The magnitude of Vb is the internal volume minus the sum total of the volumes associated with the vent duct(s), bracing and speaker.  By assuming a magnitude for Vb, the compliance ratio, alpha, is then determined (=Vas/Vb).  The magnitude of Ql is a function of the total enclosure losses associated with absorption, leakage and the vent(s).  Of the three, leakage losses are significant and Ql values between 5-20 are typical.  By using the spreadsheet calculator, a range of enclosure Ql values, for a given set of system parameters is easily model as shown below (left).  The impedance modulus, as a function of the electrical equivalent resistance, Rel, associated with the given Ql, is shown below (right).  As is evident in the graph, the motional impedance of the driver increases as the resistive losses associated with leakage decrease (green Ql=5, fuscia Ql=10, red Ql=20). 

The equations and coefficient used in the spreadsheet are consistent in form with those published by Small.  We have provided copies of Small's papers in the dropdown menu below and, for comparison purposes, included the earlier papers of A. N. Thiele.  Both papers examine the 4th order high-pass filter transfer function that is derived from equivalent circuits.  For example, multiplying the top and bottom of Thiele's Eq. (21) by p⁴T_o⁴  (i.e. multipication by unity), and setting the coefficients x1, x2 and x3 to a1, a2 and a3, respectively, the result is Small's Eq. (57).  Small extends Thiele's work by considering efficiency, large-signal behavior, enclosure losses and interactions between the diaphgram and vent.