increasing and nearly the entire path length from x 0 to x 2 u03c4 u03b4 u03bd can be placed in region u201c b u201d where the velocity is decreasing as illustrated with reference to fig2 c and 2 d in this circumstance the relative positions of the u03c4 and u03c4 u03b4 can change significantly as pressure is applied to the backside of the die with u03b4 equaling the change in time delay between first and second taps 46 48 due to pressure on the die the time separation between taps is u03b4 u03b4 and the frequency change between nulls is 1 u03b4 u03b4 since u03b4 and u03b4 can be designed to have the same or opposite sign the frequency null can even approach infinity so the shift in null frequency with changes in pressure can be tremendously large orders of magnitude more sensitive than a shift in resonant frequency or time delay as is used in other saw sensor approaches such is design dependant and can be set to any convenient range an advantage for embodiments of the invention over typical passive saw devices by way of example a passive saw sensor may be a one port device connected to an antenna in the discussion thus far it is implicitly assumed that the input transducer is connected in parallel with the two.