Short subwoofer distance

This is a little more difficult to diagnose.  It usually happens when the sound from the subwoofer arrives at the microphone faster than speed of sound in air.  How is that possible?  Sound travels faster in solids than it does in air and so, for example, a floor vibration may reach the microphone faster and produce a shorter distance result.  If this happens it is a good idea to identify the problem surface and try to move the subwoofer to a different location or isolate it from the vibrations.