calculating speaker inductive
Figure 1. A Zobel network. Source: Wavecor.

You are describing a Zobel network. From the article linked in Figure 1:


  In some speaker designs it is an advantage to use a so called Zobel network (figure on the right) to linearize the impedance of a drive unit.
  The effect of a correctly designed Zobel network is shown below on the impedance curve.
  Whether to use a separate network to equalize the high frequency impedance will depend on the crossover topology used, on personal design preferences, etc. 
  
  A Zobel network is a series resistor-capacitor (Req-Ceq) network that is connected in parallel with a loudspeaker driver in order to neutralize the effects of the driver’s voice coil inductance Le (see note below about Le).
  If a Zobel, or other kinds of networks, are used for driver impedance linearization/modification, it is important that the network be connected right on the driver terminals, between the driver and the crossover.




Figure 2. Frequency response modification without and with Zobel network.