Seeking Clarification of Resonance Frequency Equation

Dear Sir / Madam,

I have read through SMACNA's HVAC Systems - Duct Design, chapter 11 section 5 Insertion Loss External Duct Lagging.

I have a problem with equation 11-81 to determine the resonance frequency, fr (Hz), associated with the interaction between the duct wall and outer covering as below :

f_r = 156 [ (P₂/P₁ + M₂/M₁) x (P₁/M₂) x S]^1/2

where P1 is the perimeter of the duct (inches), P2 is the perimeter of the outer covering (inches), M, is the mass per unit area of the duct (lb/ft2), and M2 is the mass per unit area of the outer covering (lb/ft2). S is the cross-section area (in2) of the absorbent material.

It seems like the equation is not correct. Could somebody advise me.

Thank you.