Missile Flight Sound Levels—Telemetering Techniques and Interpretation of Data Free

Increased emphasis concerning the effects of noise on missile performance has resulted in measurements of flight sound levels utilizing various techniques. In this paper the application of broad band FM telemetering is shown to provide reliable flight measurements. Representative data acquired by use of a condenser microphone system which directly modulated a FM telemetering transmitter is illustrated. Narrow band power spectrum analysis is used to resolve the acoustic signal from system electrical noise. This technique is demonstrated by use of specific examples of telemetered data. Also shown is the correlation of sound level fluctuations and missile maneuvers. It is theorized that these fluctuations result from attached shock wave relocation. Finally, periodic fluctuations in the measured signal strength becomes shown to exist when telemetered signal strength is marginal. These fluctuations result from interference cancellation of the signal when there are two paths to the receiver antenna—one direct path and one reflected off the ground.