The generation of propeller modulation sounds for sonar trainers

Abstract

Sounds of propeller modulation were synthesized using linear predictive coding (LPC) techniques resident in digital signal processing hardware for the purpose of training submarine sonar operators. The combination of LPC techniques and digital sound generation produces realistic propeller modulation sounds which can be conveniently varied by the instructor to characterize the type, speed, and depth of surface and underwater vehicles. The synthesized sounds were generated with digital LPC (all-pole) filters similar to those commonly used in speech synthesizers. The coefficients to drive the LPC filters were calculated from spectral models which were formulated from the cavitation index, KTIP. The cavitation index was calculated based on the instructor's commands of the vehicle type, speed, and depth. Three different LPC filter designs were implemented to evaluate the computational efficiency and sound realism of each approach. The first design was a standard 12th-order LPC filter which was not capable of changing the spectrum over short time periods. The second design was a low order LPC filter with coefficients which changed quickly with time. The last design was a baseband LPC filter modulated by a time-varying sinusoidal carrier. Various propeller modulation sounds will be played to demonstrate the quality and flexibility of these synthesizer techniques for training sonar operators. [Work performed in support of NUWES, Keyport, WA.]