amplifier 291 the function of alarm invert switch 294 is to allow an inverted mode to be selected in which an alarm is created at all times unless a defect is present and detected in structural material 14 amplifier 296 is connected to the output of switch 294 and serves to amplify the magnitude of the signal produced by switch 294 amplitude phase select switch 298 serves to place test set 22 in a phase sensitive or amplitude sensitive mode meter 300 is connected to switch 298 through battery test switch 302 meter 300 corresponds to the meter 26 on test set 22 see fig1 the function of battery test switch 302 is to cause meter 300 to momentarily indicate the state of charge in the battery 270 a function of the switch 298 is to select between the display of an amplitude sensitive signal on bus 293 and a phase sensitive signal on bus 250 for display on meter 300 that is the condition of switch 298 determines whether the deflection of meter 300 is related to an amplitude measurement or to a phase measurement when switch 298 is activated so that meter 300 displays a deflection related to the amplitude of signals received by microphone 222 amplifier 291 may be adjusted with linear logarithmic select.