Noise Radiation from Jet Aircraft in Flight Free

The radiation characteristics of a jet aircraft in flight (directivity patterns for octave bands) have been determined as a function of flight speed. Accurate control of flight path and velocity and correlation of the aircraft's momentary position with each moment of the pressure time curve had to be achieved before reliable results were obtained. The maximum over‐all sound pressure level, which occurs for a stationary jet at an angle of 140° to 150° from the aircraft's nose, decreases with increasing flight speed with a reduction of the total sound power output by up to 6 db. This decrease in noise power output is less than expected theoretically from the reduced relative velocity of the gas jet to the surrounding atmosphere. The noise radiated toward the front of the aircraft increases with increasing speed and above Mach number 0.7 the maximum of the over‐all sound pressure appears at an angle of 20° to 30° from the aircraft's nose. This is accompanied by an increase in the total noise output, which is attributed to radiation from the turbulent flow generated by the moving aircraft as demonstrated by high‐speed glides.