extremely small percentage of the total static pressure p s and may as indicated be on the order of 5 psi or less the thin diaphragm associated with the transducer 21 will respond only to the dynamic pressure p d as will be explained as seen in the figure transducer 21 has the static plus the dynamic pressure applied to the top surface and is indicated again by p s p d coupled to the bottom surface of the diaphragm is a tube or reference tube of an exceedingly long length designated by reference numeral 18 the tube 18 is coupled to the bottom surface of the diaphragm essentially the tube 18 receives at an inlet both the static and dynamic pressure which is p s p d the tube as shown is in helical form it is well known that the resonant frequency f of such a tube as for example an organ pipe is given by f c 41 where c is the speed of sound and 1 is the length of the tube for instance in air where the speed of sound is approximately 1200 feet per second a tube length of 2 u00bd feet will give a resonant frequency of 120 hz thus tube 18 acts as a low pass filter and will only pass frequencies which are below 120 hz in this manner the dynamic frequency which is 5000 hz or greater will not pass through the tube 18 therefore the underside of the diaphragm associated with transducer 21 only receives the status pressure p s the static pressure p s subtracted from the static pressure plus the dynamic pressure p s p d supplied to.