Rest assured, there is nothing incorrect about the posted equations, I've been using them in this form for more than a decade. The volume units used in the equations is cubic meters.  Since there are 1000 liters in a cubic meter, you need to multiply by 1000 to convert from cubic meters to liters.  Now if you wanted to use published specs (usually quoted in liters) in the equations provided, you would divide the Vas(liters) by 1000 to get Vas in cubic meters.  Units can be challenging even for engineers, it may help to work through a few examples using published specs to get a handle on things.  Drivers from Peerless, Vifa, Scan Speak and SEAS tend to have very complete parameter sets that make sense using these equations.  Some manufacturers, especially car audio companies, publish specs that are incomplete or inconsistent and will give problems when working through the equations.