AN3340

Electrical model of a loudspeaker

Doc ID 18453 Rev 1

7/24

4

Electrical model of a loudspeaker

As described in the previous chapter the electrical, magnetic and mechanical parts of the

speaker work together to convert an electrical signal into air pressure waves. The speaker

can be represented by an electrical equivalent circuit to simulate the effect of its individual

components.

Figure 4 depicts the simplest electrical model of a speaker in a perfect infinite baffle to

decouple front and rear air-pressure waves.

Figure 4.

Simple electrical model of a loudspeaker

In this simple model the electrical parts and the mechanical parts are separate. In detail the

components present in this model are:

For the electrical part:

–

Rdc simulates the DC resistance of the voice coil

–

Ls is the inductance of the voice coil

For the mechanical part:

–

Cr represents the cone and is mostly linked to the cone mass

–

Lr represents the surround and is related to the suspension compliance

–

Rr represents the suspension losses.

The impedance of this equivalent circuit is given by:

It is quite easy to calculate or measure the parameters Rdc and Ls but it is not so simple to

determine the values of the mechanical parts. Parameters Cr, Lr and Rr can be measured

only with specific mechanical tests.

The task can be simplified by using the above equation for impedance. Measurements can

be performed by driving the voice coil with a AC voltage generator with fixed amplitude and

reading the modulus and phase of the current.

By measuring the impedance at five different frequencies a set of five linear simultaneous

equations can be written and solved for the five unknown parameters.

A intelligent selection of the five frequencies can simplify the calculation.

Electrical part

Mechanical part

Z(

ω)

Rdc

j

ωLs

1

1

Rr

------ j

ωCr

1

ωLr

----------

–

⎝⎠

⎛⎞

+

-----------------------------------------------

++

=