Measuring the impedance of a speaker
The better way is if you have a sweep tone gen (like Audacity in log scale) then use a large series R  >>100 ohms, such that current is constant and does not change much with freq, then scope voltage on speaker, Z(f)=U(f)/I  for I = constant average.  then record with AUX input and observe on Audacity

(menu) > Generate > Chirp and choose options shown below.