HARMONIC DISTORTION IN MOVING COIL TRANSDUCERS CAUSED BY GENERALIZED COULOMB DAMPING

A generalized Coulomb damping model for transducer friction is present ed and applied to an underwater sound moving coil transducer. Measured harmonic amplitudes in the acoustic output of a certain moving coil t ransducer showed characteristics that indicated they were caused by fr iction. However, calculations based on the usual definition of Coulomb damping predicted only odd harmonics while the measurements contained both odd and even harmonics. A generalization of Coulomb damping was developed in which the friction force depends on position and directio n of motion. This damping model predicts both even and odd harmonics a nd gives satisfactory agreement with the data. This friction-generated distortion is independent of drive level and can be important since i t puts a lower bound on the distortion-free dynamic range.