6Ideally, when you use the spreadsheet, you should use the actual measured impedance of the loudspeakers at the frequency of interest, rather than the quoted nominal impedance. Using the test set shown above makes this quite easy.
http://www.bestsellers-store.com/audiospeaker
Remember that once a Zobel or notch filter has been determined for a driver, that becomes part of the driver. The network and driver must be treated as one, since the network's purpose is to remove some objectionable characteristic of the attached driver - most commonly unwanted impedance variations.
To use the test set, follow these steps ...
Disconnect the speaker, and set the voltage to a convenient level (say 1V)
Connect the speaker, and sweep the frequency to locate the centre of the "flat" range, where the voltage does not change appreciably
Measure the voltage
Impedance may be calculated, based on the known input voltage and the voltage measured across the loudspeaker. Using the long way (because it is easier) ...
Vr = Vin - Vs
Where Vr is voltage across resistor, Vin is unloaded voltage, Vs is voltage across speaker
I = Vr / R
Where I is current, and R is the value of the resistor (10 ohms is suggested)
Z = Vs / I
Assume an input of 1V, R = 10 ohms and Vs = 400 mV
Vr = Vin - Vs = 1 - 0.4 = 0.6 V
I = Vr / R = 0.6 / 10 = 0.06 A
Z = Vs / I = 0.4 / 0.06 = 6.67 Ohms
This calculation is also done by the spreadsheet
http://www.bestsellers-store.com/audiospeaker
