Abstract: An acoustic transducer has an output which is fed through a preamplifier to a multiple of parallel filters. Each parallel filter passes a band of signals having frequencies within a predetermined range of frequencies. Each band of signals is converted from analog to digital and input to a signal processing unit. The peak amplitude value and the average value of the output from each parallel filter is determined. The peak amplitude value and average amplitude value are compared to determine if a pretrip condition occurs. If a pretrip condition occurs, the average value is saved and compared with the peak amplitude value for a select number of time periods to determine if a trip condition exists. A special test can be used which alters the number of time periods used in determining if a trip condition exists, based on a comparison the peak amplitude value between two of the bands.

Abstract: In accordance with the invention a wall assembly for displaying visual messages, has a front portion, a display area in the front portion, the display area providing an opening through the wall assembly, a projection screen for covering the opening, and a display panel movable from behind part of the front portion into the display area. The wall assembly can be placed before an audience and used to display visual messages which may be shown for example by projection onto a screen, or by writing on a display panel, with a facility for covering the display area when that area is not in use.