

Figure 5 shows the trace of the impedance of a typical loudspeaker. The impedance peak at

the resonant frequency can be clearly seen in the red trace. The blue trace shows the phase

which is 0° at the resonant frequency.

Figure 5.

Speaker impedance vs frequency

-100

+100

-80

-60

-40

-20

+0

+20

+40

+60

+80

P

H

A

S

E

d

e

g

10

30

12

14

16

18

20

22

24

26

28

Z

O

h

m

10

20k

20

50

100

200

500

1k

2k

5k

10k

f

Hz