Resonance and Voice Coil Reactance Equalization

 

This topic is discussed in much greater detail in Vance Dickason's book, LOUDSPEAKER DESIGN COOKBOOK. The sections are:

7.50 Driver Load and Compensation Circuitry

7.51 Series Notch Filters (EQ. 1, 2 and 3)

7.52 Impedance Equalization

The page numbers are not mentioned as they will change among editions; the section numbers should remain the same. This writing used Edition 4.

If plotting an impedance curve is possible, such as with a computer aided speaker analyzer, finding Rc becomes simplified by using a 25W potentiometer and adjusting to obtain the smoothest impedance response. (see fig.2 for location of Rc)

The least expensive computer aided speaker analyzer known to me is the Dayton Audio DATS V3

EQs 1, 2 and 3 form a CLR series notch filter to reduce or eliminate the impedance rise at resonance. EQ4, impedance equalizer CR circuit  will level the impedance rise with frequency due to voice coil reactance.

Both filters are connected in parallel with their respective driver.  see fig.2

 

 

TABLE 1

In all equations, Re=voice coil dc resistance, Qes=electrical Q, Qms=mechanical Q,

Fs is the resonant frequency, Le=voice coil inductance

EQ. 1

C=capacitance in resonance filter

 
EQ. 2

L=inductance in resonance filter

 
EQ. 3a, 3b

Resonance equalization

Rc is the series resistance of the filters

 
EQ. 4

Voice Coil reactance equalization

 

 

 

 

 

 

 

PHOTO 1

The test setup.

Ignore the component values shown as they pertain to this speaker. They will, however, give one an idea as to how high these component values can be.

The two blue capacitors are each 2000uF polarized and wired in series + to - to form a 1000uF non-polarized capacitor.

 

 

 

FIGURE 1

This is the open air impedance response

 

 

 

FIGURE 2

The component values for the CR filter are 33uF (measured) and 9.2W (measured)

 

 

FIGURE 3

This is the equalized impedance response using both filters. The peak to peak level variation is under 3W.

If you have the time and patience, by tweeking the values of the CLR filter, it may be possible to raise the dip below 300hz bringing the overall impedance very close to 8W.

This particular CLR filter in this example used 18mH instead of the designed 17.3mH. The total measured capacitance is 1168uF instead of the designed 1183uF and the measured resistance of the Rc potentiometer is 6W

The component values for the CR filter in fig.2 are 33uF (measured) and 9.2W (measured)

 

 

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