Abstract: A cinema sound system for unperforated screens includes for each stereophonic channel a floor positioned direct radiator bass speaker unit radiating into quarter space and an upper frequency speaker unit mounted above the screen. Each upper frequency speaker unit includes a middle frequency driver mounted in a sealed rear enclosure which is attached to the throat of a middle frequency horn. A constant directivity high frequency horn with a high frequency driver attached to a rear end is mounted coaxially in the middle frequency horn. Sharp cutoff active crossover filters divide the input signal into low, middle, and high frequency band signals which are separately power amplified. The middle frequency horn is adapted to function as a direct radiator at a lower end of the middle band and as a sectoral horn above an unloading frequency of the middle frequency horn.

Abstract: A device for detecting the oscillating condition caused by the acoustic coupling between a loud speaker and a microphone at those frequencies which do not satisfy the Nyquist stability criteria, and for thereafter decreasing the system loop gain. The detection circuitry consists of a bank of phase lock loop detectors. Each phase lock loop within the bank covers a limited frequency range such that together the bank covers the entire audio band. Upon detecting an oscillation in the particular phase lock loop frequency passband, that phase lock loop control voltage is automatically internally adjusted to align the phase lock loop frequency with the oscillating frequency. An abnormal phase lock loop control voltage signifies the presence of an unwanted oscillating signal and is detected by circuitry which includes a diode peak detector followed by a pair of voltage comparators. Upon detecting the ringing condition, the loop gain is decreased until the ringing condition ceases.