Automatic Equalization of Resonances in Sonic Fatigue Chambers

Abstract

As constructed, all acoustical chambers and most acoustic transducers exhibit sharp resonant peaks, resulting in a concentration of energy over a few narrow frequency bands. Furthermore, these resonant frequencies vary for different specimens placed in a chamber. In order to achieve the desired “flatness” with respect to frequency it is necessary to create electronically the inverse of this frequency characteristic at the input to the system. Early types of equalizing circuits required laborious manual adjustment of many interacting controls to achieve equalization for each new specimen. The instrument described in this report automatically compensates the chamber for both sine and random wave testing. The system consists of 27 separate closed servo loops, each one controlling a 13 octave segment of the total frequency band. The resulting amplitude-controlled segments are then summed and applied to the acoustic driver. The problem of phase cancelation between adjacent 13 octave bandpass filters is overcome through the use of special linear-phase bandpass filters and a toroidol phase compensating transformer. By using a unique solid-state circuit to compensate each frequency segment, it is possible to equalize all resonance occurring in a 200-cu-ft acoustic chamber within 10 sec after the application of a wide-band random noise input.