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Here is scheme in order to measure the impedance of a driver or a box using a MLS signal. At first, you have to generate a MLS signal. Then, you have to connect everything like on the following scheme in order to get the impedance response of the MLS. In order to do it better, you may construct your own connection box with pre-wired loudspeakers and RCA terminals and the resistor inside.
On one of your channel, you have to get the output of your amplifier and on the other channel the signal of the driver or box. Between the output of the amplifier and the driver, there is a resistor of your choice. You just have to know the Ohm value of it...
Once you have recorded the WAV file, you will be able to compute the impedance. Click 'Analyze' -> 'Impedance'.
The 'original signal' contains the output of the amplifier with the higher amplitude.
The measured signal correspond to the lower amplitude (potential of the driver).
Write the name of the output file and the value of the resistor and you will be ready to compute the impedance.
BE CAREFULL : When you are doing this kind of measures, your amplifier is directly linked to your sound card. For this reason, if you don't want to damage it, you have to choose the lowest output level for the amplifier at first, and then to increase progressively the sound to get the right input level.